Track assembly for a storage system

Abstract

A track system for a grid framework structure 14 configured to guide a load handling device (30, Fig 3) operative to move one or more containers (10), comprises a supporting framework structure comprising a plurality of storage columns, each of which are arranged to accommodate a stack of storage containers. Track system comprising a plurality of tracks arranged to form a grid pattern comprising a plurality of grid cells, each of the plurality of tracks comprising a plastic material and mounted on supporting framework structure such that each stack of storage containers is arranged below a respective grid cell of the plurality of grid cells Each of the plurality of tracks has a surface resistance less than or equal to I x 10 10 ohms (Q) or a surface resistivity less than or equal to I x 10 10 ohm.meter (Q.m) and a light reflectance value (LRV) greater than 30%.

Description

Technical Field The present invention relates to the field of a storage or fulfilment system in which stacks of bins or containers are arranged within a grid framework structure, more specifically to a track or track assembly configured to guide one or more load handling devices operative to move one or more containers stored in the storage or fulfilment system. Background Storage systems comprising a three-dimensional storage grid structure, within which storage containers / bins are stacked on top of each other, are well known. PCT Publication No. WO2015 / 185628A (Ocado) describes a known storage and fulfilment system in which stacks of bins or containers are arranged within a grid framework structure. The bins or containers are accessed by load handling devices 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, known as bins 10, are stacked on top of one another to form stacks 12. The stacks 12 are arranged in a grid framework structure 14 in a warehousing or manufacturing environment. The grid framework structure 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 for storage of a stack of containers. Figure 1 is a schematic perspective view of the grid framework structure 14, and Figure 2 is a 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), and the product items within a bin 10 may be identical, or may be of 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 detailed description of the grid framework structure is discussed in WO 2021175873 (Ocado Innovation Ltd); the contents of which are incorporated herein by reference. 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 plurality of horizontal grid structures 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 vertical movement of the bins 10. The top level of the grid framework structure 14 comprising a track system 15 which includes a plurality of rails or tracks 22 arranged in a grid pattern across the top of the stacks 12. Referring additionally 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 (for example, an X-direction) across the top of the grid framework structure 14, and a second set 22b of parallel rails 22, arranged perpendicular to the first set 22a, guide movement of the load handling devices 30 in a second direction (for example, a Y-direction), perpendicular to the first direction. In this way, the rails 22 allow movement of the robotic load handling devices 30 laterally in two dimensions in the horizontal X-Y plane, so that a load handling device 30 can be moved into position above any of the stacks 12. The upright columns of the grid framework structure are interconnected at their top ends by the rails or tracks intersecting in the grid. The intersections of the rails or tracks in the grid structure are generally termed ‘nodes’ of the grid structure. For the purpose of the present invention, the node is the area of the track system where the plurality of rails or tracks intersect in the grid pattern. Typically, the first and second set of rails comprise individual elongated rail or track sections that are interconnected together in the first and second direction at the interconnections where the track or rail sections meet at the top ends of the upright columns. The rails or tracks typically comprise an elongated element which is profiled to guide a load handling device on the grid structure and are typically profiled to provide either a single track surface so as to allow a single load handling device to travel on the track, or a double track so as to allow two load handling devices to pass each other on the same track. In the case where the elongated element is profiled to provide a single track, the track comprises opposing lips (one lip on one side of the track and another lip at the other side of the track) along the length of the track to guide or constrain each wheel from lateral movement on the track. In the case where the profile of the elongated element is a double track, the track comprises two pairs of lips along the length of the track to allow the wheels of adjacent load handling devices to pass each other in both directions on the same track. To provide two pairs of lips, the track typically comprises a central ridge or lip and a lip either side of the central ridge. In all cases, when traversing on the grid structure, the wheels of the load handling device are constrained on both sides or faces of the wheels of the load handling device. WO2018 / 146304 (Autostore Technology AS) teaches a rail arrangement for wheeled vehicles in a storage system, where the rail arrangement comprises a first set of parallel rails and a second set of parallel rails. The first and second sets of parallel rails form a grid where the second set is arranged perpendicular to the first set and intersect the first set at their crossroads, thus forming a grid of parallel rails. The crossroads of the intersecting rails correspond to the interconnections of the upright columns. Each of the rails of both sets of rails comprises two parallel tracks adapted for guiding the wheels of the vehicles or load handling devices. The rails or tracks comprise a number of longitudinal segments or sections with two edge ridges running along each longitudinal edge of the longitudinal segments and a central ridge running parallel with the edge ridges. The area between the ridges forms the tracks for receiving and guiding the wheels of the vehicles. The width of the central ridge is adapted to ensure that two vehicles can pass each other when running on the tracks in different directions on the same segment. The edge ridges of each intersecting rail are in contact with each other, forming a comer ridge. The comer ridges are arranged tightly connected in order to prevent the vehicle from snagging at the joints. In order for the vehicles to have a smooth drive across the intersections, the comer ridges are rounded at the insides. A known load handling device 30 shown in Figure 4 and 5 comprising a vehicle body 32 is described in PCT Patent Publication No. WO2015 / 019055 (Ocado), hereby incorporated by reference, where each load handling device 30 only covers one grid space of the grid framework structure 14. Here, the load handling device 30 comprises an upper portion comprising a rechargeable power source and a controller for controlling the one or more motors of the load handling device and a lower portion comprising a wheel assembly comprising a first set of wheels 34 consisting of a pair of wheels on the front of the vehicle body 32 and a pair of wheels 34 on the back of the vehicle 32 for engaging with the first set of rails or tracks to guide movement of the device in a first direction, and a second set of wheels 36 consisting of a pair of wheels 36 on each side of the vehicle 32 for engaging with the second set of rails or tracks to guide movement of the device in a second direction. Each of the sets of wheels are driven to enable movement of the vehicle in X and Y directions respectively along the rails. One or both sets of wheels can be moved vertically to lift each set of wheels clear of the respective rails, thereby allowing the vehicle to move in the desired direction on the grid. The load handling device 30 is equipped with a lifting device or crane mechanism to lift a storage container from above. The crane mechanism comprises a winch tether or cable 38 wound on a spool or reel (not shown) and a grabber device 39. The lifting device comprises a set of lifting tethers 38 extending in a vertical direction and connected nearby or at the four comers of a lifting frame 39, otherwise known as a grabber device (one tether near each of the four comers of the grabber device) for releasable connection to a storage container 10. The grabber 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 and 2. The wheels 34, 36 are arranged around the periphery of a cavity or recess, known as a container-receiving recess or container receiving space 41, in the lower part. The recess is sized to accommodate the container 10 when it is lifted by the crane mechanism, as shown in Figure 5 (a and b). When in the recess, the container is lifted clear of the rails beneath, so that the vehicle can move laterally to a different location. On reaching the target location, for example another stack, an access point in the storage system or a conveyor belt, the bin or container can be lowered from the container receiving portion and released from the grabber device. Typically, one or more load handling devices remotely operable on the grid structure is configured to receive instructions from a master controller, to retrieve a storage container from a particular storage location within the grid framework structure Wireless communications and networks may be used to provide the communication infrastructure from the master controller via one or more base stations to the one or more load handling devices operative on the grid structure. A controller in the load handling device in response to receiving the instructions 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 storage column at a particular location on the grid structure. The instruction can include various movements in an X-Y direction on the grid structure. Once at the storage column, the lifting mechanism is then operated to grab the storage container and lift it into a container receiving space in the body of the load handling device where it is subsequently transported to another location on the grid structure commonly known as a drop off port. The storage container is lowered to a suitable pick station so as to allow retrieval of the item from the storage container. Movement of the load handling devices on the grid structure or track system also involves the load handling devices being instructed to move to a charging station which is usually located at the periphery of the grid structure. When the load handling device is moving on the tracks, it is controlled to accelerate from a start position and decelerate to a stop position. The start and stop positions will depend on the trajectory or route set up for the load handling device prior to picking up a storage container from one storage column in the grid framework structure and placing it in another storage column. A detailed description of controlling the movement of the load handling device on the grid structure or track system is taught in the International Patent Applications, WO 2022136454 (Ocado Innovation Ltd); WO 2022136475 (Ocado Innovation Ltd); WO2022136480 (Ocado Innovation Ltd); and WO2022136482 (Ocado Innovation Ltd). To enable the controller to locate the position of the load handling device relative to the track system, each load handling device may be provided with a position sensor. Typically, the position sensor arranged on the body of the load handling device detects and works on the principle of detecting light reflected from the tracks when the load handling device encounters an intersection of the plurality of tracks. WO2018082972 (Autostore Tech AS) teaches a method and vehicle for tracking the position of a remotely operated vehicle following a set route relative to tracks laid out on a frame structure forming a grid, the vehicle having first and second sets of wheels connected to drives for moving the vehicle in corresponding x- and y-directions on the grid, the method comprising: receiving information of the number of track crossings to pass between start and stop positions in x- and y- directions according to the set route; directing sensors attached to the vehicle at the tracks along the route of the vehicle; detecting and monitoring track crossings passed when moving the vehicle in the x- and y-directions according to the set route, and transmitting a signal to a controller, controlling the drives of the wheels of the vehicle, when the number of track crossings passed is close to the total number of track crossings to pass between the start and stop positions in respective x- and y- directions along the set route. The sensors are optical sensors for detecting reflection of light from the tracks. Light is reflected from tracks when a robot is moving along the tracks in x- or y-directions. When the robot is passing a track crossing, the reflected intensity of the detected light will change. Detection of track crossings is based on measured intensity of reflected light. For example, when the robot is moving through a track crossing, the intensity of the light reflected will drop since no light is reflected. Multiple rails or tracks are necessary to build the track system. The greater the number of rails or tracks necessary to build the track system, the more complicated the assembly of the track system. In a majority of cases, there is a two to one relationship between the number of rails or tracks at each of the nodes or the intersections of the tracks or rails in the track system - in the sense that multiple rails or tracks are connected together at each node of the track system. For example, in WO2018 / 146304 (Autostore Technology AS), when making the intersections between the first and second sets of rails or tracks, the second set of rails or tracks all comprise a recess into which the first set of rails or tracks may be arranged. Additionally, to provide a plurality of rectangular or square shaped grid cells, multiple different sized track or rail sections are connected together in the grid structure. For example, for each grid cell there is a rail or track section extending in one direction of one length and another track or rail section extending in a second direction of a different length. The different lengths of the rail or track sections meet at a node in the grid structure where they intersect. The need to have different lengths of rail or track sections complicates the assembly of the track or rail sections in a grid pattern. The tracks are largely fabricated from metal, e.g. aluminium, usually by an extrusion process and therefore, the problem of light reflected from tracks being detected by the optical sensor is not a major concern. Extrusion consists of forcing a metallic work piece through a shaped die to reduce its cross section and convert it to the desired shape. However, the dimensional tolerances of extrusion are poor in comparison to other forming processes such as injection moulding and additive manufacturing such as 3D printing. Since the track sections are fabricated to tight dimensional tolerances to provide a continuous uninterrupted track surface when the individual track sections are assembled together on the grid structure, the extruded track sections are subsequently followed by a machining process. Moreover, extrusion is not able to produce the various cut outs and islands in the profile of the track without a subsequent machining process. WO 2022034190 (Ocado Innovation Limited) teaches a track system for a grid framework structure that is configured to guide a load handling device operative to move one or more containers, said grid framework structure comprising a plurality of grid members converging or intersecting at nodes in a grid pattern to form a grid structure comprising a plurality of grid cells. The track system comprises a plurality of track sections mountable to the grid structure to form a track surface upon which the load handling devices may move on the grid structure, wherein each track section of the plurality of track sections is formed as a unitary body so as to provide a track surface extending in transverse directions. In other words, each of the plurality of track sections can be cross shaped, having a first track section element extending in the first direction and a second track section element intersecting with the first track section element and extending in the second direction. Each of the plurality of plastic tracks is formed from a plastic material. In comparison to the use of metal in the fabrication of the tracks, the use of plastic material to form the tracks enables the tracks to be fabricated from lower cost fabrication methods, e g. injection moulding, and is much lighter than metal. However, the problem with the use of plastic is that the tracks are susceptible to generate static electricity when interacting with the robot wheels operating on the track system. To mitigate the build-up of static electricity on the tracks, the tracks are made conductive by incorporating a conductive material such as carbon into the plastic to safely dissipate the build-up of charge on the tracks to ground. However, the problem of incorporating carbon material into the plastic material to render them conductive is the ability of the position sensor to determine the position of the load 5 handling device relative to the track system due to the inability of the tracks to reflect light with sufficient intensity to be detected by the position sensor. A track system is thus required that benefits from the use of plastic material but yet allow a load handling device to determine its position relative to the track system. Summary of the Invention The present invention has mitigated the above problem by providing a track system comprising a plurality of tracks arranged in a grid pattern, wherein each of the plurality of tracks is formed from a plastic material having a material characteristic that is able to dissipate static electricity and yet be sufficiently reflective to enable light to be reflected from the tracks with sufficient intensity to be able to be detected by an optical sensor. To dissipate static electricity, each of the plurality of tracks is formed from a plastic material comprising an anti-static additive. More specifically, the present invention provides a track system for a grid framework structure configured to guide a load handling device operative to move one or more storage containers, the grid framework structure comprising a supporting framework structure comprising a plurality of storage columns, each of the plurality of storage columns being arranged to accommodate a stack of storage containers; said track system comprising: - a plurality of tracks arranged to form a grid pattern comprising a plurality of grid cells, said track system is configured to mounted on the supporting framework structure such that each stack of storage containers is arranged below a grid cell of the plurality of grid cells; characterised in that each of the plurality of tracks has a surface resistance less than or equal to 1 x 1010 ohms (Q) or a surface resistivity less than or equal to 1 x 1010 ohm.meter (Q.m) and a light reflectance value (LRV) greater than 30%. Light Reflectance Value (LRV) is a measurement of the light that is reflected from a surface when illuminated by a light source and is a characteristic of a material. It is expressed as a number (or percentage value) of 1-100; the higher the number, the more reflective the surface. Typically, dark colours such as dark grey and black have a LRV less than 30% and light colours such as white have a LRV greater than 30%. For example, bright white has an LRV of 85% and black has a LRV of about 7%. Dark colours are poor at reflecting white light and considering that the position sensor works on the principle of reflecting light from the tracks, the use of such dark colours would significantly reduce the intensity of light reflected from the track. In a worst case scenario, the reduced amount of light reflected from the track would result in the inability of the position sensor mounted to the load handling device to determine the position of the load handling device relative to the track system. Typically, carbon black is added to the plastic material to make the plastic material conductive and prevent the build-up of static electricity on the tracks. However, the addition of carbon black to the plastic material in the fabrication of the tracks results in the tracks being dark in colouration and therefore, would inherently have a low LRV. In an aspect of the present disclosure, a non-carbon anti-static additive is added to the plastic material. The use of a noncarbon anti-static additive increases the availability of plastic materials having a relatively high LRV in comparison to darker materials. Optionally, each of the plurality of tracks comprises Acrylonitrile Butadiene Styrene (ABS plastic). An example of an anti-static ABS material used in the fabrication of the plastic tracks is traded under the name Pre-Elec® ESD22226 from Premix. Optionally, each of the plurality of tracks has a surface resistivity in the range 1 x 105 Q.m (ohm meter) to 1 xlO10 Q.m (ohm meter) to dissipate static electricity. Preferably, the surface resistivity is in the range IxlO8 to IxlO10 Q.m. In accordance of the present invention, the surface resistivity is measured by measuring the resistance from two discrete points on a section of a track using a surface resistivity meter using a Bondline resistivity meter, Swindon, Wiltshire, United Kingdom. Typically, an upper limit of IxlO10 Q.m is able to dissipate the build-up of static electricity on the tracks. To provide increased reflection of light from the tracks, optionally, each of the plurality of tracks is substantially white. The use of plastic material to fabricate the tracks allows the tracks to be fabricated to tighter tolerances that can be achieved by extrusion alone. The use of plastic material to fabricate the tracks of the present invention allows the tracks to be injection moulded. Unlike extrusion, injection moulding allows parts to be formed to very tight tolerances, removing or mitigating the need to carry out excessive machining on the finished part. In addition, injection moulding allows one or more profiles to be incorporated to the track in precise or intricate detail, which is essential to guide the wheels of the load handling device on the track without the possibility of derailing. For example, depending on the number of profiles, the track can be a single or double track where the profile of the single track comprises opposing lips, i.e. a lip either side of the track, to guide or constrain each wheel on the track. In the case of the profile of a double track, the track comprise two pairs of opposing lips along the track to allow load handling devices to pass each other in both directions, i.e. having at least a central ridge separating two tracks either side of the central ridge. The tracks of the present invention are not limited to a single track or a double and can comprise one or more profiles to provide one or more track surfaces. Optionally, the track system further comprises a plurality of track supports arranged in the grid pattern of the track system to define a track support structure, said plurality of tracks being mountable to the track support structure. Optionally, each of the plurality of tracks comprises means for snap-fitting onto one or more of the plurality of track supports. The present applicant has realised that by mounting the track to a track support, whereby the track support bears at least a portion or the full weight of the load handling device operative on the track system, the track mounted thereon can be fabricated from less structurally supporting materials that can be moulded to very tight dimensional tolerances. This is because the track does not need to be composed from materials that gives it sufficient load bearing capability, e.g., metal. Optionally, the plurality of tracks is arranged so that the distal ends of adjacent tracks is between or intermediate the intersections of the plurality of the plurality of track supports. In the present invention, the distal ends of adjacent tracks meet between their respective intersections. Optionally, the distal ends of adjacent tracks meet half way or mid-point between their respective intersections. For the purpose of the present invention, the term “meet” covers abutment or connection or engagement between adjacent track sections. This has the advantage that only tracks a single size would be necessary to cover a substantial portion of the track supports instead of having multiple separate parts, thereby improving the manufacturability of the tracks, i.e. a single tool design or one size fits all. Optionally, the plurality of tracks comprises a plurality of track sections, each of the plurality of track sections comprises: a) a first track section element extending in a first direction; and b) a second track section element intersecting with the first track section element and extending in a second direction, the second direction being transverse to the first direction. In other words, each of the plurality of track sections can be cross shaped, having a first track section element extending in the first direction and a second track section element intersecting the first track section element and extending in the second direction. The first and second track section elements can also be termed transverse portions or branches of the track section. Being formed as a single or unitary body allows the track section to be mounted at each of the nodes of the track support structure where the track supports intersect, and therefore the track section is able to extend in both the first and the second direction of the track support structure. This removes the need to have separate tracks that separately extend in the first and second directions as found in prior art solutions. However, the present invention is not limited to having a one to one relationship between the number of track sections and the number of nodes of the track support structure. For example, a single track section formed as a unitary body can be configured to extend across multiple nodes of the track support structure and yet provide a track surface extending in transverse directions. In the case where the grid cells are square, preferably, each track section of the plurality of track sections can have rotational symmetry in a horizontal plane with an order of rotational symmetry of four. Having an angle of rotational symmetry of 90°, the track section of the present invention can be rotated four times and still coincide with itself. This provides the flexibility of mounting the track section of the present invention to the track support structure in multiple different orientations and thereby removing the “jigsaw” effect of assembling the track, i.e. limited to one orientation. The present disclosure provides a grid framework structure for supporting one or more robotic load handling devices operative on the grid framework structure, the grid framework structure comprising: a supporting framework structure comprising a plurality of storage columns, each of the plurality of storage columns being arranged to accommodate a stack of storage containers; a track system comprising a track system according to the present disclosure, said track system being mounted on the supporting framework structure such that each stack of storage containers is arranged below a respective grid cell of the plurality of grid cells of the track system . The present invention further provides a storage and retrieval system comprising: i) a grid framework structure according to the present disclosure; ii) a plurality of stacks of storage containers arranged in storage columns located below the track system, wherein each storage column is located vertically below a respective grid cell of the plurality of grid cells; iii) at least one load handling devices for lifting and moving a storage containers stacked in the stacks, the at least one load handling device being remotely operated to move laterally on the track system above the storage columns to access the containers through the grid cells, said at least one load handling device comprising: a) a wheel assembly for guiding the load handling device on the track system; b) a container-receiving space located above the track system; c) a container lifting mechanism comprising a grabber device for releasable connection to a storage container, said container lifting mechanism being arranged to lift a single container from a stack into the container-receiving space; d) at least one position sensor for determining the position of the at least one load handling device relative to the track system, said at least one position sensor having a predetermined detection range in the range 5mm to 100mm. Optionally, the position sensor has a predetermined detection range in the range 10mm to 5 50mm. Optionally, the position sensor is a photoelectric sensor operating in the visible red light and / or infrared light frequency range. Optionally, the visible red light has a wavelength of 624nm and the infrared light has a wavelength of 850nm. An example of such a position sensor is a limited reflective photoelectric sensor having a detection range of 10mm to 50mm traded under the name E3FA-VP21 from Omron Corporation, Kyoto, Japan. 10 Description of Drawings Further features and aspects of the present invention will be apparent from the following detailed description of an illustrative embodiment made with reference to the drawings, in which: Figure lisa schematic diagram of a grid framework structure according to a known system, Figure 2 is a schematic diagram of a top down view showing a stack of bins arranged within the supporting framework structure of Figure 1. Figure 3 is a schematic diagram of a known storage system comprising a load handling device operating on the grid framework structure. Figure 4 is a schematic perspective view of the load handling device showing the container lifting mechanism gripping a container from above. Figure 5(a) and 5(b) are schematic perspective cut away views of the load handling device of Figure 4 showing (a) a storage container accommodated within the container receiving space of the load handling device and (b) the container receiving space of the load handling device. Figure 6 is a perspective view of a second example of a grid framework structure. Figure 7 is a perspective view of a track support element of the track system. Figure 8 is a perspective view of a track or rail element. Figures 9(a and b) is a perspective view of a track or rail system configured to be mounted to the track support structure, where (a) show a track section being mounted to the track support structure; and (b) the track section being mounted to the track support structure. Figure 10 is a perspective view showing a section of an underlying track support structure at a node of intersecting track supports. Figure 11 is a perspective view of a top plan view of a track section according to an embodiment of the present disclosure. Figure 12 is a perspective view of the underside of the track section shown in Figure 9 showing the plurality of tabs for connecting to the track support structure shown in Figure 10. Figure 13 is a perspective view of a load handling device parked on a grid cell of the track system. Figure 14 is a perspective view showing a wheel of the wheel assembly of the load handing device parked on the track. Figures 15(a and b) are schematic drawings showing (a) the wheels on the side of the load handling device in the raised position above the tracks; and (b) the wheels on the side of the load handling device in the lowered position to engage with the tracks. Figures 16(a and b) are perspective views of the lower portion of the load handling device showing (a) a set of wheels engaged with a section of the track; and (b) the load handling device with the set of wheels in the raised position moving away from the section of the track. Figure 17 is a plot showing the frequency of interruptions of the intensity of light reflected from the tracks as the load handling device moves along the tracks in the X direction or the Y direction. Detailed Description It is against the known features of the storage system such as the grid framework structure and the load handling device described above with reference to Figures 1 to 5, that the present invention has been devised. A grid framework structure can be divided into a supporting framework structure 42, 142 in the lower portion of the grid framework structure and a track system 15, 115 in the upper portion of the grid framework structure (see Figure 1 and 6)). The supporting framework structure 42 comprises a plurality of vertical members or vertical uprights 16 arranged to provide a plurality of storage columns 44 for the storage of storage containers 10 in one or more stacks 12. Each of the vertical uprights 16 are generally tubular. In transverse crosssection in the horizontal plane of the storage column 44 shown in Figure 2, each of the vertical uprights 16 comprises a hollow centre section 46 (typically a box section) with one or more tote guides 48 mounted to or formed at the corners of the hollow centre section 46 that extends along the longitudinal length of the vertical upright 16 for guiding the movement of the containers along the storage column 44. A storage column 44 corresponds to a single grid cell. A method for erecting the grid framework structure shown in Figure 1 involves erecting the plurality of vertical uprights 16 one-by-one in a “stick by stick” approach and then mounting the track system 15 on the supporting framework structure 42. An example of the “stick by stick” build of the grid framework structure is taught in WO2021175873 (Ocado Innovation Ltd), the details of which are incorporated herein by reference. Alternatively, the supporting framework structure 142 can be erected from prefabricated modular structural components 46, i.e. a three dimensional arrangement of prefabricated frames 46, where each prefabricated frame comprises a plurality of vertical uprights 48 braced by one or more bracing members 50 (See Figure 6). The prefabricated modular structural components are load bearing in the sense that when assembled together to form the grid framework structure, the prefabricated modular structural components provide a three dimensional load bearing structure to support one or more load handling devices moving on the track system 115. The use of prefabricated modular structural components to erect the grid framework structure allows the grid framework structure to be assembled at a much faster rate than the traditional ‘stick-built’ approach where individual vertical uprights are initially erected one by one on the floor, and then subsequently mounting the track supports to the upper end of the vertical uprights. A detailed description of erecting the supporting framework structure from prefabricated modular structural components is discussed in WO 2022034195 (Ocado Innovation Limited); the contents of which are incorporated herein by reference. However, the present disclosure is not limited to the supporting framework structure discussed above and can be any type of supporting framework structure comprising a plurality of storage columns that is able to store a plurality of storage containers in one or more stacks of the storage containers. Supported by the supporting framework structure 42, 142 is a track system 15, 115 comprising a plurality of tracks 22 comprising a first set of tracks 22a extending in a first direction and a second set of tracks 22b extending in the second direction, the second direction being substantially perpendicular to the first direction such that the plurality of tracks 22 are arranged in a grid pattern comprising a plurality of grid cells. A load handling device 30 is operative to move on the grid framework structure by being guided by the plurality of tracks 22. The track system 15, 115 may further comprise a track support structure comprising a plurality of track supports extending in the first direction and in the second direction. The plurality of tracks 22 can be integrated into the plurality of track supports as a single body, e.g. by extrusion. In the present disclosure, the plurality of tracks 22 are mounted to the track support structure, e.g. by a snap-fit joint. The track support can also be sub-divided into discrete track support elements 56 that are linked or fixedly connected together to form the track support. An individual track support element 56 used to make up a track support is shown in Figure 7. The track support in transverse cross section can be a solid support of C-shaped or U-shaped or I-shaped cross section, or even double-C or double-U shaped support. To complete the track system 15, 115 once the track support elements 56 are interlocked together in a grid pattern comprising track supports extending in the first direction and track supports extending in the second direction, a track 22a, 22b is mounted to the track support elements 56. The track 22a, 22b is either snap-fitted and / or fitted over the track support in a slide fit arrangement. Like the track support, the track comprises a first set of tracks 22a extending in the first direction (e.g. X direction) and a second set of tracks 22b extending in the second direction (e.g. Y direction), the first direction being perpendicular to the second direction. A first set of tracks 22a is sub-divided into multiple track elements 58 in the first direction such that, when assembled, adjacent parallel track elements 58 in the first direction are offset by at least once grid cell. Similarly, a second set of tracks 22b is sub-divided into multiple track elements 58 in the second direction such that, when assembled, adjacent track elements in the second direction are offset by at least one grid cell. Further detail of the arrangement of the track elements on the track support structure is detailed WO2021175873 (Ocado Innovation Ltd), the details of which are incorporated herein by reference. An example of a single track element 68 is shown in Figure 8. As with the track support elements, multiple track elements in the first direction and the second direction are laid together to form a track in both directions. The fitting of the track element 58 to the track support comprises an inverted U-shaped cross-sectional profile that is shaped to cradle or overlap the top of the track support. One or more lugs extending from each branch of the U shape profile engage with the ends of the track support in a snap fit arrangement. Equally plausible is that the track 22a, 22b can be integrated into the track supports rather than being separate components. Instead of individual track elements making up the track system, the plurality of tracks 122 can be sub-divided into a plurality of track sections 158, each track section 158 formed as single unitary body and comprising track elements or portions 122a, 122b extending in the direction of the underlying track supports 18, 20 so as to provide a track surface that extends in the first direction and in the second direction, i.e. each track section 158 having connecting portions or elements 122a, 122b extending in transverse directions (see Figure 9(a and b)). For the purpose of explanation of the present invention, the connecting portions or track section elements 122a, 122b can be termed ‘branches’ that extend in transverse directions from the nodes 50. Like the individual track elements 58 discussed above, the plurality of tracks 122a, 122b are configured to be mounted to the track support structure comprising a first set of track supports 18 extending in the first direction and a second set of track supports 20 extending in the second direction (see Figure 9(a and b). To secure the plurality of tracks 22 to the track support structure, each of the track sections 158 can be snap fitted to the track support structure. The plurality of track supports 18, 20 extending in the first and second directions can comprise one or more opening or slots 60 to enable the plurality of tracks to be snap fitted to the track support structure (see Figure 10). In the particular embodiment of the present disclosure, the underside of the track section 158 shown in Figure 12 comprises one or more lugs or tabs 62 that are configured to be snap fitted to the track support 18, 20. The one or more lugs 62 can comprises a bead or protruding edge 64 that is arranged to deflect and be received in one or more openings 60 in opposing side walls (or vertical elements) of the track support 18, 20 in a snap fit arrangement as shown in Figures 8(a) and 8(b). The particular snap fit feature shown in Figure 12 is a cantilever snap fit. However, other forms of snap fit connections commonly known in the art for securing the track section to the track support are applicable in the present invention. Equally, other forms of securing the track section to the track support besides a snap fit joint are applicable in the present invention, e.g. the use of fasteners or an adhesive. There are two types of tracks for guiding one or more load handling devices on the grid framework structure and these are usually termed a double track or a single track. A double track allows two adjacent load handling devices to pass each other on the same track. To allow two load handling devices to pass each other on the same track, typically the dual track comprises two paths or track surfaces 66, 68 separated by a central ridge or rib 70 as clearly shown in Figures 8 and 11. To separately guide two load handling devices on the same track, each set of wheels of adjacent load handling devices is guided by a pair of lips 70, 72, 74 either side of the track (the central ridge 70 being shared by the two paths). In other words, each lip of the pair of lips 70, 72, 74 of the track defines a guide surface extending from the track surface for guiding on both sides or faces (inner and outer faces of the wheels) of the wheel, i.e. two pairs of guide surfaces on each track with a pair for each wheel. The guide surface provided by the pair of lips for guiding the load handling device along the track is shown in Figures 8 and 11. The track of the present disclosure is not limited to a double track and the track can be one or more tracks comprising one or more track surfaces provides by one or more depressions or ridges. For example, the track can be a single track comprising a single track surface formed from a pair of lips either side of the track surface for guiding a single wheel along the track. Typically, when moving on the track system, the load handling device receives instructions from a control system or controller to pick up or deposit a storage container from or to a target storage column in the grid framework structure. Such instructions comprise instructing a load handling device to travel along a particular trajectory or route on the track system to the target storage column. Depending on the position of the target storage column in the grid framework structure, this may involve the load handling device moving across several grid cells in the X and Y Cartesian directions on the track system. This also involves the load handling device accelerating from a start position at its current position and decelerate to a stop position. To determine the position of the load handling device relative to the track system, the load handling device comprises at least one position sensor 76, 78, usually an optical sensor mounted to the body 32 of the load handling device 30, more specifically to a wheel mount 80 located in the lower portion of the load handling device. As the first 34 and second 36 sets of wheels are configured to move in a vertical direction when changing direction on the track system, the at least one position sensor is mounted to at least two sides of the body 32 of the load handling device 30 as shown in Figure 13. A first position sensor 76 is mounted to one side of the load handling device so as to sense movement of the load handling device in the X-direction on the track system and a second position sensor 78 mounted to the other side of the load handling device so as to sense movement of the load handling device in the Y-direction on the track system. The first or the second position sensors are actuated or become active when the load handling device is moving in the X-direction or in the Y-direction in order to count the number of grid cells crossed. To actuate the first or second position sensors 76, 78 when the load handling device is moving in either the X-direction or the Y-direction respectively, the first and second position sensors 76, 78 are mounted to each of the wheel mounts 80 of the first and second sets of wheels 34, 36. Considering that the wheels assembly of the load handling device comprises a pair of wheels on the front of the vehicle body 32 and a pair of wheels 34 on the back of the vehicle 32 for engaging with the first set of rails or tracks to guide movement of the device in a first or X direction, and a second set of wheels 36 consisting of a pair of wheels 36 on each side of the vehicle 32 for engaging with the second set of rails or tracks to guide movement of the device in a second or Y direction, each of the wheel mounts 80 typically supports two wheels as shown in Figure 13. The first and the second position sensors 76, 78 are actuated or activated during a directional change of the load handling device on the track system. A pair of wheel mounts 80 on opposite sides of the body of the load handling device are actuated or become active at the same time when they are lowered from the body 32 into engagement with the tracks during a directional change operation on the track system. It is important that the load handling device is correctly positioned on a target grid cell before the first or the second sets of the wheels can be lowered into engagement with the tracks. The target grid cell is defined as the grid cell of the track system that is directly above the target storage column discussed above. When lowering the first or the second sets of wheels into engagement with the tracks, the wheels 82, in particular the tyres 84, should ideally sit squarely on the track surface 66, 68 between the opposing lips 70, 72 of the track surface as shown in the cross-sectional view of a track in Figure 14. The operation of lowering the first or second sets of wheels so that they sit squarely on the track surface is termed “parking” of the wheels on the tracks. Typically, there is a tight dimensional tolerance between the width of the wheel 82 and the width of the track surface. Nominally, the distance, X, between the edge of the wheel and the lip of the track when the wheel is lowered onto the track surface is less than or equal to about 4mm (see Figure 14). Any deviation from this distance, there is the risk that the wheels may park on one of the lips 70, 72, 74 of the track and in a worst case scenario may cause the load handling device to derail when moving on the track system. As a result, it is paramount that the load handling device is correctly positioned on the target grid cell when parking the wheels on the tracks. In a particular example of the present disclosure, the position sensors work on the principle of detecting light reflected from the tracks as the load handling device moves along the tracks in either X-direction or the Y-direction. In the example of the present disclosure, the position sensor 76, 78 is an optical photoelectric sensor comprising a LED (light emitting diode) light source emitting light in the visible red light wavelength range. Examples of such sensors include but is not limited to a photoelectric sensor traded under the name E3FA-VP21 from Omron Corporation, Kyoto, Japan but other optical sensors are permissible in the present disclosure that are configured to function as a position sensor mounted on the load handling device. E3FA-VP21 has a detection range of 10mm to 50mm beyond which the optical sensor has a limited detection range of the reflected light and operates in the red LED wavelength (624nm). The detection range, d, of the position sensor covers the range of vertical movement of the first or second sets of wheels in the raised and lowered position as demonstrated in Figures 15(a and b). This enables the position sensor to receive a reflected signal when the first or second sets of wheels is in the raised or lowered position relative to the tracks. The detection range, d, is chosen to prevent light reflected from below the tracks, e.g., the storage containers in a storage column, being invertedly detected by the position sensor resulting in a false reading to be registered by the control system. As the position sensor is mounted to the respective wheel mounts 80 supporting the first or second sets of wheels, the position sensor moves in a vertical direction when the first or second sets of wheels are raised or lowered to engage with the track below during a directional change. Referring to Figures 15(a and b), the distance between the position sensor and the track (or surface of the track) when the first or the second sets of wheels is in the raised position is referred to as “y” (see Figure 15a) and the distance between the position sensor and the track (or surface of the track) when the first or second sets of wheels is in the lowered position to engage with the track is referred to as “x” (see Figure 15b). As the position sensor is mounted to the wheel mount 80, the distance, “y”, is greater than the distance, “x”. The detection range, d, of the position sensor is chosen so that it is greater than the distance, y, to cover any marginal errors when the first or second sets of wheels is in the raised position but not too great to receive reflections from below the track, e.g., the storage containers below the track. The detection range of the position sensor is very much dependent on the range of vertical movement of the first or the second sets of wheels and / or the position of the position sensor mounted to the wheel mount. Mathematically the detection range, d, can be expressed as x <d <(y + s); where e is the marginal error that the detection range should cover when the first or second sets of wheels is in the raised position. In the particular example of the present disclosure, the detection range, d, is chosen to be from 10mm to 50mm. A change in intensity of light reflected from the tracks is used to provide an indication of the number of grid cells crossed by the load handling device when moving the X-direction or the Y-direction on the track system. As the load handling device crosses a track, the light reflected from the track will drop as the depth in the grid cell exceeds the detection range, d, of the position sensor which in turn results in a change in intensity of light returned to the position sensor. There are various techniques to measuring the number of grid cells crossed by the position sensor when the load handling device moves along the track system. In one example, the position sensor is configured to measure the number of track crossings passed when moving in the X and / or Y directions on the track system as taught in WO 2018 / 082972 (Autostore Technology AS). A track crossing is the area of the track system where the plurality of tracks intersects, i.e. it is the area of the crossing of the X and Y tracks. Such an area is defined as a “node” of the track system and is depicted in Figures 9(a and b) by the reference 50. Light is reflected from the lips of the tracks when the load handling device is moving in either the X or Y directions along the track. When the load handling device is moving through a track crossing, the intensity of light reflected will drop since no light is reflected. The drop in intensity of reflected light provides an indication of travelling a single grid cell and thus, the frequency of the interruptions in the intensity of the reflected light provides an indication of the number of grid cells travelled by the load handling device. In another example, the at least one position sensor 76, 78 is mounted to each of the sides of the load handling device and is configured to sense the number of tracks crossed by the leading edge and / or the trailing edge of the loading handling device when moving in either the X or Y directions on the track system. This is exemplified by the schematic drawings of the load handling device 30 on a portion of the track system in Figures 16(a and b). In Figure 16a, the position sensor 76, 78 detects light reflected from the track 58 when the load handling device is centred on the grid cell. In this example, the position sensor is positioned to detect light reflected from the surface of one of the lips 72, 74 at the edge of the track. As the load handling device moves in the X or Y direction towards an adjacent grid cell, the position sensor moves away from the track towards the adjacent grid cell as shown in Figure 16b. As a result, the intensity of the reflected light will drop since no light is reflected from within the grid cell. When the load handling device is positioned above the adjacent grid cell such that the position sensor is positioned above a track, the intensity of the light detected by the position sensor increases as light is reflected from the track in the adjacent grid cell. The number of interruptions of the reflected light or the change in frequency of the light intensity of light reflected from the tracks when the load handling device moves across a plurality of grid cells when moving in the X direction or the Y direction provides an indication of the number of grid cells crossed by the load handling device. Figure 17 is a plot of the intensity of reflected light detected by the position sensor versus the time. In the plot, the intensity of light is measured by the at least one position sensor at the leading edge and / or the trailing edge of the load handling device. The leading edge and / or the trailing edge of the load handling device will depend on the direction of movement of the load handling device on the track system and corresponds to measurements from the position sensor at one of the sides of the load handling device crossing a track, in particular a lip of the track. The spacing between each interruption of light intensity (denoted Y in Figure 17) corresponds to a signal grid crossed. In a particular example of the plot shown in Figure 17, the frequency by which the intensity of reflected light changes as the load handling device moves in either the X direction or Y direction along the tracks is fourteen and therefore, is an indication of the number of grid cells crossed by the load handling device. Having at least one position sensor mounted to each of the sides of the load handling device can also used to determine whether the load handling device is positioned squarely or centrally on a grid cell, i.e., below a storage column. On reaching a grid cell, the first and second sets of wheels are temporarily parked on the tracks at the same time or simultaneously, i.e., in their lowered or engaged position on the tracks, prior to either raising the first or second sets of wheels depending on the direction of movement of the load handling device on the tracks (i.e. the X direction or the Y direction). The instant that the position sensors from all four sides of the load handling device detects a reflected light from their respective track portions, from particular their respective lips, provides an indication that the load handling device is positioned squarely or centrally on the grid cell. If, for instance, one or more of the position sensors do not detect the light reflected from their respective track portions despite the load handling device being at a standstill at its target location or prior to changing direction on the track system is an indication that the load handling device is not seated squarely or centrally on the grid cell. When using tracks primarily composed of plastic, there is a risk of the build-up of static charge on the tracks due to the interaction with the wheels or tyres of the load handling device. This is addressed by making the plastic tracks conductive using carbon as taught in WO 2022034190 (Ocado Innovation Limited). Typically, to dissipate static electricity, the surface resistance of the track should not exceed 1011 Q (ohms), preferably, not exceed 1010 Q (ohms). Typically, carbon reduces the surface resistance of the plastic tracks between 102 Q to 10” Q. The surface resistivity is measured by a surface resistivity meter (SRM100 - Half Decade Resistance Meter) by Bondline, Swindon, Wiltshire, United Kingdom. Due to the inherent black colouration of carbon, the colouration of the plastic track comprising carbon tends to be dark. As a result, the intensity of light reflected from the track is low or even non-existent to the extent that the position sensor based on illuminating the track with visible red light is unable to detect the reflected light. Failure of the position sensor to sense the reflected light from the track may result in the load handling device not being able to follow a predetermined trajectory or route set by the control system. In a worse case scenario, the load handling device may overshoot or undershoot a target grid cell or even not being able to determine its position relative to the track system as described above. Considering that the position sensor works on the principle of sensing light reflected from the tracks in order to determine whether the load handling device is seated squarely on a grid cell or is moving away from the grid cell, there is a need to provide a track system that has a reflective surface so that light reflected from the track can be sensed by the position sensor discussed above. In the present disclosure, each of the plurality of tracks is formed from a plastic material but instead of the excessive use of carbon to render the surface of the track electrically conductive, the plastic track is light coloured to reflect light with sufficient intensity to be detected by the position sensor but yet the surface of the track be sufficiently electrically conductive to dissipate static electricity. To reflect light in the visible red region with sufficient intensity to be detected by the position sensor, the surface of the plastic track having a physical parameter that is characteristic of the reflectance of material is measured. Considering that the plurality of tracks is manufactured on a large scale at a relatively low cost, ideally, the composition of the plastic track has a measurable physical parameter that is characteristic of the reflectance of the material. For the purpose of the present disclosure, particularly in the visible red region, the Light Reflectance Value (LRV) was considered as the measurable physical parameter that is characteristic of the reflectance of the material used in the plastic track. The Light Reflectance Value (LRV) is a measurement of light that is reflected from the surface when illuminated by a light source in the visible region of the wavelength and is a characteristic of the colour of the material. It is expressed as a number (or percentage value) of 1-100; the higher the number, the more reflective the surface and vice-versa. It is found that the position sensor was able to sense light reflected from light coloured materials in comparison to dark coloured materials such as black colour. However, the physical parameter associated with the reflectance of the material is not limited to LRV and can be based on the emissivity values of the plastic material and largely depends on the wavelength of light emitted by the position sensor. For example, where the position sensors are based on sensing the reflectance of infrared light or laser, then the measured parameter that is characteristic of the reflectance of the material is emissivity. In the present disclosure, the position sensors operate in the region of visible red and infrared light, in which case, the LRV is considered a good measurement of the reflectance of the surface of the material. The test method used to measure the LRV of the surface of the track involved illuminating the surface of the track with visible red light and detecting the reflected light using a reflectance spectrophotometer, e.g., Ocean SR or Ocean HDX having a light with a wavelength in the range 380nm to 780nm. At one extreme, tests have shown that the intensity of light reflected from the black track comprising carbon with a measured resistivity of 105 Q.m that is sufficient to dissipate static electricity and a measured LRV of less than 30%, e.g. about 5% was poor to the extent that the position sensor was unable to determine the position of the load handling device relative to the track system. A grey coloured track comprising a lower quantity of carbon with a measured resistivity of 1010£lm to render it sufficiently conductive to dissipate static electricity and a measured LRV of greater than 30%, e.g. about 40% was able to detect the light reflected from the track and therefore, the load handling device was able to determine its position relative to the tracks. At the other extreme, a white coloured track with a measured intensity of 108Q.m sufficient to dissipate static electricity and a measured LRV of greater than 30%, e.g. about 85%, the position sensor was able to detect the light reflected from the track and therefore, the load handling device was able to determine its position relative to the tracks. Thus, light coloured plastic tracks are more conducive to reflecting light from the optical position sensor with sufficient intensity to be detected by the optical position sensor in comparison to dark coloured palstics. In the example of the present disclosure, a light-coloured track that is sufficiently conductive to dissipate static electricity and yet be able to reflect light with sufficient intensity to be detected by the position sensor comprises Acrylonitrile Butadiene Styrene (ABS). To dissipate static electricity preferably the ABS material comprises an antistatic additive. Commercially available plastic materials that are sufficiently light coloured to reflect light with sufficient intensity to be detected by the position sensor and have anti-static properties include but is not limited to Pre-Elect® from Premix Group, Finland or Sicoflex® AG314L from Ravage, Europe. Ideally, for the position sensor operating in the visible red region of the light spectrum to sense light reflected from the track, the LRV should be greater than 30%, preferably, greater than 50%. Examples of the colour of such materials having a LRV greater than 30% include but is not limited to blue, orange, grey and white and examples of dark coloured materials having a LRV less than 30% include black and brown. However, the present disclosure is not limited to light coloured materials being suitable to be used for fabricating the track since the reflective properties from the surface of dark coloured materials can be increased such that their respective LRV exceeds 30%. Having different coloured plastic tracks has other advantages besides being able to reflect visible light. One of the risks of items or goods that will be ingested by animals, e.g., food or medicine, is the risk of physical contamination of items with a foreign body. To address this risk, more and more companies, invest in equipment detecting contamination such as metal detectors or X-ray machines. This equipment is good for detecting metals (metal detector) or contamination of high density (X-ray), but on production areas there are many other materials used which cannot be detected neither by metal detectors nor X-ray machines. To address this risk detectable versions of objects used on production areas have been developed. Its detectability is achieved by using special metal detectable plastic or by incorporating into object aluminium or metal parts. Another solution to detect potential physical contamination is to use contrasting colour which normally does not occur in natural food products. As blue colour does not naturally occur in food products it is very often used to produce objects used in the food industry. This contrasting colour enables visual detection of potential food contaminants. Where the items are stored in the grid framework structure discussed above, it may be necessary that any contaminants from the grid framework structure are easily detectable in the storage container. One of the components of the grid framework structure that may be responsible for generating contaminants in the storage containers is the track system due to their continuous interaction with the load handling device, more specifically, the wheels of the load handling decide. In the case of plastic tracks discussed above by having a contrasting colour of the plastic track having a LRV greater than 30%, e.g., blue, enables the visual detection of contaminants responsible from the track system being detected.

Claims

1. A track system for a grid framework structure 14 configured to guide a load handling device 30 operative to move one or more containers 10, the grid framework structure comprising a supporting framework structure comprising a plurality of storage columns, each of the plurality of storage columns being arranged to accommodate a stack of storage containers; said track system comprising:-a plurality of tracks arranged to form a grid pattern comprising a plurality of grid cells, each of the plurality of tracks comprising a plastic material, said track system is configured to be mounted on the supporting framework structure such that each stack of storage containers is arranged below a respective grid cell of the plurality of grid cells;characterised in that each of the plurality of tracks has a surface resistance less than 1 x 1010 ohms (□) or a surface resistivity less than or equal to 1 x 1010 ohm.meter (Q.m) and a light reflectance value (LRV) greater than 30%.

2. The track system of claim 1, wherein each of the plurality of tracks is substantially white.

3. The track system of claim 1 or 2, wherein each of the plurality of tracks comprises Acrylonitrile Butadiene Styrene (ABS plastic).

4. The track system of any of the preceding claims, wherein each of the plurality of tracks is injection moulded.

5. The track system of any of the preceding claims, wherein the track system further comprises a plurality of track supports arranged in the grid pattern of the track system to define a track support structure, said plurality of tracks being mountable to the track support structure.

6. The track system of claim 5, wherein each of the plurality of tracks comprises means for snap-fitting onto one or more of the plurality of track supports.

7. The track system of claim 5 or 6, wherein the plurality of tracks is arranged so that the distal ends of adjacent tracks is between or intermediate the intersections of the plurality of the plurality of track supports.

8. The track system of any of the preceding claims, wherein each of the plurality of tracks is formed as a unitary body.

9. The track system of claim 8, wherein the plurality of tracks comprises a plurality of track sections, each of the plurality of track sections comprises:a) a first track section element extending in a first direction; andb) a second track section element intersecting with the first track section element and extending in a second direction, the second direction being transverse to the first direction.

10. A grid framework structure for supporting one or more robotic load handling devices operative on the grid framework structure, the grid framework structure comprising:a supporting framework structure comprising a plurality of storage columns, each of the plurality of storage columns being arranged to accommodate a stack of storage containers;a track system comprising a track system as defined in any of the claims 1 to 9, said track system being mounted on the supporting framework structure such that each stack of storage containers is arranged below a respective grid cell of the plurality of grid cells of the track system; .

11. A storage and retrieval system comprising:i) a grid framework structure as defined in claim 10;ii) a plurality of stacks of storage containers arranged in storage columns located below the track system, wherein each storage column is located vertically below a grid cell;iii) at least one load handling devices for lifting and moving a storage containers stacked in the stacks, the at least one load handling device being remotely operated to move laterally on the track system above the storage columns to access the containers through the grid cells, said at least one load handling device comprising:a) a wheel assembly for guiding the load handling device on the track system;b) a container-receiving space located above the track system;c) a container lifting mechanism comprising a grabber device for releasable connection to a storage container, said container lifting mechanism being arranged to lift a single container from a stack into the container-receiving space,d) at least one position sensor for determining the position of the at least one load handling device relative to the track system, said at least one position sensor having a predetermined detection range in the range 5mm to 100mm.

12. The storage and retrieval system of claim 11, wherein the predetermined detection range is in the range 10mm to 50mm.

13. The storage and retrieval system of claim 11 or 12, wherein the position sensor is a photoelectric sensor operating in the visible red light and / or infrared light frequency range.

14. The storage and retrieval system of claim 13, wherein the visible red light has a wavelength of 624nm and the infrared light has a wavelength of 850nm.30

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