Method and system for determining belt tension

Abstract

Determining a tension of a drive belt, wherein a motor configured to drive the drive belt is oscillated across a range of frequencies so as to determine an amplitude of the motor across the range of f

Description

Technical Field The present invention relates to a method and system for determining tension in drive belts, such as those 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 loadhandling devices are further described in WO2015 / 019055A1. Within the storage and fulfilment system, it is important that the load-handling devices perform optimally. In particular, drive belts used in a load-handling device should operate under an optimal tension. It is against this background that the present invention has been devised. Summary In a first aspect, there is a system for determining a tension of a drive belt, the system comprising: a drive belt; a motor configured to drive the drive belt; and a controller configured to: input a signal to the motor to oscillate the motor across a range of frequencies; determine an amplitude of the motor across the range of frequencies; identify a relative peak amplitude of the motor across the range of frequencies; and map a frequency of the relative peak amplitude of the motor to a tension of the drive belt. This means the system can use existing components alone to determine tension of the drive belt; no external hardware is required to determine drive belt tension. The signal may be configured to oscillate the motor across the range of frequencies at a fixed torque. This means the resonance frequency of the drive belt can be isolated by sweeping the range of frequencies. The signal may be configured to effect an angular displacement of the motor, wherein the angular displacement decreases with increasing frequency. This means the drive belt will oscillate at its resonance frequency. The controller may be configured to determine an amplitude of the motor by determining an angular displacement of the motor. The amplitude of the motor may define an extent of a revolution or revolutions of the motor. This means the extent to which the motor is oscillating can be quantified, and thus indicate the relative peak amplitude. The controller may be configured to identify a relative peak amplitude by identifying a peak amplitude in a frequency region of the range of frequencies, wherein the peak amplitude within the region meets a threshold with respect to an amplitude at a start frequency and / or an end frequency of the region. This means the relative peak amplitude can be reliably identified. Oscillating the motor may comprise oscillating a rotor, or a shaft, or a stator of the motor, wherein a drive wheel or a drive pulley of the belt may be connected to the rotor, the shaft, or the stator. This means the motor oscillations can be applied to the drive belt. The controller may be configured to map the frequency of the relative peak amplitude to a drive belt tension by using a look-up table or an equation. This allows the belt tension to be determined in minimal time. The look-up table or equation may be derived from an empirical process, the empirical process comprising determining a plurality of relative peak amplitude frequencies for a respective plurality of measured drive belt tension values; mapping the plurality of relative peak amplitude frequencies to the respective plurality of measured drive belt tension values, and storing the mapping in the look-up table, or characterising the mapping using the equation. This means the determined tension value is as accurate as possible. The equation may comprise y = ax2 + bx + c wherein / comprises the tension of the belt, x comprises the frequency of the relative peak amplitude, and a, b, and c are calibration constants. The calibration constants may be dependent on the system. This means the equation can be used to determine the belt tension for any system. The system may further comprise a sensor configured to determine an amplitude of the motor across the range of frequencies. The sensor may comprise an encoder of the motor, or an accelerometer, or a gyroscope, or an inertial measurement unit. This allows specific components of the system to be re-used to determine drive belt tension. The drive belt and motor may be located on a first side of the system, and the sensor comprises the encoder of the motor. This means the tension of a drive belt driven by and located near the motor can be accurately determined. The drive belt may be located on a first side of the system and the motor may be located on a second side of the system opposite the first side, wherein the sensor may comprise the accelerometer, or the gyroscope, or the inertial measurement unit. This means the tension of a drive belt drive by but located farther from the motor can be accurately determined. The system may further comprise an automatic tensioning device, wherein the controller is configured to use the automatic tensioning device to adjust the tension of the drive belt based on the belt tension and current operating parameters of the system. This means the drive belt tension can be dynamically adjusted to the required value. A load-handling device may comprise the above aspects. The load handling device may be configured to lift and move 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 may comprise: a body or skeleton mounted on a first set of wheels being configured to engage with the first set of parallel tracks and a second set of wheels being configured to engage with the second set of parallel tracks; and a drive assembly comprising a first system according to the above aspects, wherein the drive belt and the motor of the first system are configured to drive the first or second sets of wheels to move the load-handling device along the first or second set of parallel rails respectively, and wherein the controller of the first system is configured to determine the tension of the drive belt of the first system; and / or a direction-change assembly comprising a second system according to the above aspects, wherein the drive belt and the motor of the second system are configured to raise or lower the first set of wheels, and or lower or raise the second set of wheels with respect to the body or skeleton to engage and disengage the wheels with the parallel tracks, and wherein the controller of the second system is configured to determine the tension of the drive belt of the second system; and / or a container-lifting assembly comprising a third system according to the above aspects, wherein the drive belt and the motor of the third system are configured to raise or lower a gripping device in the vertical direction, and wherein the controller of the third system is configured to determine the tension of the drive belt of the third system. This means the tension of each drive belt used in the load-handling device can be determined. In second aspect, there is a method for determining tension of a drive belt in a system, wherein the system comprises the system of any preceding aspect, the method comprising using the controller to: input a signal to the motor to oscillate the motor across a range of frequencies; determine an amplitude of the motor across the range of frequencies; identify a relative peak amplitude of the motor across the range of frequencies; and map a frequency of the relative peak amplitude to a tension of the drive belt. 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 second aspect. In a fourth aspect, there is a data processing system comprising a controller configured 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 containerlifting means in a raised and a lowered configuration; Figure 6 is a schematic illustration of a load-handling device with a direction-change mechanism; Figure 7 shows an example drive assembly used in a load-handling device; Figure 8 shows an example container-lifting assembly; Figure 9 shows a system according to an embodiment; Figure 10 shows a method for determining a drive belt tension within a system, according to an embodiment; Figure 11 shows how a resonance frequency of a drive belt can be detected using an encoder, according to an embodiment; and Figure 12 shows the correlation between drive belt tension values determined by the method of figure 10 and measured belt tension values, according to an embodiment. 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. 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 (an example of which is shown in figure 6), 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 (an example of which is shown in figure 7), 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 (an example of which is shown in Figure 8), e.g. a crane mechanism, to lift a storage container from above. The lifting device comprises a winch tether or cable 38 wound on a spool or reel and a gripper device 39. The lifting device 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 gripper device 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 lifting frame. The gripper 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 Figures 1 and 2. For example, the lifting frame 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 lifting frame 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 gripper device 39 above the stack 12. The gripper device 39 is then lowered vertically in the Z-direction to engage with the bin 10 on the top of the stack 12, as shown in Figures 4 and 5B. The gripper device 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 load-handling device 30 can be moved to a different position in the X-Y plane, carrying the bin 10 along with it, to transport the bin 10 to another location. 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 grabber device 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 (“non-target 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 a master controller, 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 master controller, 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 lifting mechanism can be operated to grip and lift the storage container 10. 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 load-handling 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 direction-change assembly (further described in PCT Publication No. WO2021175922A1 (Ocado) and PCT application no. PCT / EP2022 / 073670 (Ocado)) is shown in figure 6. As can be seen in figure 6, a first pair of direction-change mechanisms 110 are positioned on opposed faces within the body or skeleton 102 of the loadhandling device for controlling the position of the first set of wheels 36, and a second pair of direction-change mechanisms 110 are positioned on orthogonal opposed faces within the body or skeleton of the load-handling device for controlling the position of the second set of wheels 38. Thus, each face of the load-handling device comprises a directionchange mechanism 110. The pairs of direction-change mechanisms 110 are coupled via a transfer or drive belt 108 that substantially circumnavigates the load-handling device skeleton 102, and is mechanically coupled to the direction-change mechanisms. The output of the direction-change mechanisms is transferred to the wheels 34, 36 via a chassis which translates the horizontal movement of the direction-change mechanisms to a vertical movement of the wheels. In some arrangements, the direction-change mechanism may be attached to a rod arrangement extending along a face of the loadhandling device 30 between each of the horizontal edges of the load-handling device 30 via a glide bearing. In turn the rod arrangement may be attached to corner pieces at first and second ends. The wheel sets 34, 36 can be moved in unison, for example via a motor (not shown) and the drive belt 108 to engage x- and or y- direction wheel sets with the rails of a storage system grid. Activating the motor in a clockwise direction may move a wheel mount on the face upwards to raise the wheels on the face, and lower the wheels on the face, perpendicular to the first face - or vice versa. In some examples of the direction-change assembly, the transfer or drive belt 108 may pass over one or more idler pulleys for monitoring the rotation rate when moving between positions to engage the x- and y-directions to provide instant detection of drive belt 108 failure. If the drive belt 108 or another part of the direction-change assembly were to fail, then this information could be fed back to and exploited by the central control facility to prevent bot collisions. An example drive assembly 700 for the sets of wheels 34, 36 is shown in figure 7. A drive belt assembly is provided for each set of wheels 34, 36. The drive belt assembly comprises a drive belt pulley gear arrangement for engaging with a toothed edge of a pair of wheels 34, 36 on one side of the load-handling device 30, as illustrated in figure 6. A toothed drive belt 710 engages with both of the wheels 720 (which correspond to one pair of wheels of the set of wheels 34 or 36). The drive belt 710 is guided by a slave-wheel 730 mounted on the load-handling device, and two tensioning wheel arrangements 740. The tensioning wheel arrangements are movably mounted to the load-handling device with springs (not shown), and are intended to keep the drive belt taut and maintain engagement of the drive belt with the wheels. A drive wheel 750 is provided, mounted to the load-handling device. The drive wheel is linked to the axle or rotor of a motor (not shown in figure 7). The load-handling device is provided with drive assemblies 700 for each pair of wheels. The pairs of wheels on opposed sides comprise a set of wheels 34, 36. The drive wheels on opposed sides of the load-handling device may share a common motor axle so that each pair of wheels are driven at the same time and at the same speed. As a result only a single motor is required to drive the load-handling device 30 forward and reverse in a first x-direction, and only a single motor is required to drive the load-handling device 30 forward and reverse in a second y-direction. This arrangement may advantageously reduce the cost in terms of space in the load-handling device and the number of parts required. The first set of wheels 34 and the second set of wheels 36 may be selectively driven under the control of the load-handling device. In the arrangement of figure 7, it will be appreciated that when the set of wheels 34, 36 are moved out of position for engagement with the grid track, the drive belt may become slack because the distance between the upper portion of the drive belt assembly 770 and the wheels changes as the wheels are lowered and raised. Accordingly, depending on the direction-change assembly selected, additional tensioning mechanisms may be required. For example, an idler pulley on a mechanical linkage connected to the direction-change assembly may be employed to keep the notional drive belt length constant throughout the full range of motion of the direction-change assembly. An example container-lifting assembly (further described in PCT application no. PCT / EP2022 / 081364 (Ocado)) is shown in figure 8. In the embodiment of Figure 8, a lifting assembly 80 has four spools 801, 802, 803, and 804 to wind and unwind respective tethers 38. Spools 801 and 802 are on drive shaft 805, whereas spools 803 and 804 are on drive shaft 806. Drive shafts 805 and 806, when driven by a motor, are configured to rotate in opposite directions. By rotating drive shafts 805 and 806 in opposite directions, respective tethers 38a-d can be located at or near the corners of the lifting assembly, as with the embodiments above. In particular, as shown in figure 8, the point at which each tether winds or unwinds to or from a spool is at or near a respective corner of the lifting assembly. This allows the tethers to connect to the container gripping device 39 at a respective corner of the gripping assembly, which increases stability when raising and lowering the container gripping device 39. Figure 8 shows one example of how drive shafts 805 and 806 can be rotated in opposite directions. Drive shafts 805 and 806 are connected to pulleys 810 and 811 respectively. Pulley 807 (or 809) is linked to the axle / shaft / rotor of a motor (not shown in figure 8). Drive belt 808 transmits the torque to pulleys 809, 810, and 811 in a way that ensures spools 801 and 802, and spools 803 and 804 rotate in opposite directions. In particular, pulleys 807 and 809 are arranged about pulley 811 to effect its opposite rotation to pulley 810. It will now be appreciated that load-handling device 30 has three systems, each of which can use a drive belt and a motor: the direction-change assembly; the drive assembly; and the container-lifting assembly. Whilst setting a high drive belt tension ensures no drive belt slip, this decreases the power efficiency of the various systems of the loadhandling device. The drive belt tension of each system should be regularly monitored and adjusted based on the monitoring. In each of the three systems, the motor may include an encoder to monitor the movement of the motor. The encoder may comprise a rotary electromechanical device that generates pulses when the motor rotates. For example, a pulse is generated for a predetermined amount of angular rotation of the motor. A tool (such as an electronic tension gauge by OPTIBELT™) is used to measure belt tension in Hertz, Hz, and the tension can be adjusted by moving a tensioning pulley on which the drive belt runs. The tensioning pulley can be releasably fixed, via a screw or bolt for example, in a number of positions to vary the drive belt tension. This is both a time consuming and an inefficient process that involves the use of an expensive tool to measure belt tension. Operational downtime is unavoidable with this process. It should be appreciated that these problems are common to all systems in which drive belt tension should be regularly monitored and adjusted based on the monitoring. The above is merely illustrative of how an example belt drive belt can be used in a load-handling device. The ability to measure drive belt tension in any system whilst avoiding the above problems is desirable. Figure 9 shows a schematic 900 of load-handling device 30 in accordance with the invention. One or more motor encoders 920 (such as the encoder described above) are used with one or more drive assemblies 910, such as that shown in figure 7. It will be appreciated that system 910 could instead be the direction-change assembly (such as that shown in Figure 6) or the container-lifting assembly (such as that shown in figure 8). In Figure 9, only a pair of wheels 934 for moving the load-handling device 30 in the X-direction on the grid structure are shown. However, the complete set of wheels for each of the X- and Y-directions, e.g. the first set 34 and second set 36 of wheels described in earlier examples, can be included. One or more vibration detection devices 940 can be included. One or more automatic tension systems 930 can be included. Load-handling device 900 can use processor or controller 950 to receive and transmit data from and to each of the one or more drive assemblies 910, one or more encoders 920, one or more automatic tension systems 930, and the one or more vibration detection devices 941. This data can be stored in storage 960. The data in storage 960 can be periodically transmitted for further processing via one or more networks, such as base stations. Figure 10 shows the steps of a method 1000 for determining a tension of a drive belt used in a system, wherein the system comprise a drive belt and motor configured to drive the drive belt. It would be appreciated that the method of figure 10 could be carried out using a controller of the system (the controller of a load-handling device of figure 9 for example). In step 1010, an input signal is applied to the motor to oscillate the motor across a range of frequencies. Step 1010 results in an angular displacement 770 of the motor shaft or rotor, such as ±x° about a 0° reference point, as shown in figure 7. It will be appreciated the stator of the motor (i.e. the shaft or rotor is fixed) can instead be displaced ±x° about a 0° reference point. A drive wheel or a drive pulley of the drive belt may be connected to the rotor, the shaft, or the stator. The extent of x° will depend on the current frequency of oscillation: x° will decrease with increasing frequency. The oscillation can be effected using appropriate drive signals from the controller to a motor, such as an electric motor. A fixed torque may be applied across the frequency range to ensure that the effect of frequency variation alone can be detected. In this way, the signal motor moves in accordance with a sinusoidal wave with variable frequency, where the amplitude is the fixed torque. Step 1010 also results in the resonance or oscillation 780 of the drive belt, as shown in figure 7. That is, the drive belt forms a wave about its otherwise stationary (i.e. absent the applied oscillation) axis. In step 1020, an amplitude of the motor output across the range of frequencies is determined. In other words, the angular displacement of the motor shaft or rotor or stator is observed at each frequency. At or about a certain oscillation frequency of the motor, the drive belt will oscillate at or about its resonant frequency. The range of frequencies applied to the motor are selected to ensure the drive belt oscillates at or about its resonant frequency. The resonant frequency of the drive belt depends on the tension of the drive belt. For a given system, a range of possible drive belt tensions is known so the range of frequencies applied to the motor can be empirically determined to ensure that drive belt oscillates at or about its resonant frequency. In other words, the drive belt will have a range of possible tension values (measured in Hertz) that correlate to the range of frequencies applied to the motor. In the example of a load-handling device and the drive assembly, the range of frequencies is 60 Hz to 130 Hz. However, it has to be stressed that the range of frequencies will depend on the configuration of the system such as the space between adjacent pulleys, and system load, for example. However, the skilled person will be able to select the range of frequencies applied to the motor as a matter of course such that it includes the possible range of resonance frequencies. To assist the ease of detection, as per step 1030 below, it is preferable that the range of frequencies ensures the drive belt oscillates at its lowest resonant or fundamental frequency or first harmonic of the drive belt. At step 1030, a relative peak amplitude of the motor across the range of frequencies is identified. This relative peak amplitude indicates that the drive belt is oscillating at or about its resonant frequency. A plot 1100 of motor amplitude against input frequency is shown in figure 11. As the input frequency (shown on the x-axis) increases, the output amplitude of the motor (shown on the y-axis) decreases. In figure 11, the amplitude of the motor is expressed in terms of motor revolutions, i.e. the extent of revolutions of the shaft, rotor, or stator, which is derived from the angular displacement of the motor shaft, rotor, or stator. As shown in figure 11, there is a relative peak (shown by the circled region 1100) in the otherwise substantially downwards straight line trend. The relative peak amplitude can be identified by identifying a peak amplitude in a frequency region of the range of frequencies, wherein the peak amplitude within the region meets a threshold with respect to an amplitude at a start frequency and / or an end frequency of the region. The peak amplitude results in a notable rate of increase in the output amplitude of the motor shaft. The use of a threshold means that minor deviations from the substantially downwards straight line trend are not erroneously identified as the frequency of the relative peak amplitude. The threshold can be set depending on the system and its configuration. Additionally or alternatively to using a threshold, the controller may analyse the graph of figure 11 using image analysis techniques or equivalent pattern recognition techniques to identify the relative peak. One implementation can involve analysing data logs to identify the largest relative peak. Generally, any appropriate sensor can be used to detect the relative peak amplitude. The example of figure 10 uses an encoder of the motor to directly obtain the output amplitude of the motor. As an alternative or equivalent to using the encoder, a vibration detection device (940 for example), such as an accelerometer, or a gyroscope, or an inertial measurement unit, can be used to detect the output amplitude. Such a vibration detection device can already be present within the system, such as a load-handling device. As the motor oscillates, a vibration detection device will detect acceleration or movement in a given axis (such as one aligned with a portion of the belt that moves backwards and forwards along that axis) according to the frequency of oscillation. The detected acceleration or movement along a given axis is correlated to the angular displacement of the motor shaft, rotor, or stator as it oscillates. In other words, the detected acceleration or movement can be used to derive the output amplitude of the motor that is otherwise detected by the encoder. The output amplitude of the motor derived from the detected acceleration or movement can be used to detect a relative peak amplitude in the same manner as described in respect of figure 10. That is, a peak amplitude in a frequency region of the range of frequencies, wherein the peak amplitude within the region meets a threshold with respect to an amplitude at a start frequency and / or an end frequency of the region. The peak amplitude results in a notable rate of increase in the output amplitude of the motor shaft. The use of a threshold means that minor deviations from the substantially downwards straight line trend are not erroneously identified as the frequency of the relative peak amplitude. The threshold can be set depending on the system and its configuration. Additionally or alternatively to using a threshold, the controller may analyse the graph of figure 11 using image analysis techniques or equivalent pattern recognition techniques to identify the relative peak. One implementation can involve analysing data logs to identify the largest relative peak. It will be appreciated from the above examples that in general any sensor can be used provided the sensor output can be mapped against the range of frequencies input to the motor such that a relative peak amplitude of the motor can be identified. The encoder and vibration detection device can both be used to verify the frequency of the relative peak amplitude. If both the encoder and vibration device detect substantially the same frequency of the relative peak amplitude, this increases confidence that the correct frequency of the relative peak amplitude has been identified. Additionally, the vibration detection device can provide back-up should the encoder fail and vice versa. This means the system can still determine the frequency of the relative peak amplitude without first having to replace a faulty encoder or vibration detection device. Advantageously, the encoder may be optimal in detecting the tension of a drive belt to which the motor is closer located, whereas the vibration detection device may be optimal in detecting tension of a drive belt driven by but located farther from the motor. For example, as explained in relation to figure 7, drive wheels 36 or 38 on opposed sides of the load-handling device can share a common motor axle so that each pair of wheels 150 are driven at the same time and at the same speed by the motor. In practice, the motor may be located at or near one side, a drive side, of the load-handling device. It has been found that the encoder accurately detects the frequency of the relative peak amplitude of a drive belt on the drive side of the load-handling device. It has also been found that the vibration detection device accurately detects the frequency of the relative peak amplitude at the side of the load-handling device opposite the drive side. Thus, vibration detection devices can be located where they can sense vibration in a drive belt driven by but located farther from the motor (i.e. a non-drive side of the system or loadhandling device). That is, when a first drive belt and the motor are on a first side of the system or load-handling device, an encoder of the motor is used, and / or when a second drive belt is located on second side of the system or load-handling device, opposite the first side of the system or load-handling device (on which the motor driving the second drive belt is located), a vibration detection device is used. In step 1040, the frequency of the relative peak amplitude is mapped to a tension of the drive belt in Hertz (i.e. the resonant frequency of the drive belt). In effect, an output of the motor is used to determine the resonance frequency of the drive belt. The mapping of resonant frequency to belt tension is the result of an empirical process in which steps 1010 to 1030 are repeated for multiple different belt tensions thus giving rise to respective relative peak amplitude frequencies. Then, the relative peak amplitude frequencies can be mapped to respective drive belt tension values measured by an electronic tension gauge, such as that produced by OPTIBELT™. Assuming the empirical process is carried out using a sufficient range of both relative peak amplitude frequencies and measured frequencies at a sufficient degree of granularity, a look-up table can be created. From this look-up table, any relative peak amplitude frequency value determined from step 1030 can be mapped to a belt drive tension value (i.e. a resonant frequency of the drive belt). Extrapolation and / or interpolation can be used as appropriate. Alternatively or additionally, a mathematical relationship or equation that characterises the mapping of the relative peak amplitude frequencies to the measured frequencies, can be derived. One such mathematical relationship or equation is a second degree polynomial indicated by the equation below where: y = ax2 +bx + c (1) where y comprises the tension of the belt in Hertz, x comprises the frequency of the relative peak amplitude, and a, b, and c are calibration constants. The calibration constants are dependent on a system in which the drive belt is driven by the motor, and can easily be derived using the above empirical process. In the example of the drive assembly of the load-handling device, the calibration constants were determined as: a=0.0123; b=-1.68; c=74.3. In principle, the above empirical process can be run on any system to map the detected relative peak amplitude frequencies to measured values. A mathematical equation or look-up table that characterises the mapping can be readily derived. Whilst belt tension can be measured in Hertz, it can also be measured in Newtons, N, and the two values can be interchanged using a known formula. One such known formula is: T = 4ml2 f2 (2) where T is drive belt tension in Newtons, m is drive belt mass in Kilograms per metre, f is the drive belt tension in Hertz (i.e. the resonant frequency of the drive belt), and I is drive belt span which is calculated: CD2 -(D- d)2 J 4 (3) where CD is the distance in metres between the centre of two pulleys between which the drive belt tension is measured, D is the diameter in metres of the first pulley of the two pulleys, d is the diameter in metres of the second pulley of the two pulleys. It will be also be appreciated that the resonant frequency of the drive belt can be converted to Newtons using (2) and (3). Similarly, the relative peak amplitude frequency can also be converted directly to Newtons using the empirical process described above. The method of figure 10 can be used to determine drive belt tension using existing hardware of the system. The motor, encoder, and vibration sensing device are present in a large number of systems. No electronic tension gauge or system downtime is required. The method of figure 10 has been found to be extremely accurate. Figure 12 shows a strong correlation between drive belt tension resonant frequencies determined / predicted using the method of figure 10 and drive belt resonant frequencies measured by an electronic tension gauge, such as that produced by OPTIBELT™. The measured and predicted belt tension resonant frequencies are shown on the y-axis. The relative peak amplitude frequencies used to derive the determined / predicted belt tension resonant frequencies are shown on the x-axis. All of the determined or predicted values are within 5% of the measured values. Additionally, the method of figure 10 can be carried out in an order of seconds. The method of figure 10 finds use in a number of applications. One such application is to run the method as part of a system boot sequence. Once it has been determined that the belt tension is optimal, the system can proceed to normal operation. Another application is to adjust the tension of the drive belt based on the determined tension and current operating parameters of the system. The system could include an automatic tensioning system, such as linking one of the tensioning wheel arrangements 740 to a stepped motor that can move the tensioning wheel arrangement to increase or decrease drive belt tension to an optimal value. Another example of an automatic tensioning system is a linear actuator that moves a tensioning pulley on which the drive belt operates. Movement of the tensioning pulley via the linear actuator can increase the drive belt tension. In general the automatic tensioning system moves a pulley or drive belt guide to increase the tension of the drive belt. The automatic adjustment of the belt tension could be a dynamic real-time process which accounts for the current operating parameters of the system. In general, the belt tension is adjusted based on the current system load, whereas the heavier the load the greater the drive belt (of the drive assembly or container-lifting assembly) tension required. For example, a load-handling device when travelling with or lifting a heavy pay load, such as container 10 with heavy items, will require a higher drive belt tension compared to when travelling with or lifting a low payload. The automatic tensioning system can thus use the method of figure 10 to determine an adjustment is required to the belt drive tension. The adjustment can then be verified using the method of figure 10. Dynamic adjustment of the drive belt tension based on current operating conditions helps ensure both drive belt longevity and efficiency. Yet another application is determining if there are defects such as loose parts or mechanical failures within the system, such as a load-handling device. A normal system with no defects when executing the method of figure 10 should be return a single relative peak amplitude in line with figure 11 for example. That is, the system returns a normal frequency response. Should the system not return the normal frequency response, and there are a number of relative peaks and a different slope to the response, this may indicate a system defect that warrants further investigation. For example, this may be indicative of a loose bolt or failed joint in a load-handling device. 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 thatx 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 of is given by way of example only and that various modifications may be made by those skilled in the art. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of this invention. The following is a non-exhaustive list of embodiments which may be or are claimed: 1. A system for determining a tension of a drive belt, the system comprising: a drive belt; a motor configured to drive the drive belt; and a controller configured to: input a signal to the motor to oscillate the motor across a range of frequencies; determine an amplitude of the motor across the range of frequencies; identify a relative peak amplitude of the motor across the range of frequencies; and map a frequency of the relative peak amplitude of the motor to a tension of the drive belt. 2. The system of embodiment 1, wherein the signal is configured to oscillate the motor across the range of frequencies at a fixed torque. 3. The system of any preceding embodiment, wherein the signal is configured to effect an angular displacement of the motor, wherein the angular displacement decreases with increasing frequency. 4. The system of any preceding embodiment, wherein the controller is configured to determine an amplitude of the motor by determining an angular displacement of the motor. 5. The system of any preceding embodiment, wherein the amplitude of the motor defines an extent of a revolution or revolutions of the motor. 6. The system of any preceding embodiment, wherein the controller is configured to identify a relative peak amplitude by identifying a peak amplitude in a frequency region of the range of frequencies, wherein the peak amplitude within the region meets a threshold with respect to an amplitude at a start frequency and / or an end frequency of the region. 7. The system of any proceeding embodiment, wherein oscillating the motor comprises oscillating a rotor, or a shaft, or a stator of the motor, and optionally wherein a drive wheel or a drive pulley of the belt may be connected to the rotor, the shaft, or the stator. 8. The system of any preceding embodiment, wherein the controller is configured to map the frequency of the relative peak amplitude to a drive belt tension by using a lookup table or an equation. 9. The system of embodiment 8, wherein the look-up table or equation is derived from an empirical process, the empirical process comprising: determining a plurality of relative peak amplitude frequencies for a respective plurality of measured drive belt tension values; mapping the plurality of relative peak amplitude frequencies to the respective plurality of measured drive belt tension values; and storing the mapping in the look-up table; or characterising the mapping using the equation. 10. The system of embodiments 8 or 9, wherein the equation comprises: y — ax2 + bx + c wherein y comprises the tension of the belt, x comprises the frequency of the relative peak amplitude, and a, b, and c are calibration constants. 11. The system of embodiment 10, wherein the calibration constants are dependent on the system. 12. The system of any preceding embodiment, the system further comprising a sensor configured to determine an amplitude of the motor across the range of frequencies. 13. The system of embodiment 12, wherein the sensor comprises an encoder of the motor, or an accelerometer, or a gyroscope, or an inertial measurement unit. 14. The system of embodiment 13, wherein the drive belt and motor are located on a first side of the system, and the sensor comprises the encoder of the motor. 15. The system of embodiment 13, wherein the drive belt is located on a first side of the system and the motor is located on a second side of the system opposite the first side, wherein the sensor comprises the accelerometer, or the gyroscope, or the inertial measurement unit. 16. A system of any preceding embodiment, wherein the system further comprises an automatic tensioning device, wherein the controller is configured to: use the automatic tensioning device to adjust the tension of the drive belt based on the belt tension and current operating parameters of the system. 17. A load-handling device comprising the system of any preceding embodiment. 18. The load-handling device of embodiment 17, wherein the load-handling device is configured to lift and move 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 configured to engage with the first set of parallel tracks and a second set of wheels being configured to engage with the second set of parallel tracks; and a drive assembly comprising a first system according to embodiments 1 to 16, wherein the drive belt and the motor of the first system are configured to drive the first or second sets of wheels to move the load-handling device along the first or second set of parallel rails respectively, and wherein the controller of the first system is configured to determine a tension of the drive belt of the first system; and / or a direction-change assembly comprising a second system according to embodiments 1 to 16, wherein the drive belt and the motor of the second system are configured to raise or lower the first set of wheels, and or lower or raise the second set of wheels with respect to the body or skeleton to engage and disengage the wheels with the parallel tracks, and wherein the controller of the second system is configured to determine a tension of the drive belt of the second system; and / or a container-lifting assembly comprising a third system according to embodiments 1 to 16, wherein the drive belt and the motor of the third system are configured to raise or lower a gripping device in the vertical direction, and wherein the controller of the third system is configured to determine a tension of the drive belt of the third system. 19. A method for determining a tension of a drive belt in a system comprising a drive belt, a motor configured to drive the drive belt, and a controller, the method comprising using the controller to: input a signal to the motor to oscillate the motor across a range of frequencies; determine an amplitude of the motor across the range of frequencies; identify a relative peak amplitude of the motor output across the range of frequencies; and map a frequency of the relative peak amplitude of the motor to a tension of the drive belt. 20. The method of embodiment 19, wherein the signal is configured to oscillate the motor across the range of frequencies at a fixed torque. 21. The method of embodiments 19 or 20, wherein the signal is configured to effect an angular displacement of the motor, wherein the angular displacement decreases with increasing frequency. 22. The method of embodiments 19 to 21, wherein the controller is configured to determine an amplitude of the motor by determining an angular displacement of the motor. 23. The method of embodiments 19 to 22, wherein the amplitude of the motor defines an extent of a revolution or revolutions of the motor. 24. The method of embodiments 19 to 23, wherein the controller is configured to identify a relative peak amplitude by identifying a peak amplitude in a frequency region of the range of frequencies, wherein the peak amplitude within the region meets a threshold with respect to an amplitude at a start frequency and / or an end frequency of the region. 25. The method of embodiments 19 to 24, wherein oscillating the motor comprises oscillating a rotor, or a shaft, or a stator of the motor and optionally wherein a drive wheel or a drive pulley of the belt may be connected to the rotor, the shaft, or the stator. 26. The method of embodiments 19 to 25, wherein the controller is configured to map the frequency of the relative peak amplitude to a drive belt tension by using a look-up table or an equation. 27. The method of embodiment 26, wherein the look-up table or equation is derived from an empirical process, the empirical process comprising: determining a plurality of relative peak amplitude frequencies for a respective plurality of measured drive belt tension values; mapping the plurality of relative peak amplitude frequencies to the respective plurality of measured drive belt tension values; and storing the mapping in the look-up table; or characterising the mapping using the equation. 28. The method of embodiments 26 or 27, wherein the equation comprises: y = ax2 + bx + c wherein y comprises the tension of the belt, x comprises the frequency of the relative peak amplitude, and a. b, and c are calibration constants. 29. The method of embodiment 28, wherein the calibration constants are dependent on the system. 30. The method of embodiments 19 to 29, wherein the system further comprise a sensor, wherein the method further comprises using the controller to determine an amplitude of the motor across the range of frequencies. 31. The method of embodiment 30, wherein the sensor comprises an encoder of the motor, or an accelerometer, or a gyroscope, or an inertial measurement unit. 32. The method of embodiment 31, wherein the drive belt and motor are located on a first side of the system, and the sensor comprises the encoder of the motor. 33. The method of embodiment 31, wherein the drive belt is located on a first side of the system and the motor is located on a second side of the system opposite the first side, wherein the sensor comprises the accelerometer, or the gyroscope, or the inertial measurement unit. 34. The method of embodiments 19 to 33, wherein the system further comprises an automatic tensioning device, wherein the method further comprises using the controller to use the automatic tensioning device to adjust the tension of the drive belt based on the belt tension and current operating parameters of the system. 35. A method of operating a load-handling device using the method of any preceding embodiment. 36. The method of embodiment 35, wherein the load-handling device is configured to lift and move 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 configured to engage with the first set of parallel tracks and a second set of wheels being configured to engage with the second set of parallel tracks; and a drive assembly comprising a first drive belt and a first motor configured to drive the first or second sets of wheels to move the load-handling device along the first or second set of parallel rails respectively, and wherein a first method according to embodiments 19 to 34 is used to determine a tension of the first drive belt; and / or a direction-change assembly comprising a second drive belt and a second motor configured to raise or lower the first set of wheels, and or lower or raise the second set of wheels with respect to the body or skeleton to engage and disengage the wheels with the parallel tracks, and wherein a second method according to embodiments 19 to 34 is used to determine a tension of the second drive belt; and / or a container-lifting assembly comprising a third drive belt and third motor configured to raise or lower a gripping device in the vertical direction, wherein a third method according to embodiments 19 to 34 is used to determine a tension of the third drive belt. 37. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of embodiments 19 to 36. 38. A data processing system comprising a controller configured to carry out the method of embodiments 19 to 36.

Claims

1. A load-handling device configured to lift and move storage containers stacked in a grid framework structure comprising:5 a first set of parallel rails or tracks and a second set of parallel rails or tracksextending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical10 direction through the plurality of grid spaces, the load-handling device comprising:a body or skeleton mounted on a first set of wheels being configured to engage with the first set of parallel tracks and a second set of wheels being configured to engage with the second set of parallel tracks; anda drive assembly comprising a drive belt and a motor configured to drive the first15 or second sets of wheels to move the load-handling device along the first or second set of parallel rails respectively, and wherein the load-handling device further comprises a controller configured to:input a signal to the motor to oscillate the motor across a range of frequencies; determine an amplitude of the motor across the range of frequencies;20 identify a relative peak amplitude of the motor across the range of frequencies;andmap a frequency of the relative peak amplitude of the motor to a tension of thedrive belt.25 2. The load-handling device of claim 1, wherein the signal is configured to oscillatethe motor across the range of frequencies at a fixed torque.

3. The load-handling device of any preceding claim, wherein the signal is configured to effect an angular displacement of the motor, wherein the angular displacement30 decreases with increasing frequency.

4. The load-handling device of any preceding claim, wherein the controller isconfigured to determine an amplitude of the motor by determining an angular displacement of the motor.

355. The load-handling device of any preceding claim, wherein the amplitude of the motor defines an extent of a revolution or revolutions of the motor.

6. The load-handling device of any preceding claim, wherein the controller is5 configured to identify a relative peak amplitude by identifying a peak amplitude in a frequency region of the range of frequencies, wherein the peak amplitude within the region meets a threshold with respect to an amplitude at a start frequency and / or an end frequency of the region.10 7. The load-handling device of any proceeding claim, wherein oscillating the motorcomprises oscillating a rotor, or a shaft, or a stator of the motor, wherein a drive wheel or a drive pulley of the belt is connected to the rotor, the shaft, or the stator.

8. The load-handling device of any preceding claim, wherein the controller is15 configured to map the frequency of the relative peak amplitude to a drive belt tension bycmusing a look-up table or an equation.

9. The load-handling device of claim 8, wherein the look-up table or equation is derived from an empirical process, the empirical process comprising:20 determining a plurality of relative peak amplitude frequencies for a respectiveplurality of measured drive belt tension values;mapping the plurality of relative peak amplitude frequencies to the respective plurality of measured drive belt tension values; andstoring the mapping in the look-up table; or25 characterising the mapping using the equation.

10. The load-handling device of claims 8 or 9, wherein the equation comprises:y = ax2 + bx + c30wherein y comprises the tension of the belt, x comprises the frequency of the relative peak amplitude, and a, b, and c are calibration constants.

11. The load-handling device of claim 10, wherein the calibration constants are35 dependent on the system.

12. The load-handling device of any preceding claim, the system further comprising a sensor configured to determine an amplitude of the motor across the range of frequencies.

13. The load-handling device of claim 12, wherein the drive belt and motor are located on a first side of the load-handling device, wherein the sensor comprises an encoder of the motor.

14. The load-handling device of claim 12, wherein the drive belt is located on a first side of the load-handling device and the motor is located on a second side of the loadhandling device opposite the first side, wherein the sensor comprises an accelerometer, or a gyroscope, or an inertial measurement unit.

15. The load-handling device of any preceding claim, further comprising an automatic tensioning device, wherein the controller is configured to:use the automatic tensioning device to adjust the tension of the drive belt based on the belt tension and current operating parameters of the load-handling device.

16. A method for determining tension of a drive belt in a system, wherein the system comprises the load-handling device of any preceding claim, the method comprising using the controller to:input a signal to the motor to oscillate the motor across a range of frequencies; determine an amplitude of the motor across the range of frequencies;identify a relative peak amplitude of the motor across the range of frequencies; andmap a frequency of the relative peak amplitude to a tension of the drive belt.

17. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of claim 16.

18. A data processing system comprising a controller configured to carry out the method of claim 16.

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