A method of monitoring a filter of an air-moving device

Abstract

A method comprises: obtaining a series of measurements of one or more operating parameters of the air-moving device, such as a floor cleaner; determining inlet restriction conditions of the air-moving device based on the series of measurements, each inlet restriction condition being associated with one or more of the obtained series of measurements; determining a series of filter loading values based on the obtained series of measurements, each filter loading value being based on one or more of the obtained measurements and a pre-determined relationship for the determined inlet restriction condition associated with the one or more obtained measurements, the pre-determined relationship relating values of the measured operating parameter to respective filter loading values for that inlet restriction condition; detecting a first filter status if the most recent filter loading value of the series of filter loading values is less than a first threshold value; and detecting a second filter status if the most recent filter loading value of the series of filter loading values is greater than or equal to the first threshold value. described. Also disclosed is a method of detecting a filter fault.

Description

BACKGROUND There is a general desire to improve air-moving devices, such as vacuum cleaners, in a number of ways. For example, improvements may be desired in terms of efficiency, manufacturing cost, flexibility of use and reliability. SUMMARY According to an aspect of the invention, there is provided a method of monitoring a filter of an air-moving device, such as a vacuum cleaner. The method comprises: obtaining a series of measurements of one or more operating parameters of the air-moving device; determining one or more inlet restriction conditions of the air-moving device based on the series of measurements (e.g. each inlet restriction condition being associated with one or more of the obtained series of measurements); determining a series of filter loading values based on the obtained series of measurements, each filter loading value being based on one or more of the obtained measurements and a pre-determined relationship for the determined inlet restriction condition associated with the one or more obtained measurements (the pre-determined relationship relating values of the measured operating parameter to respective filter loading values for that inlet restriction condition); detecting a first filter status if the most recent filter loading value of the series of filter loading values is less than a first threshold value; and detecting a second filter status if the most recent filter loading value of the series of filter loading values is greater than or equal to the first threshold value. In this manner, the method is able to track changes in the filter loading values and thereby monitor a state of the filter(s) during operation of the air-moving device. The method makes advantageous use of the fact that the build-up of particulate matter on the filter(s) presents an additional airflow restriction through the device and therefore the amount of particulate affects the measured operating parameters according to a predetermined relationship for a particular inlet restriction condition. In this context, the filter loading may be a level of loading of one or more filters indicative of an amount of dirt or dust collected in the filter. The filter loading value may also be a measure of the additional airflow restriction presented by the filter(s) and the particulates accumulated thereon, varying from a minimum restriction (e.g., for new filter(s)) to a maximum restriction (e.g., for fully loaded filter(s)). The first filter status may therefore correspond to an effective working state of the filter(s). The second filter status may correspond to an ineffective state of the filter(s) In examples, the method may further comprise: outputting a filter cleaning prompt signal in dependence on the second filter status of the filter. In this manner, the method may be used to instruct an operator to clean the filter(s) upon determining the second filter status (when the filter(s) become ineffective). Optionally, the filter cleaning prompt signal may be terminated in dependence on subsequently detecting the first filter status. Optionally, the method may further comprises detecting a filter refreshment, such as a filter wash. The filter refreshment may be detected: by determining a successive pair of the filter loading values; and detecting the filter refreshment in dependence on a second filter loading value of that pair being less than a preceding first filter loading value of that pair, or in dependence on the second filter loading value being less than or equal to a threshold percentage of the first filter loading value. Alternatively or additionally, the filter refreshment may be detected in dependence on obtaining a refreshment confirmation signal from a filter sensor or a user interface device. The filter loading should reduce following a filter refreshment. Hence, by detecting the filter refreshment, the device can be configured to expect a reduction in the filter loading value and to identify an issue if this is not the case. In examples, the method may further comprise terminating the filter cleaning prompt signal in dependence on detecting: the first filter status, after the second filter status; and / or the filter refreshment. In this manner, the device may signal to the operator that the device is ready for continued operation and, optionally, that the filter(s) have been suitably refreshed for continued operation. Optionally, the method may further comprise detecting a third filter status if: following the detected filter refreshment, the second filter loading value of that pair is less than the first threshold value but greater than or equal to a second threshold value; or two or more filter refreshments are detected in the determined series of filter loading values, and each filter loading value succeeding an earliest one of the filter refreshments is greater than or equal to a second threshold value. The third filter status may indicate that the filter has expired and cannot be recovered. In other words, the third filter status may indicate that despite cleaning the filter(s), the filter(s) continue to present a significant airflow restriction having degraded. In this respect, the invention is particularly advantageous as particulate matter accumulated at the filter(s) can typically be cleaned off with reasonable effectiveness by rinsing and / or washing the filter(s). However, with each cycle of particulate loading and washing, the fibres of the filter(s) may degrade and the recovery is reduced, increasing a base restriction (i.e. following a wash) that the filter(s) represent to the airflow. Detecting the third filter status may therefore indicate that further filter refreshment will not solve the problem. In examples, the method may further comprise detecting a fourth filter status, instead of the second filter status, if the most recent one or more filter loading values, after detecting the third filter status, are greater than or equal to the first threshold value. The fourth filter status may be an expired and ineffective state of the filter(s), for example, indicating to an operator that the filter cannot be adequately recovered and is impairing the operation of the device. Optionally, the method further comprises: activating a first trigger if the second filter loading value associated with the filter refreshment is less than the first threshold value but greater than or equal to a second threshold value, deactivating the first trigger if a more recent filter refreshment is detected and the second filter loading value associated with that filter refreshment is less than the second threshold value; activating a second trigger if a further one or more filter refreshments are detected, while the first trigger is active, and the second filter loading value associated with the most recent refreshment is greater than or equal to the second threshold value; and detecting the fourth filter status, instead of the second filter status, if the most recent one or more filter loading values are greater than or equal to the first threshold value, while the second trigger is active. In this manner, the use of triggers reduces the memory requirements of the device. In particular, the device only needs to store the filter loading value preceding the filter refreshment for comparison to the filter loading value following the filter refreshment and it can be checked whether the first and / or second triggers are active to determine the filter status. In examples, the method further comprises: outputting a filter replacement prompt signal in dependence on detecting the third or fourth filter status. The filter replacement prompt signal serves to indicate expiry of the filter(s) to an operator, prompting replacement. The filter replacement prompt signal may be output upon detecting the expired filter(s) or subsequently, when the filter(s) become ineffective, to make full utilisation of the filter(s). Optionally, the filter replacement prompt signal may be terminated in dependence on subsequently detecting the first filter status. Optionally, the method further comprises detecting a filter replacement, after detecting the third or fourth filter status, the filter replacement being detected: by determining a successive pair of the filter loading values; and detecting the filter replacement in dependence on a first filter loading value of that pair being greater than the second threshold value and a subsequent second filter loading value of that pair being less than the second threshold value; and / or in dependence on obtaining a replacement confirmation signal from a filter sensor or a user interface device. The filter loading should reduce below the first threshold following a filter replacement. Hence, by detecting the filter replacement, the device can be configured to expect a corresponding reduction in the filter loading value and to identify an issue if this is not the case. In examples, the method may further comprise terminating the filter replacement prompt signal in dependence on subsequently detecting: the first filter status; and / or the filter replacement. In this manner, the device may signal to the operator that the device is ready for continued operation and, optionally, that the filter(s) have been renewed. Optionally, each of the series of filter loading values may be determined by: obtaining one or more measurements of at least one operating parameter of the air-moving device and a signal indicative of an inlet restriction condition of the air-moving device associated with the one or more measurements; and determining that filter loading based on those one or more measurements and a pre-determined relationship relating values of that operating parameter to filter loading values when operating with that inlet restriction condition. For example, for each filter loading value, the inlet restriction condition may be obtained as an inlet restriction condition of the air-moving device when operating in a pre-determined operating condition. The pre-determined operating condition may be indicative of a type of tool attached to the device. In examples, the method may further comprise selecting, based on the pre-determined operating condition, the predetermined relationship. Hence, the tool attached to the device may be indicated by a signal and used to select the corresponding pre-determined relationship relating the measured operating parameters to the filter loading values. In examples, the method may further comprise: detecting an event indicative of a filter loading reduction; arranging the device in a calibration mode to determine an initial filter-loading value following the event, the calibration mode setting a pre-determined inlet restriction condition for the device; operating the device while in the calibration mode to determine one or more measurements of an operating parameter of the air-moving device associated with the pre-determined inlet restriction condition; determining the initial filterloading value following the detected event based on the one or more determined measurements and a pre-determined relationship for the inlet restriction condition of the calibration mode, the pre-determined relationship relating values of the operating parameter to respective filter-loading values for the calibration mode; and using the determined initial filter-loading value as a baseline value for determining the subsequent filter-loading values following the detected event. In this manner, the device prompts an operator to use the calibration mode following any significant change in the determined or expected filter loading, producing a calibrated baseline value for improved accuracy in the subsequent determinations of the filter loading. By way of example, the detected events may include the filter refreshment and / or the filter replacement. In this manner, the device may be recalibrated following each filter refreshment and / or filter replacement. In examples, the series of filter loading values may be determined at intervals. Each filter status may be detected by analysing the most recent filter loading value and / or the preceding filter loading value. A memory (of the device) may be updated at each interval to replace the filter loading value stored as the preceding filter loading value with the most recent filter loading value for analysis in a subsequent interval. In this manner, the memory requirements of the device may be minimised, and the stored value(s) can be updated with each iteration. According to another aspect of the invention, there is provided a method of detecting a filter fault of an air-moving device, such as a vacuum cleaner. The method comprises: responsive to detecting a filter refreshment, arranging the device in a calibration mode to determine an initial filter-loading value, the calibration mode setting a pre-determined inlet restriction condition for the device; operating the device while in the calibration mode to determine one or more measurements of an operating parameter of the air-moving device associated with the pre-determined inlet restriction condition; determining the initial filterloading value based on the one or more determined measurements and a pre-determined relationship for the inlet restriction condition of the calibration mode, the pre-determined relationship relating values of the operating parameter to respective filter-loading values for the calibration mode; and detecting a filter fault status in dependence on the determined initial filter-loading value being greater than or equal to a threshold filter loading value. In this manner, the method is able to determine when the filter(s) have expired and failed to recover adequately following the cleaning or refreshment. The calibration mode is used to deal with the uncertainty as it is expected that the filter loading value will reduce to an unknown value following the filter refreshment. According to a further aspect of the invention, there is provided a set of machine-readable instructions which, when executed by one or more processors, cause the method described in a previous aspect of the invention to be performed. According to yet another aspect of the invention there is provided an air-moving device, such as a vacuum cleaner. The air-moving device comprises: a processor; and a storage comprising a set of machine-readable instructions which, when executed by the processor, cause the processor to perform a method as described in a previous aspect of the invention. Within the scope of this invention, it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS Figure lisa schematic diagram of an example motor assembly of an air-moving device; Figure 2 is an illustration of an exemplary air-moving device; Figure 3 is a graph illustrating a plot of values of an operating pressure against inletrestriction values; Figure 4 is a graph illustrating further examples of plots of values of operating pressure against inlet-restriction values; Figure 5 illustrates, schematically, aspects of an example process of determining the value of a filter loading; Figure 6 illustrates, schematically, further aspects of the example process shown in Figure 5; Figure 7 illustrates, schematically, further aspects of the example process shown in Figures 5 and 6; Figure 8 is a flow chart representation of a method of monitoring a filter of the air-moving device shown in Figure 2; Figure 9 is a flow chart representation of exemplary sub-steps of the method shown in Figure 8; and Figure 10 is a graph illustrating an example of a plot of a series of filter loading values determined whilst monitoring the filter according to the method shown in Figure 8. DETAILED DESCRIPTION Figure 1 shows an example schematic representation of a motor assembly 100 of an airmoving device. The motor assembly 100 comprises a set of coils 102, a shaft 104 with magnets (not shown) mounted thereon, bearings 106 and an impeller 108. The motor assembly 100 comprises motor air inlets 110, and air outlets / a diffuser 112. The motor assembly comprises a circuit board 114 on which sensors are mounted, including an ambient temperature sensor 116 and a first pressure sensor 118. The motor assembly 100 comprises a housing 124 in which the other components are housed. The motor assembly 100 further comprises one or more filters for filtering air drawn into and passing through the motor assembly 100. In this example, the motor assembly 100 may therefore include a pre-motor filter 126 for filtering air which is drawn into the motor assembly 100 and a post-motor filter (not shown) for filtering particulate matter from the airflow after it has passed through the impeller 108. However, this arrangement is not intended to be limiting on the scope of the invention and, in other examples, the motor assembly 100 may only include any one such filter, or the motor assembly 100 may include additional filters. Figure 2 shows an example air-moving device 200 comprising the motor assembly 100. The example air-moving device 200 is a vacuum cleaner. The vacuum cleaner 200 comprises an inlet tube 202 with a tool 204 attached to a distal end of the inlet tube 202. The tool 204 is for engaging with a surface to be cleaned by the vacuum cleaner and comprises an air inlet (not shown) to the vacuum cleaner 200. The tool 204 may be active, comprising one or more mechanically operated components, for example, a rotating brush bar, to assist with cleaning tasks. Alternatively, the tool 204 may be passive and not comprise any such mechanically operated components. A passive tool may nevertheless comprise elements such as bristles or the like to assist with cleaning tasks. In examples, the inlet tube 202 or a portion thereof may be removable. A tool, such as a passive tool, may be attached to the device 200 when the inlet tube 202 or the portion thereof is removed. The vacuum cleaner 200 also comprises a dirt separating chamber 206, which may, for example, be a cyclone chamber. The vacuum cleaner 200 further comprises a processor 208 and a storage 210 for storing machine-readable instructions for execution by the processor 208 to control operation of components of the vacuum cleaner 200 including the motor 100. In examples, the machine-readable instructions when executed may cause the processor 208 to carry out any of the exemplary methods described herein and / or aspects of such methods. In use, the motor of the motor assembly 100 draws air through the air inlet to the airmoving device 200, through the air-moving device 200, and out of an exhaust. Air is drawn through the device 200 along an airflow path 128 which passes through the inlet tube 202, through the dirt-separating chamber 206, through the motor assembly 100 and exits the device 200 through an exhaust 212. Returning to Figure 1, when the motor is in use in the air-moving device 200, an electric current is passed through the coils 102, in a manner which causes a varying magnetic field. This varying magnetic field acts on the magnets on the shaft 104 to cause the shaft 104 to rotate about its longitudinal axis. This in turn rotates the impeller 108. Air, driven by the impeller 108, is drawn into the air-moving device 200 and along the airflow path 128. The airflow path 128 enters the motor assembly 100, passing through the pre-motor filter 126, which removes particulate matter from the air, and into the housing 124 through the air inlets 110 (shown, in Figure 1, as gaps in the housing 124). The airflow path 128 continues through the motor to the impeller 108 and exits the motor assembly 100 through the air outlets 112, passing through the post-motor filter. As the airflow passes through the motor assembly 100, particulates accumulate on the filter(s), increasing a restriction to the airflow through the motor assembly 100 and affecting various measurable operating parameters of the air-moving device 200. For example, the increased restriction may have a measurable impact on an airflow rate through the motor, a speed of the motor, and / or an operating pressure of the motor. In examples, such operating parameters may also be affected by, and therefore vary in dependence on, the operating conditions of the device 200, such as an ambient pressure, an ambient temperature, and a motor input power of the device 200. Moreover, a condition of an inlet restriction to the air-moving device 200 is expected to significantly impact such operating parameters, as shall be discussed in more detail below. In this context, the inlet restriction of the air-moving device 200 is a level of restriction acting on the air inlet through which air flows into the device 200. The level of inlet restriction may vary based on various factors such as obstructions blocking the flow of air into the device 200. For example, the inlet restriction may vary depending on a type of surface the vacuum cleaner 200 is being used to clean. For instance, a carpeted surface, or similar, may place a greater restriction on the flow of air into the vacuum cleaner 200 than a smooth surface, such as a wood or tile surface. The level of the inlet restriction may also vary depending on a type of tool attached to the vacuum cleaner 200. Different tools may, for example, have different geometries and thus restrict the flow of airflow into the vacuum cleaner 200 by different amounts. For example, different tools may have different air inlet diameters. Further, certain tools may include elements which obstruct the airflow into the device 200, such as bristles for cleaning carpet, while other tools may not include such elements. An inlet restriction value of the device in operation may be defined in terms of the diameter of an orifice which would provide an equivalent level of restriction to airflow into the device 200 under standardised conditions (e.g. test conditions). As an example, the vacuum cleaner 200 when being used to clean a carpeted surface may be operating under a high level of inlet restriction, which may be equivalent to operating in known conditions with an orifice plate having an orifice of small diameter restricting airflow into the vacuum cleaner 200. Conversely, the vacuum cleaner 200 when cleaning a wood surface may be operating under a lower level of inlet restriction, equivalent to that presented by an orifice of larger diameter. In examples, the inlet restriction condition may also be a known property of a distribution of inlet restriction values of the device 200. For example, a mean or mode inlet restriction value of the device 200 over a period of operation may be determined. The value(s) of the operating parameter may then be value(s) measured when the device 200 is operating with the mean or mode inlet restriction value. In order to determine the restriction presented by the filters, and the particulates accumulated thereon, one or more operating parameters of the air-moving device 200 may be measured while operating the device 200 with an associated inlet restriction condition. As mentioned previously, the operating parameter(s) may include an operating pressure of the motor of the air-moving device 200. The operating pressure of the motor is an air pressure relating to the motor when the motor is in operation, that is, when the motor is running. The operating pressure may relate to an air pressure at one or more locations along the airflow path 128. The operating pressure may be a differential air pressure. The operating pressure may, for example, be a pressure difference between an upstream and a downstream location, in the motor assembly, along the airflow path 128. In another example, the operating pressure is a difference between a first pressure measured when the motor is not running and a second pressure measured when the motor is running. The first pressure and the second pressure may be measured at the same location. A value of an operating pressure may, for example, be obtained by determining a difference between an ambient pressure measurement, taken when the motor is not running, for example, before start-up of the air-moving device 200, and a pressure measurement taken during operation of the motor. In some examples described herein, such an operating pressure is referred to as delta-P. The pressure measurement taken during operation of the motor may, for example, be taken at the air inlet 110. Alternatively, the measurement may be taken at an air outlet from the motor. In some examples, the pressure measurements used to obtain a value of an operating pressure may be taken by the same pressure sensor. This allows for a value of the operating pressure to be obtained using a single pressure sensor, which may be cost- and space- efficient. In another example, an operating pressure used in an example method may be determined based on a difference in pressures between two locations in the motor assembly 100, for example, an upstream location and a downstream location. For example, an additional pressure sensor (not shown) may be configured to take pressure measurements at a position, along the airflow path 128, downstream of the first pressure sensor 118. The additional pressure sensor may be configured to measure pressure at an air inlet 109 to the impeller 108. A difference between a pressure measured by the first pressure sensor 118 and a pressure measured by the additional pressure sensor, or, for example, a dynamic pressure value derived from these two pressures, may then be used as the operating parameter. In other examples, the operating parameter(s) may additionally or alternatively include a parameter other than an operating pressure, such as a speed of the motor of the air-moving device 200. This speed may be measured, for example, by a suitable sensor (not shown in the figures). In other examples, the operating parameter(s) may include an airflow rate through the device 200, for example. In other examples, such operating parameters may be calculated, or otherwise determined, for example based on measured power characteristics of the motor, such as a motor current and / or voltage. A relationship can be established between values of the operating parameter(s) and the restriction to the airflow through the device 200. Hence, when an inlet restriction to the device 200 is known, the state of the filters can therefore be numerically quantified in relation to the additional restriction they represent to the airflow through the device 200, and vice versa. Herein, this numerical measure of the additional restriction presented by the filters is referred to as a filter loading value. In examples, the filter loading may be a level of loading of the pre-motor filter 126. Alternatively, the filter loading may be a level of loading of the post-motor filter (not shown). In some examples, the filter-loading value may take into account a level of loading of a plurality of filters, including the pre-motor filter and the post-motor filter for example. The level of loading of a filter may be indicative of an amount of dirt or dust collected in the filter, and / or a measure of effectiveness of a filter, for example after it has been cleaned. In particular, the filter may have been cleaned but may not be ‘as new’ due to a residue of dirt or dust that can remain after cleaning, or due to degradation of the filter material. The filter loading values are therefore determined based on measurements of one or more operating parameters of the air-moving device 200 and a pre-determined relationship relating values of the operating parameter(s) to filter-loading values for a respective inlet restriction condition, as shall be described in more detail below. The pre-determined relationship used to determine the filter loading values may be selected from a plurality of pre-determined relationships, based on the inlet restriction condition (and / or the operating conditions). For example, if it is determined that a given tool is attached to the device, a first pre-determined relationship may be selected as a relationship which applies for the inlet restriction presented by that tool. Similarly, once the filter loading value has been determined, each pre-determined relationship relates values of the operating parameter(s) to values of an inlet restriction of the air-moving device 200. To give an example, each pre-determined relationship may be derived as follows (e.g. using a fresh set of filters) and may comprise a relationship defined by a curve relating values of the operating parameter and values of an inlet restriction of the air-moving device 200. The relationship between values of the operating parameter and values of the inlet restriction may be determined during a testing or design process, for example. This may involve operating the device 200 under known operating conditions, including a known value of inlet restriction, and measuring values of the operating parameter. For example, the device 200 may be operated with orifice plates having orifices of differing diameters restricting airflow into the device 200. The value of inlet restriction of the device in operation may then be defined in terms of the diameter of the orifice which would provide an equivalent level of restriction to airflow into the device 200. As an example, the vacuum cleaner 200 when being used to clean a carpeted surface may be operating under a high level of inlet restriction, which may be equivalent to operating in known conditions with an orifice plate having an orifice of small diameter restricting airflow into the vacuum cleaner 200. Conversely, the vacuum cleaner 200 when cleaning a wood surface may be operating under a lower level of inlet restriction, equivalent to that presented by an orifice of larger diameter. In this manner, a curve may be determined relating values of the operating parameter and values of the inlet restriction. The measurable operating parameters may further vary in dependence on the operating conditions of the device 200 though and do so according to a pre-determined relationship. The value of the inlet restriction may therefore be determined based on a value of the operating parameter and respective values of one or more further parameters defining the operating conditions. The further parameters may be parameters of the air-moving device 200 which influence the value of the operating parameter being measured for a given value of the inlet restriction. For example, different values for parameters such as the ambient pressure, ambient temperature, motor input power, and build tolerance of the air-moving device may result in different values of the operating parameter for the same value of inlet restriction. Ambient pressure and ambient temperature form part of the external conditions under which the device 200 is operating. In some examples, ambient pressure may be measured prior to start-up of the motor by the first pressure sensor 118. Ambient temperature may be measured by the temperature sensor 116. Motor input power is the power which is supplied to drive the motor. The motor input power may be controlled by the processor 208 and supply a DC or AC power, for example from a battery (not shown) of the device 200 or from a mains supply. The motor input power may control the suction power of the airmoving device. The build tolerance of the air-moving device 200 may account for the variability in operation between different devices. For example, various operating parameters of the device may be measured during a testing process following assembly of the device. The build tolerance of a particular device may be expressed as a percentage of a total allowable tolerance. In one example, at an end of a production line for a device, an orifice plate having an orifice of a given diameter is connected to an inlet of the device, wherein the device is known to have clean filters, i.e., the filter loading value is 0%. The ambient temperature and pressure are measured. The device is operated at a given power level and the operating parameter, e.g., delta-P, is measured. With values of the input power, ambient temperature, ambient pressure, filter loading, being measured or otherwise known, the measured delta-P is indicative of the build tolerance factor. This process may be repeated at multiple power levels and at different orifice diameters. In some examples, normalised values of the operating pressure or the other operating parameters may therefore be obtained by normalising values of the operating parameter with respect to one or more of the further parameters defining the operating conditions, such as those mentioned above. For example, a look-up table may be defined which maps respective values of build tolerance, ambient pressure, ambient temperature, motor input power, and / or a value of the operating pressure to a normalised value of the operating parameters, such as the operating pressure. In other words, normalised values of the operating parameter may be determined by using of one or more of the further parameters defining the operating conditions as normalisation parameters. In this manner, the normalisation parameters may provide an efficient way of obtaining values which map robustly and accurately to values of the inlet restriction in different operating conditions. An example of a curve relating normalised values of the operating pressure to the values of inlet restriction is shown in Figure 3. This example is for the motor of a vacuum cleaner. In the example of Figure 3, the operating parameter, shown on the y-axis, is an operating pressure, namely a delta-P value, defining a difference between an ambient pressure of the motor prior to start-up and a pressure at a motor inlet during operation. Delta-P is in units of kPa. The values of the inlet restriction are in terms of orifice diameter, in millimetres. A first curve 302 mapping values of normalised delta-P to values of inlet restriction has been obtained by a suitable testing process involving operating the vacuum cleaner under known conditions with inlet restriction provided by orifices of various diameter. Corresponding values of the diameter of the orifice and delta-P have been measured. The first curve 302 has been obtained by normalising values of delta-P with respect to values of build tolerance, ambient pressure, ambient temperature and motor input power. The first curve 302 uniquely maps a normalised value of the operating pressure to a value of the inlet restriction. However, as will be described with reference to Figure 4, the normalised value of the operating pressure does not take into account a value of the filter loading of the motor. Accordingly, different values of the filter loading will result in a different mapping between values of normalised operating pressure and values of the inlet restriction. Figure 4 shows a set of curves 302, 404, 406 relating normalised values of the operating pressure to values of the inlet restriction. Each curve corresponds to a different value of filter loading. The first curve 302 of Figure 3 is also shown in Figure 4 and corresponds to a value of filter loading of 0%. A second curve 404 corresponds to a value of filter loading of 50%. A third curve 406 corresponds to a value of filter loading of 100%. Further curves corresponding to intermediate values of filter loading may also be determined, though not shown here. Accordingly, if the inlet restriction is known, then a determined value of the operating pressure may be used as an operating point for identifying which filter loading curve 302, 404, 406 is applicable and, in turn, for determining the associated filter loading. Alternatively, if the filter loading is known, then a corresponding one of the curves 302, 404, 406 may be selected based on the filter loading value and the determined value of the operating pressure may be used to determine the inlet restriction. The normalised value of the operating pressure therefore uniquely determines a value of the inlet restriction for a known value of the filter loading, and vice versa. As noted above, even when it is determined that a given tool is attached to the device, the inlet restriction may vary according to the use of the device. For example, the inlet restriction will be relatively large when the device is used on a carpeted surface when compared to a wood floor. Accordingly, an example of a determination process for determining a value of a filter loading will now be described with reference to Figure 5. Figure 5 shows the set of normalised delta-P curves 302, 404, 406 described above with reference to Figure 4. Figure 5 shows a first probability distribution 502 of applicable inlet restriction values of the vacuum cleaner. The first probability distribution 502 represents the probability of the vacuum cleaner having a given level of inlet restriction, in terms of an equivalent orifice diameter, when operating with a first tool attached. This first probability distribution 502 corresponds to a passive tool comprising a relatively wide nozzle and a selectively engageable brush. As can be seen from Figure 5, in this example, the inlet restriction values of the first probability distribution 502 range from around 13mm to around 47mm. The minimum level of inlet restriction in the first probability distribution 502 corresponds to an orifice diameter of around 47mm, i.e., with the largest opening. Figure 5 also shows respective projections of the first probability distribution 502 onto two 302, 406 of the normalised delta-P curves. A first projection 504 is a projection of the first probability distribution 502 onto the first curve 302, which, as described above, corresponds to a filter loading of 0%. A second projection 506 is a projection of the first probability distribution 502 onto the third curve 406, which, as described above, corresponds to a filter loading of 100%. The projections 504, 506 define respective probability distributions of the measured normalised value of delta-P for filter loading values of 0% and 100% respectively. In other words, the first projection 504 defines the probability of measuring a given normalised value of delta-P when the device is operating with 0% filter loading. Similarly, the second projection 506 defines the probability of measuring a given normalised value of delta-P when the device is operating with 100% filter loading. Further projections, not shown in Figure 5, can be defined, defining the probability distributions of normalised delta-P at different values of filter loading, e.g. 25%, 50%, 75%, etc. The values of normalised delta-P defined by the curves 302, 404, 406, in general, decrease rapidly for low values of orifice diameter but begin to level off for high values of orifice diameter. This levelling off means that a given value of normalised delta-P, at high values of orifice diameter, may map uniquely to a given one of the curves 302, 404, 406. In examples, this property may be used to determine the level of filter loading from a measured value of normalised delta-P. To give a non-limiting example, from the first probability distribution 502, it is known that the minimum normalised delta-P values in the probability distributions 504, 506 correspond to the device operating with a known maximum orifice diameter, in this example of around 47mm. Accordingly, to determine a filter loading value, a minimum value of normalised delta-P during operation of the vacuum cleaner may be measured and mapped to a given one of the curves 302, 404, 406 at the known maximum orifice diameter. By determining which of the curves 302, 404, 406, the measured normalised delta-P value maps to, the value of the filter loading can be determined. For example, in Figure 5, the third curve 406 and the probability distribution 506 show that, for a filter loading value of 100%, the minimum value of normalised delta-P, which corresponds to the maximum orifice diameter value of around 47mm, is around 15.3kPa. Accordingly, if the minimum normalised value of delta-P of the vacuum cleaner is measured to be around 15.3kPa, this is indicative that the filter loading is 100%. Similarly, the first curve 302 and the probability distribution 504 show that, for a filter loading value of 0%, the minimum normalised delta-P value, again which corresponds to the maximum orifice diameter value of around 47mm, is around 11.2kPa. A measurement for the minimum normalised value of delta-P of around 11.2kPa is therefore indicative that the value of the filter loading is 0%. The normalised delta-P can therefore be acquired during operation of the vacuum cleaner 200 and a minimum value of the normalised delta-P can be determined continuously or at regular intervals, for example at 60 second intervals. At any point or interval, the minimum value determined can be mapped to a given one of the curves 302, 404, 406 at the known maximum orifice diameter, and used to determine the value of the filter loading, allowing the filter loading value to be tracked over time with successive measurements. Figure 6 illustrates another example of a determination process for determining a filter loading value. Figure 6 is similar to Figure 5 but relates to the vacuum cleaner when operating with a second tool attached. Figure 6 therefore shows a second probability distribution 602 of inlet restriction values applicable when the vacuum cleaner is in use with the second tool. Similarly as described above with reference to Figure 5, Figure 6 shows projections 604, 606 of the second probability distribution 602 onto the first curve 302 and the third curve 406 respectively. As described above with reference to Figure 5, the value of the filter loading can be determined by measuring the minimum normalised delta-P value of the vacuum cleaner in use. It can then be determined to which of a plurality of curves, e.g. the curves 302, 404, 406, this minimum value maps. The curve to which the minimum normalised delta-P value maps is indicative of the value of the filter loading. From a comparison of the second probability distribution 602 and the first probability distribution 502, it can be seen that the second tool generally provides a higher level of inlet restriction than the first tool. The second tool may, for example, be a passive, crevice tool. In this example, a minimum level of inlet restriction provided by the second tool corresponds to an orifice diameter of around 23mm, compared to a value of around 47mm for the first tool. As a consequence, the minimum normalised delta-P value for a given filter loading value is greater when using the second tool than when using the first tool. For example, when using the second tool, the minimum normalised delta-P value at a filter loading of 100% is around 16.5kPa and the minimum normalised delta-P value at a filter loading of 0% is around 13.2kPa. Figure 6 shows this difference 608 in the minimum normalised delta-P value for a filter loading of 0% when using the first tool and when using the second tool by way of an arrow. In some examples, this difference in minimum normalised delta-P values at the same filter loading value may also be used to determine when a tool which is attached to the device is changed. It will be appreciated that various different types of tool may be therefore used with the device 200 and that each of these different tools may have an associated probability distribution of inlet restriction values, which may be used in a method of determining a value of the filter loading of the device 200. Moreover, the inlet restriction probability distribution of a given tool may vary depending on the usage. However, providing a given property of the probability distribution, e.g., the minimum level of inlet restriction, does not change, the given property may be used to determine the filter loading regardless of other variations in the overall probability distribution. Figure 7 shows a plot of filter loading values, on the y-axis, and minimum normalised delta-P values on the x-axis. Figure 7 shows a first filter loading curve 702 which corresponds to the first tool and a second filter loading curve 704 which corresponds to the second tool. Figure 7 illustrates, in this non-limiting example, how a minimum normalised value of delta-P maps to a given filter loading value for the first tool and the second tool. Figure 7 shows the difference 608 between the minimum normalised delta-P values for the first tool and the second tool at a filter loading value of 0%. As described above, typically values of the filter loading change gradually as the filter gathers more dirt during use of the device. Accordingly, sudden relatively large changes in a determined value of the filter loading may be generally not expected to occur unless an operating condition of the device changes. If such a sudden change is detected, then in some examples, this change may be taken to indicate a change in an operating condition of the device. For example, a detected sudden change in a determined value of the minimum normalised delta-P may be taken to be indicative of a change in the tool which is attached to the vacuum cleaner. For example, if the measured minimum normalised delta-P value changes by the difference 608 over a relatively short period of usage of the device, this may be taken to indicate that the vacuum cleaner has transitioned from an operating state in which the first tool is attached to an operating state in which the second tool is attached. Further, the correct filter loading curve to be used to relate minimum normalised delta-P values to filter loading values can be determined based on the type of tool attached to the device, e.g., if it is known which type of tool is attached to the device. For example, after a detected change 608 in the determined minimum normalised delta-P value, the device may move from determining the value of filter loading using the first filter loading curve 702 to determining the value of filter loading using the second filter loading curve 704. Although in certain examples described above, the minimum value of the operating parameter is the value used to indicate the value of the level of the filter loading, in other examples it shall be appreciated that other values of the operating parameter may be used to indicate the filter loading value. For example, a different property other than the minimum of a probability distribution of the operating parameter, such as an arithmetic mean, mode or other property, may be determined and used as the value of the operating parameter which indicates the filter loading value. Examples of the above-described method may allow for the filter loading value to be determined based on a correspondence between filter loading values and an operating parameter of the motor. This may in some examples allow for the filter loading value to be determined without use of further additional sensors, such as pressure sensors upstream and downstream of the filter. Accordingly, having demonstrated how the filter-loading values are related to values of the measured operating parameters by a pre-determined relationship for a respective inlet restriction condition, it shall now be discussed how the filter loading values are used to monitor a state or condition of the filter(s) in embodiments of the invention. In particular, the pre-motor filter 126 and the post-motor filter are typically made of different materials. The particulate matter accumulated at the pre-motor filter 126 can typically be cleaned off with reasonable effectiveness by rinsing and / or washing the filter 126. This is called recovery or refreshment of the filter. However, with each cycle of particulate loading and washing, the fibres of the pre-motor filter 126 degrade and the recovery is reduced, increasing a base restriction (i.e., following a wash) that the pre-motor filter 126 represents to the airflow. Moreover, the post-motor filter typically displays negligible recovery with washing, and therefore the particulate matter continuously accumulates at the post-motor filter, increasing the restriction that the post-motor filter represents to the airflow. In examples, the filter status is therefore categorised according to the determined filterloading value, which may be expressed in terms of a percentage. For example, a filterloading value of a particular value or percentage may represent a level of loading of the filter which is indicative that the filter should be washed or replaced, while filter-loading values below this threshold might indicate that the filter remains in a working, usable, condition. In particular, filter-loading values that are greater than or equal to a first filterloading threshold may be associated with a state in which the filter or filters are restricting airflow by an amount which reduces the efficiency of the operation of the device by a given degree. For example, a state in which the filter or filters are not completely blocked but in which the degree of blockage is sufficient that washing or replacing of the filter(s) is desirable. Below this first filter-loading threshold, the filter may be considered suitable for continued operation, for example only impairing the efficiency of the operation of the device to an acceptable degree. To give an example, the first filter-loading threshold value may be 100% and a filterloading value of 0% may represent that the filter has a minimum level of loading. For example, a filter-loading value of 0% may correspond with a state in which the filter or filters provide a minimum level of restriction to airflow through the device, e.g., corresponding with a level of filter loading provided by a new filter. The actual degree of blockage or dirt collected by the filter or filters that is corresponded with a given filter-loading value may vary, for example, depending on what degree of blockage is considered acceptable for allowing the device to operate with sufficient efficiency. Typically, the level of filter loading may increase gradually during use of the device 200 as air passes through the device and dirt is filtered from the air and collected by the filter. When a filter is deemed in need of replacing or cleaning, it may be removed from the device 200. The filter may then be washed and replaced in the device. Alternatively, a new filter may replace the previous filter. Other threshold filter-loading values may therefore also be set for detecting when the filter has expired / cannot be adequately recovered, i.e., for detecting when the filter is not refreshable to a degree that provides acceptable performance, as shall be discussed in more detail. In order to monitor the condition of the filter(s), the filter loading value may therefore be determined continuously or repeatedly, e.g., at regular intervals, during operation of the air-moving device 200. For example, the filter loading values may be determined at regular intervals to be used in a control method of the device, controlling one or more actions, such as the input power of the motor and / or providing prompts to clean or replace the filter(s) based on the determined filter loading values. A method of monitoring the filter(s) shall therefore be discussed in more detail with additional reference to Figure 8. Figure 8 shows a flow chart representation of an example method 800 of monitoring the air-moving device 200 in accordance with an embodiment of the invention. The method 800 comprises, at step 802, obtaining a series of measurements of one or more operating parameters of the air-moving device 200. As discussed above, such operating parameters may include an operating pressure of the motor, a speed of the motor and / or an airflow rate through the motor and each operating parameter may be measured by a suitable sensor (not shown in the figures). For example, the vacuum cleaner 200 may have been turned on and used for a cleaning operation, during which a series of measurements may be acquired for one or more such operating parameters of the air-moving device. The measured operating parameters are determined in series for the purposes of monitoring changes in the state of the filter. For example, the measurements may be obtained from the respective sensors at a predetermined sampling rate, for example being acquired at a rate of one measurement per second during operation. In examples, in addition to obtaining measurements of the operating parameters, measurements of one or more of the further parameters that are indicative of the operating conditions may also be acquired during operation, such as the ambient pressure, ambient temperature, and / or motor input power, which are used to normalise the measured operating parameters, as discussed previously. At step 804, the method 800 comprises determining one or more inlet restriction conditions of the air-moving device based on the obtained series of measurements. In other words, the series of measurements are obtained, in step 802, while the airmoving device 200 is operated with one or more inlet restriction conditions. For example, the obtained series of measurements may be associated with a plurality of inlet restriction conditions where the air-moving device 200 is initially operated with a first tool attached (while a first set of measurements are obtained) and subsequently operated with no tool attached (while a second set of measurements are obtained) or whilst being used on a relatively smooth surface. Each measurement is therefore obtained with an associated inlet restriction condition and, in step 804, the method 800 comprises determining the inlet restriction condition associated with each of the obtained measurements. In step 804, the device 200 may therefore be configured to detect whether a tool is attached to the device and / or what type of tool is attached to the device whilst obtaining the series of measurements, in step 802. In another example, whether or not the device is configured to detect which tool is attached to the device, the user may be asked to indicate which tool is attached to the device. In each case, the attached tool information may be stored in a memory and updated upon detection of replacement or removal of that tool. In this manner, the attached tool information may be used to determine the inlet restriction condition associated with the measurements obtained whilst that tool is attached to the device. In examples, the inlet restriction condition associated with the obtained measurements may therefore be determined continuously or periodically. By way of example, the inlet restriction condition associated with one or more successive measurements may be obtained at regular intervals, for example by comparing such measurements to the probability distribution of inlet restriction values associated with the attached tool indicated in the attached tool information. In examples, the measurements may also be normalised to account for the operating conditions of the vacuum cleaner 200. The determination process of step 804 may therefore further comprise determining normalised values of the operating parameter(s) using the one or more further parameters as respective normalisation parameters. At step 806, the method 800 comprises determining a series of filter loading values based on the obtained series of measurements. In this respect, each filter loading value may be determined based on one or more successive measurements of one or more operating parameters of the air moving device 200. In some examples, each filter loading value may be determined based on a single or representative value of an operating parameter, which may be a minimum, mean, or mode value. For example, the device may be configured to operate for a pre-determined time to allow the value of the operating parameter to be reliably measured and the operating parameter(s) may be sampled at a particular rate during such operation, such that the representative value used to determine the filter-loading value may be based on two or more such measurements. In examples, a period of operation may be chosen that allows the operating parameter to stabilise, for example following start-up of the device. For example, the device may be configured to operate for a minimum of 3 seconds to allow the operating parameter to be measured. In certain examples, for each filter loading value, a minimum value of the one or more measurements may be used to indicate the value of the level of the filter loading. In other examples, other values of the operating parameter may be used to indicate the filter loading value. For example, a different property other than the minimum of a probability distribution of the operating parameter, such as an arithmetic mean, mode or other property, may be determined and used as the value of the operating parameter which indicates the filter loading value. In examples, the measurements may be normalised again to account for the operating conditions of the vacuum cleaner 200. In which case, the pre-determined relationships used for determining each filter loading value may be between normalised values of the operating parameter and values of the filter loading. Such normalisation parameters may include one or more of an ambient pressure; an ambient temperature; a motor input power; and a build tolerance of the air-moving device, as discussed above. These parameters may be readily determinable, for example by use of sensors, or may be pre-determined, for example by a setup procedure. Normalising the value of the operating parameter by use of these parameters may provide for effectively mapping values of the operating parameter to values of the filter loading, accounting for the operating conditions. Each filter loading value is then determined based on one or more of the obtained measurements, using the pre-determined relationship for the associated inlet restriction condition, as determined in step 804. Successive filter loading values may therefore be determined iteratively based on one or more respective measurements of the operating parameter(s) and the filter loading values may be determined, in step 806, in parallel with the acquisition of measurements, in step 802, using the associated inlet restriction conditions determined, in step 804. That is, successive filter loading values may be determined iteratively based on one or more of the measurements obtained between intervals to produce a series of filter loading values. In examples, at each interval, a memory of the device 200, such as a non-volatile memory, may be updated to store the most recent filter loading value for analysis in a subsequent interval and to replace a previously determined filter loading value. In this manner, at each interval, the memory may store the preceding filter lading value, and the filter loading value determined in step 806 may be compared to the preceding filter loading value to derive a state of the filter in step 808, as shall be discussed in more detail. In step 808, the method 800 comprises determining a status of the filter based on the determined series of filter loading values. In examples, the filter status may be determined iteratively based on the most recent filter loading value, determined in step 806, and / or one or more preceding filter loading values. For example, the filter status may be determined based on the most recent filter loading value and the preceding filter loading value, stored in the memory, forming a successive pair of values. In each case, the filter status is determined, in part, by comparing the filter loading values to respective threshold values. For example, the most recent filter loading value may be compared to a threshold filter loading value that is indicative of whether the filter requires cleaning. That is, a first threshold value may be used corresponding to a filter loading value indicative that the filter presents an unacceptable restriction to the airflow and that the filter should therefore be cleaned or refreshed. For example, the first threshold may be a filter loading value of 100%. The filter status may therefore be categorised accordingly in dependence on the comparison. For example, if the most recent filter loading value is less than the first threshold value, the filter loading may be considered acceptable for continued operation, such that there is no immediate need to refresh the filter. The filter status may therefore be categorised as a first filter status, in step 808, being a normal working (effective) state of the filter. However, if the most recent filter loading value is greater than or equal to the first threshold value, the determined filter loading may be indicative that the filter is fully loaded and detrimentally affecting the operating of the motor assembly 100. The filter status may therefore be categorised as a second filter status, in step 808, being an ineffective state of the filter. In examples, the method 800 may further comprise a step 810 of controlling one or more actions in dependence on the detected filter state. For example, the method 800 may further involve outputting a filter cleaning prompt signal from the vacuum cleaner 200, in step 810, in dependence on the second filter status of the filter. For example, the filter cleaning prompt signal may be output in response to detecting the second filter status and may be output as a single prompt or the signal may be output continuously or periodically while the filter remains in the second filter status. In examples, the filter cleaning prompt may be performed by the device itself, for example, by a user interface of the device, or by another device which may be in communication with the air-moving device 200, for example a smartphone operating an app. The filter cleaning prompt signal may be terminated in dependence on subsequently detecting the first filter status, i.e., the normal working state of the filter, based on one or more subsequent filter loading values, and / or in dependence on detecting a filter refreshment, i.e. detecting that the filter has been washed or cleaned. In the course of determining the filter status, in step 808, the method 800 may therefore be further configured to detect filter refreshments. A filter refreshment may be detected by receiving indications from a user, for example via a user interface of the device, or via another device in communication with the air-moving device 200, such as an application on a smartphone. In other examples, the device may be configured to detect when a filter is removed and / or replaced by use of a suitable sensor, such as a magnetic sensor, and the removal and replacement may be indicative of a filter refreshment. Alternatively or additionally, the filter refreshment may be detected based on the determined series of filter loading values. In particular, the device 200 may detect a filter refreshment between a successive pair of the filter loading values if a more recent, or second, filter loading value of that pair is less than a preceding or earlier first filter loading value of that pair. In examples, it shall be appreciated that the filter refreshment may only be detected if the second filter loading value is less than or equal to a threshold percentage of the first filter loading value, e.g. in order to mitigate anomalous results. However, as discussed above, with each cycle of particulate loading and washing, the fibres of the pre-motor filter 126 degrade and the recovery is reduced, increasing a base restriction (i.e. following a wash) that the pre-motor filter 126 represents to the airflow. Moreover, the post-motor filter typically displays negligible recovery with washing, and therefore the particulate matter continuously accumulates at the post-motor filter, increasing the restriction that the post-motor filter represents to the airflow. The step 808 of determining the status of the filter based on the determined series of filter loading values may therefore further comprise detecting an exhausted or expired state of the filter(s), in which the filter(s) have degraded to the extent that the filter(s) cannot be recovered by washing / cleaning. The third filter status may be detected if, following a filter refreshment, the filter loading value remains too high. In particular, the earliest filter loading value determined after a detected filter cleaning operation may therefore be compared to a threshold filter loading value indicative of an unrecoverable state of the filter(s). This threshold filter loading value may therefore form a second threshold, which is less than the first threshold, for detecting when the filter(s) is exhausted and inadequately recovers following a filter refreshment. The filter loading value, or two or more such filter loading values, determined after the detected filter refreshment may therefore be compared to the second threshold and the third filter status may be detected if the filter loading value(s) are greater than or equal to the second threshold value. It shall be appreciated that in order to determine the third filter state, the determined series of filter loading values must therefore include a successive pair of filter loading values: (i) a first or earlier filter loading value, which may be recalled from a non-volatile memory of the device 200, that is greater than the first threshold value; and (ii) a second or most recent filter loading value, determined in step 806, that is less than the first value (and the first threshold value) but greater than the second threshold value. In other examples, the device 200 may have some degree of tolerance for an ineffective filter refreshment, for example anticipating that a user may inadequately clean the filter and require further prompts. Accordingly, in examples, the third filter status may only be detected, in step 808, if a predetermined number, e.g. two or more, filter refreshments have been detected but the filter loading values remain greater than or equal to the second threshold value after each refreshment. In examples, the third filter status may therefore be detected in this manner, without storing or recording a long history of filter loading values, by instead activating respective triggers whenever: (i) a filter refreshment is detected, and (ii) the subsequent filter loading values remain greater than or equal to the second threshold value. For example, in step 808, whenever it is detected that the filter loading value has not recovered below the second filter threshold following a filter refreshment, a check may be performed to determine whether a first trigger is active. If the first trigger is inactive upon detecting this condition, the first trigger may be activated and remain activated unless the subsequent filter loading values reduce below the second threshold, e.g. following a further filter refreshment. However, if the first trigger is already active when it is detected that the filter loading value has not recovered below the second filter threshold (following a further filter refreshment), a second trigger may be activated. Activation of the second trigger may correspond to the detection of the third or unrecoverable state of the filter. In examples, the method 800 may therefore further involve outputting a filter replacement prompt signal from the vacuum cleaner 200, as one of the control actions in step 810, in dependence on the third filter status of the filter. For example, the filter replacement prompt signal may be output in response to detecting the third filter status, as a single prompt or being output continuously or periodically while the filter remains in the third filter status. Again, in examples, the filter replacement prompt may be performed by the device itself, for example, by a user interface of the device, or by another device which may be in communication with the air-moving device 200, for example a smartphone operating an app. In other examples, the filter replacement prompt signal may not be output upon determining that the filter is in an expired state, but rather the filter replacement prompt may be delayed until the filter loading values indicate that the filter is also loaded to the extent that it provides ineffective operation of the air-moving device 200. That is, the filter replacement prompt signal may be output when it is detected that the filter is in a fourth filter status, having expired and been used to the extent that it is ineffective. The fourth filter status may therefore be detected, in step 808, if the filter loading values reach or exceed the first filter threshold after the third filter state has been detected. For example, if one or more filter loading values are determined that are greater than or equal to the first threshold value while the first and second triggers are active. In this manner, the device 200 enables maximum utilization of a filter before signalling for its replacement. In examples, the filter replacement prompt signal may be terminated in dependence on subsequently detecting the first filter status, i.e., the normal working state of the filter, and / or in dependence on detecting a filter replacement. Accordingly, in the course of determining the filter status, in step 808, the method 800 is further configured to detect filter replacements. A filter replacement may be detected, and distinguished from a filter refreshment, by receiving respective indications from a user, for example via a user interface of the device, or via another device in communication with the air-moving device 200, such as an application on a smartphone. In examples, the device may be configured to detect when a filter is removed and / or replaced by use of a suitable sensor, such as a magnetic sensor, and the removal and replacement may be indicative of a filter replacement. Alternatively or additionally, the filter replacement may be detected based on the determined series of filter loading values. In particular, the device 200 may detect a filter replacement, after detecting the third or fourth filter status, if a successive pair of filter loading values include: (i) a first or earlier filter loading value that is greater than or equal to the second threshold value; and (ii) a second or subsequent filter loading value is less than or equal to the second threshold value. Again, in examples, the filter replacement may only be detected if the second filter loading value is less than or equal to a threshold percentage of the first filter loading value, e.g. in order to mitigate anomalous results. It shall be appreciated in the above that, whenever an event occurs that may result in a significant change in the level of the filter loading, such as a filter refreshment or replacement, the method 800 may further involve recalibrating the device 200 to determine an initial filter loading value of the filter(s). In particular, following a filter wash event or replacement, the filter loading value may be entirely unknown. For example, washing the filter(s) may not always reduce the filterloading value to zero, as has been mentioned, and even the replacement of one of a plurality of filter may not restore the filter loading value to zero, for example, where other filters present a restriction to the airflow. It may therefore be necessary to determine an initial filter loading value, in known conditions, for subsequent usage of the device 200. The method 800 may therefore involve an additional calibration method for determining an initial filter loading value whenever a filter refreshment or replacement is detected. The determined initial filter-loading value acts as a baseline value for the refreshed / replaced filter, on the basis of which changes in the level of filter-loading during the subsequent operation of the device may be determined. One or more methods for determining the initial filter loading value may be used, but an exemplary calibration method is shown in Figure 9. The calibration method 900 may therefore form a subsidiary part of the method 800 of monitoring the filter(s) and comprises, at sub-step 902, arranging the device in a calibration mode that sets a pre-determined inlet restriction value for the device 200. For example, in the calibration mode, the device 200 may be arranged to run under a predetermined inlet restriction state in which no tool is attached to the device 200 or in which a specific tool is attached to the device 200. The predetermined inlet restriction state, in one example, comprises the device 200 operating in free air with no tool attached. In another example, the pre-determined inlet restriction state may comprise the device 200 operating in free air with a specific tool attached. The removal of all tools from the device or the attaching of a specific tool to the device may provide a simple and easily repeatable way of providing a pre-determined inlet restriction for the device. The sub-step 902 may therefore also involve prompting a user to arrange the device 200 in the calibration mode with the pre-determined inlet restriction value, such that a calibration can be performed. For example, when the pre-determined inlet restriction value is provided by removing all tools from the device, the user may be prompted to remove all tools from the device or, where no tools were attached to the device to begin with, to indicate that no tools are attached to the device. The user may be asked to indicate when no tool is attached to the device, for example, once they have removed all tools from the device 200. Alternatively, where the pre-determined inlet restriction is provided by a specific tool being attached to the device, the user may be prompted to attach the specific tool to the device and, for example, to indicate when they have done so. The prompting and / or the receiving of an indication may be performed by the device itself, for example, by a user interface of the device, or may, for example, be performed by a different device, such as a smartphone in communication with the device operating an app. Additionally or alternatively, in some examples, the device 200 may be configured to detect whether a tool is attached to the device and / or what type of tool is attached to the device. In such examples, in the calibration mode, the device may determine the pre-determined inlet restriction value based on the detected tool or lack thereof. For example, one or more inlet restriction values for the device may be pre-determined with each pre-determined inlet restriction value corresponding to the device when operating with a given respective tool attached. The device 200 may detect whether a tool is attached to the device and, if so, which tool is attached to the device. Using this information, the device may determine the pre-determined inlet restriction value to use in the method to determine the initial filterloading value. In another example, whether or not the device is configured to detect which tool is attached to the device, the user may be asked to indicate which tool is attached to the device and the device may then use this information to determine the predetermined inlet restriction value to use in the calibration process. In another example, the device may be configured to operate with a given predetermined inlet restriction value in the calibration mode. The device may then be configured to detect whether the correct tool is attached (which may be no tool) or for the user to indicate that the correct tool is attached to allow a calibration to be performed. The method 900 also comprises, at sub-step 904, operating the device while in the calibration mode to determine one or more measurements of an operating parameter of the air-moving device 200. In this case, the measurement(s) are associated with the predetermined inlet restriction condition prescribed for the calibration mode. In some examples, the user may be prompted to operate the device. For example, the user may be prompted to operate the device once the device has received an indication that the device is arranged with the pre-determined inlet restriction value. The indication may be provided, for example, via the user providing an input confirming that the correct tool (which may be no tool) is attached to the device or via the device detecting which tool is attached to the device. In some examples, the device may be configured to operate automatically once an indication has been received that the device is arranged with the pre-determined inlet restriction value. For example, in an example where the device is configured to detect which tool is attached, when the calibration mode is initiated, the device may operate automatically as part of a calibration sequence if the device detects that the correct tool is attached. Tn sub-step 906, the method 900 involves determining the initial filter-loading value based on a pre-determined relationship for the prescribed inlet restriction condition, relating values of the operating parameter to a set of filter-loading values. It shall be appreciated that, if the inlet restriction value is not known, it may be difficult to unambiguously determine a filter-loading value from a measured value of the operating parameter. However, by arranging the device with the pre-determined inlet restriction value, a value of the operating parameter may be determined based on a relationship between values of the operating parameter and filter-loading values which is applicable when the device is operating with the pre-determined inlet restriction value. In this respect, the initial filter loading value may be determined substantially as described previously. Once the initial filter loading value has been determined, it shall be appreciated that changes of the inlet restriction, can be determined, in step 804, with relative ease. That is, once the filer loading value is known, the filter loading may be used in combination with one or more operating parameters to determine the inlet restriction condition of the device 200. Such determinations may be made, for example, with reference to the pre-determined relationships between the filter loading values, and the operating parameters for different inlet restrictions. In this manner, the overall method 800 is able to track the filter loading through different operating conditions, and as the filter is washed and replaced. The method 800 determines respective state of the filter during such use and outputs corresponding signals to prompt a user to clean or replace the filter for effective operation as required. The method 800 of monitoring the filter(s) shall now be further demonstrated with reference to the exemplary plot shown in Figure 10. Figure 10 shows a series of filter loading values, including Points Pl to P9, amongst others, determined according to the method 800 described above. As shown in this example, the device 200 is initially operated with a new or unused set of filters and one or more operating parameters are measured, in step 802, while the device has a particular inlet restriction condition, determined in step 804. One or more such measurements are then used in combination with the known inlet restriction condition to determine the first filter loading value, Pl, in step 806, based on the predetermined relationship between the filter loading values and the operating parameter(s) for the associated inlet restriction condition. The first filter loading value, Pl, corresponds to an unused set of filters, and may be a filter loading value of 0% for example. Accordingly, in step 808, the method 800 determines the first working state of the filters, indicating that the filters are sufficiently clean for continued effective use without significantly impairing the operation of the device 200. During the subsequent use of the vacuum cleaner 200, particulates accumulate on the filters, presenting a greater restriction to the airflow through the motor assembly 100 and impacting the measured operating parameters. For example, the measured motor speed may reduce for a particular input power as the airflow restriction increases. Filter loading values may therefore be determined iteratively based on one or more respective measurements for each interval and the determined filter loading values increase to the Point, P2. In this example, the filer loading values are shown in Figure 10 to increase linearly, however this linear increase is merely provided for the sake of simplicity and, in other examples, it shall be appreciated that the filter loading values may present a more complicated pattern, generally increasing with continued use. Upon determining the filter loading value at the Point P2, the method 800 determines, in step 808, that the filters have reached the second state and are now ineffective, and may impair the operation of the device 200. Accordingly, the device 200 may control the output of the filter refreshment prompt signal, in step 810, in dependence on detecting the Point P2. The filter refreshment prompt signal may be output as a single signal or continuously until a filter refreshment is detected. In this example, a filter refreshment is subsequently detected between the filter loading values P2 and P3, in response to the prompt. For example, the filter refreshment may have been detected following an indication from a user and, upon detecting the filter refreshment, the device may prompt the user to arrange the device in the calibration mode to determine the initial filter loading value at the Point P3, using the method 900. As shown in Figure 10, the filter loading value, P3, has reduced below the first filter threshold value and the device may therefore detect, in step 808, that the filter is configured in the first working state following the refreshment. Although the filter refreshment is shown to occur, in this example, upon detecting the second state of the filter, it shall be appreciated that, in other examples, a user may ignore the filter refreshment prompt and the filter loading values may therefore continue to increase beyond the first threshold filter value before a filter refreshment is detected. However, this is not shown in this example for the sake of simplicity. Notably, the initial filter loading value is less than the previous filter loading value, P2, but greater than the filter loading value at the Point Pl, determined for a new filter. This is because, despite cleaning the filter(s), the filter(s) may have degraded or the post-motor filter may have failed to recover despite being washed. Thereafter the initial filter loading value, P3, is used as a baseline for determining the subsequent filter loading values, based on further measurements of the one or more operating parameters. The filter loading value therefore increases, as between Point Pl and P2, until the filter(s) become ineffective once more, at the Point P4. This cycle of usage and recovery continues to the Point, P5. The Point P5 represents the filter loading value determined, in step 806, after a detected filter refreshment, where the initial filter loading value determined after the filter refreshment exceeds the second filter threshold. Upon determining this condition, the method 800 may therefore check whether a first trigger is activated. If the first trigger is not activated, this is indicative that the filter(s) have failed to adequately recover for the first time and, to give some tolerance, the first trigger may be set without outputting the filter replacement prompt. In examples, the device 200 may nonetheless, continue to output the filter cleaning prompt signal, in step 810, to indicate that the filter cleaning was ineffective. In any case, during continued usage of the device 200, the filter loading value increases to the first threshold value again and the filter cleaning prompt signal is output, in step 810, upon determining the filter loading value at the point P6. Again a filter refreshment is detected between the points P6 and P7, and the initial filter loading value, P7, determined after the filter refreshment indicates that the filter recovery is inadequate, remaining above the second filter threshold. The device 200 checks again whether the first trigger is active and, in this case, activates the second trigger recognising that the filter(s) have not recovered adequately on two successive occasions. The device 200 therefore determines that the filter(s) have reached the third expired state, in step 808, at the Point P7. Finally, as the filter loading value continues to increase during subsequent usage, the filter loading reaches the first threshold value again at the point P8, indicating that the filter(s) have expired and are now fully loaded and therefore ineffective, impairing the operation of the device. The device 200 therefore determines that the filter(s) are in the fourth state, in step 808, at the point P8 and the filter replacement prompt signal is output, in step 810, to an operator. In response, an operator removes the expired filter(s) and replaces the filter(s) with new filter(s). Hence, a filter replacement is detected between the filter loading values at Points P8 and P9, where the initial filter loading value determined at the Point P9 indicates that the filter loading has returned to the first working state. The filter replacement prompt is therefore terminated, the first and second triggers are deactivated, and the method 800 is repeated for the new filter(s). The method 800 therefore monitors the state of the filter and provides feedback prompts to an operator to ensure that the filter is cleaned or replaced as appropriate for effective operation of the device 200. In this manner, the method 800 maximises the utility of the filter whilst ensuring that the operation of the device 200 is not compromised. It is anticipated that embodiments of the invention will therefore provide for improved maintenance of the vacuum cleaner 200, leading to a longer life cycle of the vacuum cleaner and improved cleaning performance

Claims

1. A method of monitoring a filter of an air-moving device, the method comprising:obtaining a series of measurements of one or more operating parameters of the airmoving device;determining one or more inlet restriction conditions of the air-moving device based on the series of measurements, each inlet restriction condition being associated with one or more of the obtained series of measurements;determining a series of filter loading values based on the obtained series of measurements, each filter loading value being based on one or more of the obtained measurements and a pre-determined relationship for the determined inlet restriction condition associated with the one or more obtained measurements, the pre-determined relationship relating values of the measured operating parameter to respective filter loading values for that inlet restriction condition;detecting a first filter status if the most recent filter loading value of the series of filter loading values is less than a first threshold value; anddetecting a second filter status if the most recent filter loading value of the series of filter loading values is greater than or equal to the first threshold value.

2. A method according to claim 1, further comprising:outputting a filter cleaning prompt signal in dependence on the second filter status of the filter.

3. A method according to claim 1 or claim 2, further comprising detecting a filter refreshment, the filter refreshment being detected:by determining a successive pair of the filter loading values; and detecting the filter refreshment in dependence on a second filter loading value of that pair being less than a preceding first filter loading value of that pair, or in dependence on the second filter loading value being less than or equal to a threshold percentage of the first filter loading value; and / orin dependence on obtaining a refreshment confirmation signal from a filter sensor or a user interface device.

4. A method according to claims 2 and 3, further comprising terminating the filter cleaning prompt signal in dependence on detecting:the first filter status, after the second filter status; and / orthe filter refreshment.

5. A method according to claim 3 or claim 4, further comprising detecting a third filter status if:following the detected filter refreshment, the second filter loading value of that pair is less than the first threshold value but greater than or equal to a second threshold value; ortwo or more filter refreshments are detected in the determined series of filter loading values, and each filter loading value succeeding an earliest one of the filter refreshments is greater than or equal to a second threshold value.

6. A method according to claim 5, further comprising detecting a fourth filter status, instead of the second filter status, if the most recent one or more filter loading values, after detecting the third filter status, are greater than or equal to the first threshold value.

7. A method according to claim 6, further comprising:activating a first trigger if the second filter loading value associated with the filter refreshment is less than the first threshold value but greater than or equal to a second threshold value,deactivating the first trigger if a more recent filter refreshment is detected and the second filter loading value associated with that filter refreshment is less than the second threshold value;activating a second trigger if a further one or more filter refreshments are detected, while the first trigger is active, and the second filter loading value associated with the most recent refreshment is greater than or equal to the second threshold value; anddetecting the fourth filter status, instead of the second filter status, if the most recent one or more filter loading values are greater than or equal to the first threshold value, while the second trigger is active.

8. A method according to any of claims 5 to 7, further comprising:outputting a filter replacement prompt signal in dependence on detecting the third or fourth filter status.

9. A method according to claim 8, further comprising detecting a filter replacement, after detecting the third or fourth filter status, the filter replacement being detected:by determining a successive pair of the filter loading values; and detecting the filter replacement in dependence on a first filter loading value of that pair being greater than the second threshold value and a subsequent second filter loading value of that pair being less than the second threshold value; and / orin dependence on obtaining a replacement confirmation signal from a filter sensor or a user interface device.

10. A method according to claims 8 and 9, further comprising:terminating the filter replacement prompt signal in dependence on subsequently detecting: the first filter status; and / orthe filter replacement.

11. A method according to any preceding claim, wherein each of the series of filter loading values is determined by:obtaining one or more measurements of at least one operating parameter of the airmoving device and a signal indicative of an inlet restriction condition of the air-moving device associated with the one or more measurements; anddetermining that filter loading based on those one or more measurements and a predetermined relationship relating values of that operating parameter to filter loading values when operating with that inlet restriction condition.

12. A method according to claim 11, wherein, for each filter loading value, the inlet restriction condition is obtained as an inlet restriction condition of the air-moving device when operating in a pre-determined operating condition, wherein the pre-determined operating condition is indicative of a type of tool attached to the device.

13. The method of claim 12, wherein the method comprises: selecting, based on the predetermined operating condition, the predetermined relationship.

14. A method according to any preceding claim, further comprising:detecting an event indicative of a fdter loading reduction;arranging the device in a calibration mode to determine an initial filter-loading value following the event, the calibration mode setting a pre-determined inlet restriction condition for the device;operating the device while in the calibration mode to determine one or more measurements of an operating parameter of the air-moving device associated with the predetermined inlet restriction condition;determining the initial filter-loading value following the detected event based on the one or more determined measurements and a pre-determined relationship for the inlet restriction condition of the calibration mode, the pre-determined relationship relating values of the operating parameter to respective filter-loading values for the calibration mode; andusing the determined initial filter-loading value as a baseline value for determining the subsequent filter-loading values following the detected event.

15. A method according to claim 14, when dependent on claim 3, wherein the detected event is the filter refreshment.

16. A method according to claim 14 or claim 15, when dependent on claim 8, wherein the detected event is the filter replacement.

17. A method according to any preceding claim, wherein the series of filter loading values are determined at intervals and each filter status is detected by analysing the most recent filter loading value and / or the preceding filter loading value; wherein a memory is updated at each interval to replace the filter loading value stored as the preceding filter loading value with the most recent filter loading value for analysis in a subsequent interval.

18. A method of detecting a filter fault of an air-moving device, the method comprising: responsive to detecting a filter refreshment, arranging the device in a calibration mode to determine an initial filter-loading value, the calibration mode setting a pre-determined inlet restriction condition for the device;operating the device while in the calibration mode to determine one or more measurements of an operating parameter of the air-moving device associated with the pre-determined inlet restriction condition;determining the initial filter-loading value based on the one or more determined measurements and a pre-determined relationship for the inlet restriction condition of the calibration mode, the pre-determined relationship relating values of the operating parameter to respective filter-loading values for the calibration mode; anddetecting a filter fault status in dependence on the determined initial filter-loading value being greater than or equal to a threshold filter loading value.

19. An air-moving device comprising: a processor; and a storage comprising a set of machine-readable instructions which when executed by the processor cause the processor to perform a method according to any of claim 1 to claim 18.

20. An air-moving device according to claim 19, wherein the air-moving device is a vacuum cleaner.

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