A power control method for a motor of an air-moving device

Abstract

Controlling input power of a motor 1 200of an air-moving device 1208, such as a fan impeller of a vacuum cleaner, comprises measuring a first value of an operating parameter of the motor, the operatin

Description

Field of the Invention The present invention relates to a method to control an input power of a motor of an air-moving device, a set of machine-readable instructions for causing the method to be performed, and an air-moving device having a storage comprising such instructions and a processor configured to perform the method by executing the instructions. Background of the Invention 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 of the Invention According to a first aspect of the invention, there is provided a method to control an input power of a motor of an air-moving device, the method comprising: performing a measurement process to determine a first value of an operating parameter of the motor, wherein the operating parameter is an operating pressure of the motor or an airflow rate through the motor; performing a determination process to determine, based on the first value of the operating parameter of the motor and a first pre-determined relationship between values of the operating parameter of the motor and values of an inlet restriction of the air-moving device, a value of the inlet restriction of the air-moving device; and controlling, based on the determined value of the inlet restriction, the input power of the motor. Controlling the input power of the motor based on the determined value of the inlet restriction may allow the input power of the air-moving device to be adapted to an appropriate level for the value of the inlet restriction. For example, the appropriate level may be a level which allows a task of the air-moving device to be performed effectively and efficiently. Where the value of inlet restriction is low, for example, lower motor input power may be needed to perform a given task, whereas, where the value of the inlet restriction is high, a higher input power may be needed to perform the given task. The method may allow for the motor input power to be dynamically controlled, for example, increasing the motor input power when the value of the inlet restriction is high and reducing the motor input power when the value of the inlet restriction is low. Determining the value of the inlet restriction based on the operating parameter, e.g. an operating pressure, of the motor may allow the inlet restriction to be determined reliably, based on an observable physical parameter, without directly measuring the inlet restriction. The method may allow the inlet restriction to be used when a passive tool, which does not have any sensing means which would allow inlet restriction to be inferred, is attached to the device. The method may obviate the need for additional sensors or processing to directly measure the inlet restriction. The operating parameter may be the operating pressure and the measurement process may comprise determining the first value of the operating parameter of the motor based on: an ambient pressure measurement; and a motor-inlet pressure measurement during operation of the motor. Determining the first value of the operating parameter based on an ambient pressure measurement and a motor-inlet pressure measurement during operation of the motor may provide for the value of the first operating parameter to be a differential operating pressure which correlates in a reliable and accurate way with the inlet restriction. It may also allow measurements taken for other purposes relating to the operation of the air-moving device, for example, the ambient pressure, to be used to obtain the first value of the operating parameter of the motor. The ambient pressure measurement and the motor-inlet pressure measurement may be measured at different times by a single pressure sensor. Measuring the ambient pressure measurement and the motor-inlet pressure measurement at different times by a single pressure sensor may allow the first value of the operating parameter to be obtained by use of a single pressure sensor which may provide for a cost- and space- efficient method of determining the operating pressure. The measurement process may comprise determining the first value of the operating parameter of the motor based on: a first pressure measurement of a pressure at a first position in a motor assembly in which the motor is located; and a second pressure measurement of a pressure at a second position in the motor assembly; wherein the second position is downstream of the first position. Using a pressure measurement upstream of the motor and a pressure measurement downstream of the motor may allow for an accurate and reliable measurement of the operating parameter to be obtained in a simple manner. The first pre-determined relationship may relate values of the operating parameter of the motor and values of one or more further parameters to values of the inlet restriction of the air-moving device. The determination process may comprise: determining the value of the inlet restriction based on one or more respective further parameter values of the one or more further parameters. This may allow for the determination of the value of the inlet restriction to compensate for other parameters, for example other measurable or determinable parameters relating to the air-moving device. The one or more further parameters may comprise one or more of: an ambient pressure; an ambient temperature; a motor input power; and a build tolerance of the air-moving device. These parameters may be readily determinable, for example by use or sensors, or may be pre-determined, for example by a calibration procedure. Compensating, in the determination of the inlet restriction, for these parameters may provide for first values of the operating parameter of the motor to be effectively mapped to values of inlet restriction. The determination process may comprise: determining a first normalised value of the operating parameter by normalising the first value of the operating parameter by use of one or more respective values of the one or more further parameters; and determining the value of the inlet restriction of the air-moving device based on the normalised operating parameter. Normalising values of the operating parameter by use of one or more further parameters may provide an efficient way of obtaining values of the operating parameter which map well to values of the inlet restriction of the air-moving device. For example, normalising the first value of the operating parameter may provide for reducing a dimensionality in a look-up to be performed based on a mapping between normalised values of the operating parameter and values of the inlet restriction. The one or more further parameters may comprise a value of a filter loading of a filter of the air-moving device. This may allow for a level of filter loading of a filter of the air-moving device to be compensated for in the determination of the level of inlet restriction. This may also allow for a reliable mapping of values of the operating parameter to values of inlet restriction to be maintained even where the level of filter loading changes during use of the air-moving device. The filter loading may be a level of loading of a filter which filters particulate matter from the airflow which passes through the motor. For example, the filter loading may be a level of loading of a pre-motor filter. The level of loading may define a dynamic restriction to airflow which is provided by the filter, e.g. due to dirt collected by the filter obstructing the airflow. The determination process may comprise determining the value of the filter loading. This may allow for a reliable, current value of the value of the filter loading to be obtained during operation of the air-moving device. This may contribute to the accuracy of the mapping of values of the operating parameter to values of the inlet restriction. The determining the value of the filter loading may comprise performing a second measurement process comprising determining a first value of a second operating parameter of the air-moving device, the first value being a value of the second operating parameter when the air-moving device is operating with a first inlet restriction condition; and performing a second determination process to determine the value of the filter loading, the determination process comprising determining, based on the first value of the second operating parameter and a second pre-determined relationship relating, for the air-moving device operating with the first inlet restriction condition, values of the second operating parameter to values of the filter loading, the value of the filter loading. This may allow for an accurate value of the filter loading to be obtained without directly measuring the level of filter loading. For example, the value of the level of filter loading may be obtained without the additional sensors and processing which might be used to determine the value of the filter loading based on measurements of a pressure upstream of and a pressure downstream of the filter. The second operating parameter may be: the operating pressure of the motor of the air-moving device; a speed of the motor of the air-moving device; or an airflow rate through the motor of the air-moving device. The second operating pressure of the motor or the speed of the motor of the airmoving device may be correlated with the value of the filter loading and this correlation may be used to obtain the value of the filter loading. The second operating parameter may be the same as the operating parameter referred to above. Using the same operating parameter, may allow a value which is already obtained as part of the method of determining the value of the inlet restriction to also be used to obtain the value of the filter loading. The speed of the motor of the air-moving device may also be a parameter which is obtained for use in monitoring or control procedures of the device and therefore using this parameter to estimate filter loading may be efficient in terms of not requiring further sensors or further processing to obtain the measurements. The determining the first value of the second operating parameter may comprise: determining a plurality of values of the second operating parameter; determining a distribution of the plurality of values of the second operating parameter; determining a first property of the distribution; and determining, based on the first property of the distribution, the first value of the second operating parameter. This may provide an effective way of obtaining values of the second operating parameter which map well to values of the filter loading. By determining the first value from a property of a distribution of values of the second operating parameter, pre-determined information regarding a probability of values of the inlet restriction of the air-moving device during operation may be taken into account in order to facilitate corresponding the first value with a pre-determined value of the inlet restriction. The first property of the distribution may be a minimum value in the distribution. The minimum value in the distribution may be efficient to determine and may correlate well to values of the filter loading. The controlling may comprise adjusting the motor input power according to a set profile relating motor input power values to inlet restriction values. This may allow the input power to be controlled in a pre-determined manner based on the determined inlet restriction values in order to provide an appropriate input power for the determined value of the inlet restriction. The set profile may be a continuous profile. This may provide for the input power to be dynamically adjusted with a fine level of granularity based on the determined value of the inlet restriction. This may provide for the air-moving device to perform efficiently with an input power matched finely to all values of the inlet restriction. The set profile may comprise a plurality of discrete power levels, each of the power levels corresponding to a respective range of values of inlet restriction. This may provide for the input power to be dynamically adjusted with a lower level of granularity. For example, this may provide for the input power to remain consistent with small determined changes in the inlet restriction while changing to meet a large determined change in the inlet restriction. This may provide for a good level of efficiency while presenting a consistent experience to a user of the air-moving device during small variations in inlet restriction during operation. The method may comprise, during operation of the air-moving device, performing the measurement process, the determination process and the controlling a plurality of times. This may, for example, involve continuously performing the method to continuously control the input power. This may allow for the device to dynamically adapt to changes in the determined value of the inlet restriction. According to a second aspect of the present invention, there is provided a method to control an input power of a motor of an air-moving device. The method comprises performing a measurement process to determine a first value of an operating parameter of the motor, wherein the operating parameter is an operating pressure of the motor or an airflow rate through the motor; performing a determination process to determine, based on a previous value of the operating parameter of the motor, a baseline value of the operating parameter of the motor; and controlling the input power of the motor, based on the first value of the operating parameter of the motor and the baseline value of the operating parameter of the motor. Controlling the input power of the motor based a determined first value of the operating parameter and a baseline value of an operating parameter may allow the input power of the air-moving device to be adapted to an appropriate level in response to changes in the loading of the air-moving device, for example changing between cleaning and idle modes or changes of inlet cleaning tools. This may change the restriction of air into the air-moving device, changing the operating pressure of the motor and / or airflow rate through the motor - this corresponds to a change in a value of the inlet restriction of the air-moving device. For example, the appropriate level of the input power may be a level which allows a task of the air-moving device to be performed effectively and efficiently. Where the value of inlet restriction is low, for example, lower motor input power may be needed to perform a given task, whereas, where the value of the inlet restriction is high, a higher input power may be needed to perform the given task. The method may allow for the motor input power to be dynamically controlled, for example, increasing the motor input power when the value of the inlet restriction increases and reducing the motor input power when the value of the inlet restriction reduces. This can be determined by comparing a value of the operating parameter with a baseline value, and controlling the input power accordingly. The baseline value of the operating parameter of the motor may be determined responsive to a predetermined variance in a plurality of previous values of the operating parameter of the motor. The baseline value is dynamic and may only be determined or updated when the values of the operating parameter change by a certain amount, reducing unnecessary control changes due to transient conditions. This may enhance control stability for the air-moving device. The predetermined variance corresponds to a threshold range of previous values of the operating parameter of the motor for a predetermined period. The baseline determination process may be performed based on one or more threshold values of the operating parameter of the motor. Controlling the input power of the motor may be based on the one or more threshold values of the operating parameter of the motor. Using threshold values allows for the simple implementation of different control strategies, as these thresholds may be readily updated as required. Controlling the input power to the motor may comprise reducing the input power responsive to the first value falling below a said threshold value or increasing the input power responsive to the first value exceeding a said threshold value. This allows power to be reduced due to cessation of cleaning and / or removal of a less restrictive cleaning tool and / or replacing a restrictive cleaning tool with a less restrictive cleaning tool. In one example, the threshold values may comprise an upper threshold value and a lower threshold value, and controlling the input power to the motor may comprise reducing the input power responsive to the first value falling below the lower threshold value and increasing the input power responsive to the first value exceeding the upper threshold value. This may allow a control hysteresis to be introduced to prevent the input power from being rapidly switched when the first value of the operating parameter switches above and below a single threshold, thereby improving the stability of the air-moving device. The reducing or increasing of input power may be prevented until a predetermined period after a previous reducing or increasing of input power. This blanking allows the air-moving device to reach a steady state, for example after switch-on, before starting automated input power control. Again, this may improve stability of the air-moving device. Controlling the motor is additionally dependent on a current input power of the motor. Additional control inputs may improve the control response in some situations. The first value may correspond to a predetermined time after applying input power to the motor. Again this “start-up” behaviour may improve control stability by allowing the air-moving device to reach a steady state before starting automated input power control. The baseline value of the operating parameter may be updated using the determined first value of the operating parameter. The method may then determine a subsequent value of the operating parameter of the motor to compare with the baseline value. This allows the steady state operation of the air-moving device to adapt to its current situation, whilst still responding to more transient conditions such as a change of cleaning mode or cleaning tool. The method may additionally comprises performing a determination process to determine, based on the first value of the operating parameter of the motor and a first pre-determined relationship between values of the operating parameter of the motor and values of an inlet restriction of the air-moving device, a value of the inlet restriction of the air-moving device; and to control, based on the determined value of the inlet restriction, the input power of the motor. Controlling the input power of the motor of the air-moving device using the determined baseline value of the operating parameter and the value of the determined inlet restriction may provide improved control in some situations. Determining the value of the inlet restriction based on the operating parameter, e.g. an operating pressure, of the motor may allow the inlet restriction to be determined reliably, based on an observable physical parameter, without directly measuring the inlet restriction. The method may allow the inlet restriction to be used when a passive cleaning tool, which does not have any sensing means which would allow inlet restriction to be inferred, is attached to the device. The method may obviate the need for additional sensors or processing to directly measure the inlet restriction. The operating parameter may be the operating pressure and the measurement process may comprise determining the first value of the operating parameter of the motor based on: an ambient pressure measurement; and a motor-inlet pressure measurement during operation of the motor. Determining the first value of the operating parameter based on an ambient pressure measurement and a motor-inlet pressure measurement during operation of the motor may provide for the value of the first operating parameter to be a differential operating pressure which correlates in a reliable and accurate way with the inlet restriction. It may also allow measurements taken for other purposes relating to the operation of the air-moving device, for example, the ambient pressure, to be used to obtain the first value of the operating parameter of the motor. The ambient pressure measurement and the motor-inlet pressure measurement may be measured at different times by a single pressure sensor. Measuring the ambient pressure measurement and the motor-inlet pressure measurement at different times by a single pressure sensor may allow the first value of the operating parameter to be obtained by use of a single pressure sensor which may provide for a cost- and space- efficient method of determining the operating pressure. The measurement process may comprise determining the first value of the operating parameter of the motor based on: a first pressure measurement of a pressure at a first position in a motor assembly in which the motor is located; and a second pressure measurement of a pressure at a second position in the motor assembly; wherein the second position is downstream of the first position. Using a pressure measurement upstream of the motor and a pressure measurement downstream of the motor may allow for an accurate and reliable measurement of the operating parameter to be obtained in a simple manner. The first pre-determined relationship may relate values of the operating parameter of the motor and values of one or more further parameters to values of the inlet restriction of the air-moving device. The determination process may comprise: determining the value of the inlet restriction based on one or more respective further parameter values of the one or more further parameters. This may allow for the determination of the value of the inlet restriction to compensate for other parameters, for example other measurable or determinable parameters relating to the air-moving device. The one or more further parameters may comprise one or more of: an ambient pressure; an ambient temperature; a motor input power; and a build tolerance of the air-moving device. These parameters may be readily determinable, for example by use or sensors, or may be pre-determined, for example by a calibration procedure. Compensating, in the determination of the inlet restriction, for these parameters may provide for first values of the operating parameter of the motor to be effectively mapped to values of inlet restriction. The determination process may comprise: determining a first normalised value of the operating parameter by normalising the first value of the operating parameter by use of one or more respective values of the one or more further parameters; and determining the value of the inlet restriction of the air-moving device based on the normalised operating parameter. Normalising values of the operating parameter by use of one or more further parameters may provide an efficient way of obtaining values of the operating parameter which map well to values of the inlet restriction of the air-moving device. For example, normalising the first value of the operating parameter may provide for reducing a dimensionality in a look-up to be performed based on a mapping between normalised values of the operating parameter and values of the inlet restriction. The one or more further parameters may comprise a value of a filter loading of a filter of the air-moving device. This may allow for a level of filter loading of a filter of the air-moving device to be compensated for in the determination of the level of inlet restriction. This may also allow for a reliable mapping of values of the operating parameter to values of inlet restriction to be maintained even where the level of filter loading changes during use of the air-moving device. The filter loading may be a level of loading of a filter which filters particulate matter from the airflow which passes through the motor. For example, the filter loading may be a level of loading of a pre-motor filter. The level of loading may define a dynamic restriction to airflow which is provided by the filter, e.g. due to dirt collected by the filter obstructing the airflow. The determination process may comprise determining the value of the filter loading. This may allow for a reliable, current value of the value of the filter loading to be obtained during operation of the air-moving device. This may contribute to the accuracy of the mapping of values of the operating parameter to values of the inlet restriction. The determining the value of the filter loading may comprise performing a second measurement process comprising determining a first value of a second operating parameter of the air-moving device, the first value being a value of the second operating parameter when the air-moving device is operating with a first inlet restriction condition; and performing a second determination process to determine the value of the filter loading, the determination process comprising determining, based on the first value of the second operating parameter and a second pre-determined relationship relating, for the air-moving device operating with the first inlet restriction condition, values of the second operating parameter to values of the filter loading, the value of the filter loading. This may allow for an accurate value of the filter loading to be obtained without directly measuring the level of filter loading. For example, the value of the level of filter loading may be obtained without the additional sensors and processing which might be used to determine the value of the filter loading based on measurements of a pressure upstream of and a pressure downstream of the filter. The second operating parameter may be: the operating pressure of the motor of the air-moving device; a speed of the motor of the air-moving device; or an airflow rate through the motor of the air-moving device. The second operating pressure of the motor or the speed of the motor of the airmoving device may be correlated with the value of the filter loading and this correlation may be used to obtain the value of the filter loading. The second operating parameter may be the same as the operating parameter referred to above. Using the same operating parameter, may allow a value which is already obtained as part of the method of determining the value of the inlet restriction to also be used to obtain the value of the filter loading. The speed of the motor of the air-moving device may also be a parameter which is obtained for use in monitoring or control procedures of the device and therefore using this parameter to estimate filter loading may be efficient in terms of not requiring further sensors or further processing to obtain the measurements. The determining the first value of the second operating parameter may comprise: determining a plurality of values of the second operating parameter; determining a distribution of the plurality of values of the second operating parameter; determining a first property of the distribution; and determining, based on the first property of the distribution, the first value of the second operating parameter. This may provide an effective way of obtaining values of the second operating parameter which map well to values of the filter loading. By determining the first value from a property of a distribution of values of the second operating parameter, pre-determined information regarding a probability of values of the inlet restriction of the air-moving device during operation may be taken into account in order to facilitate corresponding the first value with a pre-determined value of the inlet restriction. The first property of the distribution may be a minimum value in the distribution. The minimum value in the distribution may be efficient to determine and may correlate well to values of the filter loading. The controlling may comprise adjusting the motor input power according to a set profile relating motor input power values to inlet restriction values. This may allow the input power to be controlled in a pre-determined manner based on the determined inlet restriction values in order to provide an appropriate input power for the determined value of the inlet restriction. The set profile may be a continuous profile. This may provide for the input power to be dynamically adjusted with a fine level of granularity based on the determined value of the inlet restriction. This may provide for the air-moving device to perform efficiently with an input power matched finely to all values of the inlet restriction. The set profile may comprise a plurality of discrete power levels, each of the power levels corresponding to a respective range of values of inlet restriction. This may provide for the input power to be dynamically adjusted with a lower level of granularity. For example, this may provide for the input power to remain consistent with small determined changes in the inlet restriction while changing to meet a large determined change in the inlet restriction. This may provide for a good level of efficiency while presenting a consistent experience to a user of the air-moving device during small variations in inlet restriction during operation. The method may comprise, during operation of the air-moving device, performing the measurement process, the determination process and the controlling a plurality of times. This may, for example, involve continuously performing the method to continuously control the input power. This may allow for the device to dynamically adapt to changes in the determined value of the inlet restriction. According to a third aspect of the invention, there is provided a set of machine-readable instructions which when executed by a processor of an air-moving device cause the air-moving device to perform a method according to the first or second aspect of the invention. According to a fourth aspect of the invention, there is provided 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 the first or second aspect of the invention. The air-moving device may be a vacuum cleaner. Optional features of aspects of the present invention may be equally applied to other aspects of the present invention, where appropriate. Brief Description of the Drawings The present invention will now be described, by way of example only, with reference to the following figures, in which: Figure 1 shows a schematic representation of an example motor assembly of an air-moving device; Figure 2 shows an example of an air-moving device; Figure 3 is a flow chart representation of a method to control an input power of a motor of an air-moving device; Figure 4 shows an example of a plot of values of an operating pressure and values of an inlet restriction in an air-moving device; Figure 5 shows further examples of plots of values of the operating pressure and values of the inlet restriction; Figures 6A to 6C show, schematically, example power control profiles for an airmoving device; Figure 7 is a flow chart representation of a method to determine a value of a filter loading of an air-moving device; Figure 8 illustrates, schematically, aspects of an example method of determining the value of the filter loading; Figure 9 illustrates, schematically, further aspects of the example method shown in Figure 8; Figure 10 illustrates, schematically, further aspects of the example method shown in Figures 8 and 9; Figures 11A and 11B illustrate yet further aspects of the example method shown in Figures 8 to 10; Figure 12 shows a schematic representation of another example motor assembly of an air-moving device; Figure 13 shows a schematic representation of another example motor assembly of an air-moving device; Figure 14 shows a schematic representation of certain components of a motor assembly of an air-moving device according to an example; Figure 15 shows a schematic representation of certain components of an example motor assembly of an air-moving device according to another example; Figure 16 is a flow chart representation of a method to control an input power of a motor of an air-moving device; Figure 17 shows an example of a plot of operating parameter values and power levels over time in an air-moving device; Figure 18 is a flow chart representation of a method to control an input power of a motor of an air-moving device; and Figure 19 is a flow chart representation of a method to control resetting the baseline operating parameter for the method of Figure 18. Detailed Description of the Invention Figure 1 shows an example schematic representation of a motor assembly 100 of an air-moving device. The motor assembly 100 comprises 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 / diffuser 112. The motor assembly comprises a circuit board 114 on which are mounted sensor 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 a pre-motor filter 126 for filtering air which is drawn into the motor in use. Figure 2 shows an example air-moving device 200 comprising the motor assembly 100. The 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, e.g. 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. The machine-readable instructions when executed may cause the processor 208 to carry out any of the example methods described herein. In use, the motor of the motor assembly 100 draws air through the air inlet to the air-moving 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. 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 the generation of a varying magnetic field. This varying magnetic field is configured to act 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. The airflow path 128 continues through the motor to the impeller 108 and, after passing over the impeller 108, exits the motor assembly 100 through the air outlets 112. Figure 3 shows a flow chart representation of an example method 300 to control an input power of a motor of an air-moving device, such as a vacuum cleaner. The method 300 comprises, at block 302, performing a measurement process to determine a first value of an operating parameter of the motor. The operating parameter may be an operating pressure or an airflow rate. The operating pressure of the motor is an air pressure relating to the motor when the motor is in operation, i.e. 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, e.g. 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. The airflow rate is a rate at which, in use, air, being drawn by the operation of the motor, flows through the motor. Examples of methods of obtaining operating pressure measurements and airflow rate measurements will be described in more detail below. At block 304, the method 300 comprises performing a determination process to determine, based on the first value of the operating parameter of the motor and a first pre-determined relationship between values of the operating parameter of the motor and values of an inlet restriction of the air-moving device 200, a value of the inlet restriction of the air-moving device 200. Values of the inlet restriction of the air-moving device 200 define 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 value 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 flow of air-flow into the device 200, such as bristles for cleaning carpet, while other tools may not include such elements. In some examples, it may not be possible or practical to directly measure a value of inlet restriction of the air-moving device 200. Accordingly, according to examples described herein, values of the operating parameter of the motor are measured and used to determine values of the inlet restriction of the device 200. The first pre-determined relationship may be defined in terms of a curve relating values of the operating parameter of the motor and values of an inlet restriction of the air-moving device 200. The pre-determined relationship between values of the operating parameter and values of the inlet restriction may be obtained, for example, by a calibration process. This calibration process may involve, for example, operating the device 200 under known operating conditions, including a known value of inlet restriction, and measuring values of the operating parameter. This may be done by operating the device 200 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. The first pre-determined relationship may relate values of the operating parameter of the motor and values of one or more further parameters to values of the inlet restriction of the air-moving device. The value of the inlet restriction may then be determined based on a first value of the operating parameter and respective values of the one or more further parameters. The further parameters may be parameters of the air-moving device 200 which influence the value of the operating pressure which is 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, filter loading of a filter of the motor, 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 filter loading may be a level of loading of a filter which filters particulate matter from the airflow which passes through the motor. For example, 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 a post-motor filter or may take into account a level of loading of a plurality of filters, e.g. a pre-motor filter and a post-motor filter. The level of loading of the filter may define how much dirt has been collected by the filter. In examples, this may be expressed in terms of the amount of dirt the filter may collect before it is deemed in need of replacing or cleaning. For example, a filter loading of 100% may represent that the filter has collected an amount of dirt such that it is deemed in need of replacing or cleaning. A filter loading level of 0% may represent that the filter has collected no dirt, e.g. because it has been fully cleaned or newly replaced. Typically, the level of filter loading may increase steadily during use of the device 200 as air passes through the device and dirt is filtered from the air. 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 calibration 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. The first pre-determined relationship may in some examples define a multidimensional look-up table mapping values of the operating parameter and values of one or more further parameters to values of the inlet restriction. In one example, the first pre-determined relationship defines a six-dimensional look-up table which maps respective values of build tolerance, ambient pressure, ambient temperature, motor input power, filter loading and operating parameter, e.g. the operating pressure, to a value of the inlet restriction. In another example, a look-up table of lower dimensionality may be used in which normalised values of the operating parameter are mapped to values of the inlet restriction. The normalised values of the operating parameter may be obtained by normalising values of the operating parameter with respect to one or more further parameters, such as those mentioned above. For example, a fivedimensional look-up table may be defined which maps respective values of build tolerance, ambient pressure, ambient temperature, motor input power, and a value of the operating pressure to a normalised value of the operating pressure. A further, two-dimensional, look-up table may then be used to obtain a value of the inlet restriction from the normalised value of the operating pressure and a value of the filter loading. The lower dimensionality of the look-up table in this example means that the calculation is simpler. However, the accuracy of the determined inlet restriction value is highly dependent on the accuracy of the normalisation process. In contrast, using a look-up table of higher dimensionality without performing a normalisation process means that the calculation may be more computationally expensive but the accuracy of the output is not dependent on the accuracy of any normalisation process. In another example, the first pre-determined relationship may be represented using multi-dimensional curve fits or an artificial neural network. Such representations may in some examples be more efficient than a look-up table in terms of the amount of memory required. An example of a first pre-determined relationship relating normalised values of the operating pressure to the values of inlet restriction is shown in Figure 4. This example is for the motor of a vacuum cleaner. In the example of Figure 4, the operating pressure, shown on the y-axis, is 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 402 mapping values of normalised delta-P to values of inlet restriction has been obtained by a suitable calibration 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 402 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 402 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 5, 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 5 shows a set of curves 402, 504, 506 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 402 of Figure 4 is also shown in Figure 5 and corresponds to a value of filter loading of 0%. A second curve 504 corresponds to a value of filter loading of 50%. A third curve 506 corresponds to a value of filter loading of 100%. To determine the value of the inlet restriction, using the set of curves 402, 504, 506, one of the curves 402, 504, 506 may be selected based on a given value of the filter loading. The normalised value of the operating pressure then uniquely determines a value of the inlet restriction for the known value of the filter loading. In some examples, the method 300 involves determining the value of the filter loading. An example of a method for determining the value of the filter loading is described in detail below. Returning to Figure 3, at block 306, the method 300 comprises controlling, based on the determined value of the inlet restriction, the input power of the motor. The controlling may comprise adjusting the motor input power according to a set profile relating motor input power values to inlet restriction values. For example, a set profile may be defined which maps determined values of the inlet restriction to values of the input power. The set profile may be used to determine a desired input power from a determined value of the inlet restriction. The input power to the motor may then be adjusted to the determined desired input power. In some examples, the set profile comprises a plurality of discrete power levels, with each of the power levels corresponding to a range of values of inlet restriction. The number of discrete power levels may be, for example, two or three or more. In other examples, the set profile may be a continuous profile. For example, the power profile may comprise a curve relating a range of values of inlet restriction to values of input power. Examples of such profiles are shown in Figures 6A-C. In Figures 6A-C, values of motor input power are shown on the y-axis. Values of inlet restriction are on the x-axis and are defined in terms of an orifice diameter, in same manner as in Figures 4 and 5. A first example power profile is shown in Figure 6A. In Figure 6A, the set profile comprises two discrete power levels: a first power level 602 and a second power level 604. The first power level 602 corresponds to a first, higher, power P1 and the second power level 604 corresponds to a second, lower, power level P2. A range of low values of orifice diameter map to the first power level 602 while higher values of orifice diameter map to the second power level 604. Accordingly, when the inlet restriction of the air-moving device is highly restricted, i.e. the value of the orifice diameter is low, the method determines that the first, higher, input power P1 should be used. This allows the suction power of the device to be increased when the airflow into the device is highly restricted. When the level of inlet restriction is lower, i.e. the value of the orifice diameter is higher, the method determines that the second, lower, input power P2 should be used. Using a lower power at lower levels of restriction may, when it is practical to do so, allow the device to operate with lower power consumption while still providing adequate suction power. This may allow for less energy to be used by the device 200 and, for example, for a battery-powered device to operate for a longer period before the battery becomes depleted. A power profile may also define transition points between different power levels. The transition points may differ depending on whether the transition is a transition from a lower to a higher power level or a transition from a higher to a lower power level. For example, a transition “up” from a lower to a higher level may occur at a lower orifice diameter than a transition “down” from a higher to a lower power level. This may help prevent the power level transitioning between power levels more often than is desired, for example when there are small changes in the determined orifice diameter value around a boundary between power levels. Figure 6A shows examples of such transition points represented by dotted arrows 606a, 606b. As can be seen from Figure 6A, a first transition 606a from the lower power level 604 to the higher power level 602 occurs at a lower orifice diameter value than a second transition 606b from the higher power level 602 to the lower power level 604. Figure 6B shows a second example power profile. This second example power profile comprises three discrete power levels 608, 610, 612, compared with the two discrete power levels of the first example power profile of Figure 6A. As with the example of Figure 6A, different transition points, represented by dotted arrows, are defined between the power levels 608, 610, 612, depending on whether the transition is an “up” transition or a “down” transition. Figure 6C shows a third example power profile. The power profile of Figure 6C is continuous. That is, there are no transition points between discrete power level wherein the power level is discontinuous for varying orifice diameter. The power profile of Figure 6C comprises a first section 614 corresponding to low orifice diameters and a second section 616 corresponding to higher orifice diameters. The first section 614 in this example is flat and maps a range of values of orifice diameter to a single, high, power value. The second section 616 defines a curve which maps increasing orifice diameter values to decreasing values of power. As will be understood, various other types of mapping of inlet restriction to input power may be used. For example, a power profile may comprise one or more continuous sections where the power level varies smoothly for varying orifice diameter and / or one or more discontinuous sections wherein a change in orifice diameter corresponds to a transition between one discrete power level and another discrete power level. The method 300 may be performed a plurality of times during operation of the airmoving device. For example, the input power of the motor may be controlled continuously based on the determined level of the inlet restriction. For example, the value of the inlet restriction may be determined at regular intervals according to the steps described above and the value of the input power controlled accordingly. As such, the input power may, for example, be constantly be adjusted to be appropriate for the level of inlet restriction with which the device is operating. The input power may accordingly be being constantly adjusted to an appropriate level for the task being performed by the air-moving device. As mentioned above, determining the value of the inlet restriction based on the operating parameter of the motor may allow the inlet restriction to be determined reliably and accurately, based on an observable physical parameter, without directly measuring the inlet restriction. The method may also allow the inlet restriction to be determined for use in controlling the input power without the need for additional sensors or processing to directly measure the inlet restriction. Figure 7 shows a flow chart representation of an example method 700 to determine a value of the filter loading of the air-moving device 200. In some examples, the method 700 is performed to determine the value of the filter loading which is used in a method of determining the input power of the motor. For example, the example method 700 may be performed as a part of the example method 300 of Figure 3. The method 700 comprises, at block 702, performing a measurement process comprising determining a first value of a second operating parameter of the airmoving device, the first value being a value of the second operating parameter when the air-moving device is operating with a first inlet restriction condition. The second operating parameter may be an operating pressure of the motor of the air-moving device 200. For example, the operating pressure may be delta-P or any of the other types of operating pressure described above. For example, the operating pressure may be a normalised value of the operating pressure, for example, a normalised delta-P value. In some examples, the operating pressure may be the same operating pressure which is used in the method 300 of determining the value of the input power of the motor. This may be efficient since only one type of operating parameter may be needed in order to determine the filter loading and the input power. In other examples, a different type of operating pressure may be used to determine the value of the filter loading compared to the type of operating pressure used in the method of determining the input power of the motor. In some examples, the operating parameter may be 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 may be an airflow rate. Airflow rate may in some examples be determined based on pressure operating pressure measurements, as will be described below according to an example. The first inlet restriction condition may be indicative of a minimum level of an inlet restriction of the air-moving device 200. For example, the first value of the second operating parameter may be a value of the second operating parameter measured when the air-moving device 200 is operating with a minimum level of inlet restriction, or, equivalently, with a maximum equivalent orifice diameter. This minimum level of inlet restriction may correspond to the device 200 operating in free air. That is, the minimum level of inlet restriction may be the level of inlet restriction acting on the device 200 when a tool of the vacuum cleaner is not engaged with a surface, such that there is no external obstruction to the flow of air into the device 200. In other examples, the first inlet restriction condition may 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 first value of the second operating parameter may then be a value measured when the device 200 is operating with the mean or mode inlet restriction value. The method 700 also comprises, at block 704, performing a determination process to determine the value of the filter loading of the filter of the air-moving device 200. The determination process comprises determining, based on the first value of the second operating parameter and a first pre-determined relationship relating, for the air-moving device 200 when operating with the first inlet restriction condition, values of the second operating parameter to values of the filter loading, the value of the filter loading. An example of a determination process for determining a value of a filter loading will now be described with reference to Figure 8. Figure 8 shows the set of normalised delta-P curves 402, 504, 506 described above with reference to Figure 5. Figure 8 shows a first probability distribution 802 of applicable inlet restriction values of the vacuum cleaner. The first probability distribution 802 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 802 corresponds to a passive tool comprising a relatively wide nozzle and a selectively engageable brush. As can be seen from Figure 8, in this example, the inlet restriction values of the first probability distribution 802 range from around 13mm to around 47mm. The minimum level of inlet restriction in the first probability distribution 802 corresponds to an orifice diameter of around 47mm. Figure 8 also shows respective projections of the first probability distribution 802 onto two 402, 506 of the normalised delta-P curves. A first projection 804 is a projection of the first probability distribution 802 onto the first curve 402, which, as described above, corresponds to a filter loading of 0%. A second projection 804 is a projection of the first probability distribution 802 the third curve 506, which, as described above, corresponds to a filter loading of 100%. The projections 804, 806 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 804 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 806 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 8, 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 402, 504, 506, 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 402, 504, 506. In examples, this property may be used to determine the level of filter loading from a measured value of normalised delta-P. For example, from the first probability distribution 802, it is known that the minimum normalised delta-P values in the probability distributions 804, 806 correspond to the device operating with a known maximum orifice diameter, in this example of around 47mm. 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 given one of the curves 402, 504, 506 at the known maximum orifice diameter. By determining which of the curves 402, 504, 506, the measured normalised delta-P value maps to, the value of the filter loading can be determined. For example, in Figure 8, the third curve 506 and the probability distribution 806 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 402 and the probability distribution 804 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%. Figure 9 illustrates another example of a method of determining a filter loading value. Figure 9 is similar to Figure 8 but relates to the vacuum cleaner when operating with a second tool attached. Figure 9 shows a second probability distribution 902 of inlet restriction values applicable when the vacuum cleaner is in use with the second tool. Similarly to as described above with reference to Figure 8, Figure 9 shows projections 904, 906 of the second probability distribution 902 onto the first curve 402 and the third curve 506 respectively. As described above with reference to Figure 8, 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 402, 504, 506, 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 902 and the first probability distribution 802, it can be seen that the second tool generally provides a higher level of inlet restriction than the first tool. For example, 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 9 shows this difference 908 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 be used to determine when a tool which is attached to the device is changed. Figure 10 shows a plot of filter loading values, on the y-axis, and minimum normalised delta-P values on the x-axis. Figure 10 shows a first filter loading curve 1002 which corresponds to the first tool and a second filter loading curve 1004 which corresponds to the second tool. Figure 10 illustrates how a minimum normalised value of delta-P maps to a given filter loading value for the first tool and the second tool. Figure 10 shows the difference 908 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 908 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 which is 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 908 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 1002 to determining the value of filter loading using the second filter loading curve 1004. Figures 11A and 11B show examples of probability distributions 1102, 1104 of normalised delta-P values when the device 200 is in use with a given tool, e.g. with the first tool. Figure 11A corresponds to the vacuum cleaner operating with a filter loading value of 0%. Figure 11B corresponds to the vacuum cleaner operating with a filter loading value of 100%. In an example, to determine such a probability distribution, a fixed number of counters is defined, e.g. 100 counters may be defined. Two one-dimensional arrays are defined, a pressure bin array and a FIFO, first in, first out, array. The FIFO array has a length equal to the fixed number of counters. A value of delta-P is measured and normalised. A counter is added to the pressure bin corresponding to the measured and normalised value. Once all of the counters have been allocated to bins, with the next measurement of normalised delta-P, the oldest allocated counter, which may be the counter in the final position of the FIFO array, is moved to the bin corresponding to the latest measured normalised delta-P value. In this way, a permanent rolling distribution can be maintained. The rolling distribution may be queried at any time and at any frequency. In the example where the minimum value of normalised delta-P is the value which is determined and used to indicate the filter loading value, the lowest populated bin is determined in order to determine the minimum value of normalised delta-P. In some examples, a minimum counter threshold may be set under which counters in a bin are not tallied, such that the lowest populated bin is the lowest bin with at least the threshold number of counters. This may act as a noise filter. In some examples, the distribution may be filtered, e.g. by determining an exponential moving average, to further remove noise. In other examples, the FIFO array may not be defined and, for example, once all of the counters have been allocated, the counters may be removed from the pressure bins and the allocation of counters may start again. Figures 11A and 11B show, by way of example, minimum normalised delta-P values 1106 of the distributions 1102, 1104 which are indicative of filter loading values of 0% and 100% respectively. Figures 11A and 11B also show by way of example a threshold counter value 1108 below which counters are not tallied for the purposes of determining the lowest populated bin. The above example has been described with reference to two particular tools. However, it will be appreciated that various different types of tool may be 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. Although in certain examples described above, the minimum value of the second operating parameter is the value used to indicate the value of the level of the filter loading, in other examples other values of the second 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 second operating parameter, such as an arithmetic mean, mode or other property, may be determined and used as the value of the second 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. The value of the filter loading may be used for various purposes. For example, as described above, in certain examples, determining an inlet restriction value may require correcting for a filter loading value. In another example, the filter loading value may be used to provide an alert. For example, when the filter loading value reaches a given threshold an alert may be issued indicating that the filter should be washed or replaced. The method 700 may also be performed a plurality of times, e.g. at regular intervals, during operation of the air-moving device. For example, the filter loading value may be determined at the same regular intervals as the operating parameter which is used to control the input power of the motor. As above, this may be the same operating parameter as the second operating parameter in some examples, e.g. both may be a delta-P. Figure 12 shows another example schematic representation of a motor assembly 1200. The motor assembly 1200 comprises features corresponding to those of the motor assembly 100 described above with reference to Figure 1, which, where labelled, are labelled with like reference numbers. The pre-motor filter is not shown in Figure 12, for the sake of clarity. The motor assembly 1200 further comprises a second pressure sensor 1220 and wiring 1222 which electrically connects the second pressure sensor 1220 to circuit board 1214. The motor assembly 1200 further comprises an inlet tube 1226 providing a fluid connection, through housing 1224, from the second pressure sensor 1220 to an inlet 1230 of impeller 1208. This allows the second pressure sensor 1220 to take measurements of a pressure at the impeller inlet 1230. As can be seen by the schematic representation of Figure 12, a cross-sectional area of the motor is narrower at the impeller inlet 1230 than at the motor inlet 1210. Figure 13 shows another example motor assembly 1300. The motor assembly 1300 is the same as the motor assembly 1200 of Figure 12 with the exception that, in the motor assembly 1300 of Figure 13, the second pressure sensor 1320 is located on the circuit board 1314. A channel or duct 1322 provides a fluid connection between the second pressure sensor 1320 and the impeller inlet 1330. The channel 1322 allows the second pressure sensor 1320 to take measurements of a pressure at the impeller inlet 1330 without the second pressure sensor being located at the impeller inlet 1330. Such a channel may be provided by various means. In one example, the channel 1326 may be formed by a pipe. The pipe may, for example, extend along an exterior surface of the housing 1324 and extend through a hole 1326 in the housing to provide the fluid connection from the second pressure sensor 1320 to the impeller inlet 1330. At an end of the channel 1326 at which the second pressure sensor 1320 is located, an air-tight seal may be formed around the second pressure sensor 1320. The seal may, for example, comprise a circular, e.g. EPDM, foam seal sealing the pipe to a location on the circuit board 1314 at which the second pressure sensor 1320 is located. A similar seal may be formed around the second pressure sensor 1220 of the motor assembly 1200 of Figure 12. In another example the channel may be integral with the housing of the motor assembly. Figure 14 shows an example schematic representation of such a housing 1424 with a channel 1422 extending through the housing 1424. In such an example, the housing 1424 may be formed by an injection moulding process, with the channel through the housing 1424 formed during the injection moulding, e.g. by the use of removable pins 1430a, 1430b during the injection moulding. In use, in the manner described with reference to Figure 13, a hole 1426 through the housing 1424 opens into an impeller inlet of the motor. In use, the second pressure sensor is located at an upstream end 1428 of the channel 1422. This provides a fluid connection via the channel 1422 to the second pressure sensor and allows the second pressure configured to take pressure measurements of the pressure at the impeller inlet. This may allow the second pressure sensor to be conveniently located. The second pressure sensor may, for example, be located on a circuit board of the device. Further, forming the channel 1422 integrally with the housing 1424 may be cost effective and convenient. In another example, the channel may be formed between an exterior surface of the housing and a mount located against the exterior surface of the housing. Figure 15 schematically illustrates such an example. In Figure 15, a mount 1532, e.g. made of rubber, has a groove 1534 therein. The mount 1532 seals in an airtight manner against an exterior surface of the housing 1524 which has a hole 1526 which, in use, leads to the impeller inlet. A channel 1522 is created by a gap provided by the groove 1534 in the mount 1532 between the mount 1532 and the exterior surface of the housing 1524. In both of the examples of Figure 14 and Figure 15, in use, the second pressure sensor is sealed in an air-tight manner to the channel 1422,1522 at the upstream end 1428,1528 of the channel 1422, 1522. For example, in the example of Figure 15, the mount 1532 may be a rubber mount which forms a seal around the second pressure sensor. In each of the example motor assemblies 1200, 1300, the first pressure sensor 1218, 1318 is positioned to take pressure measurements at the air inlet 1210, 1310. The pressure measurements taken by the first pressure sensor 1218,1318 include an ambient pressure pa which is measured prior to start-up of the motor. Further, the pressure measurements taken by first pressure sensor 1218, 1318 include measurements of a first pressure pi taken during running of the motor. The temperature sensor 1216, 1316 is configured to measure an ambient temperature Ta. The second pressure sensor 1220, 1320 is configured to take pressure measurements of a second pressure p2 at the impeller inlet 1230, 1330 during running of the motor. Each of the ambient pressure pa, the first pressure pi and the second pressure p2 are absolute pressures. In an example, measurements taken by the first pressure sensor 1218, 1318 the second pressure sensor 1220, 1320 and the temperature sensor 1216, 1316 are used to determine a dynamic pressure value. In one example, a dynamic pressure measurement is determined as follows. A gauge static pressure pstatic in the motor is determined by subtracting the ambient pressure pa from the first pressure pi. The first pressure pi is typically lower than the ambient pressure pa because the running of the motor causes a partial vacuum to be generated within the motor housing 1324. A gauge total pressure ptotai at the impeller inlet is determined by subtracting the second pressure p2 from the first pressure pi. The total pressure ptotai at the impeller inlet is made up of the static pressure pstatic and a dynamic pressure payn. The second pressure p2 is typically lower than the first pressure pi due to the lower cross-sectional area and associated higher air velocity at the impeller inlet 1230, 1330 as compared with at the motor inlet 1210, 1310. The dynamic pressure pdyn at the impeller inlet 1230, 1330 is determined by subtracting the static pressure pstatic in the motor from the total pressure ptotai at the impeller inlet 1230, 1330. The dynamic pressure pdyn may also be referred to as an air velocity pressure. The dynamic pressure pdyn, the first pressure pi and the temperature Ta are input into a density ratio formula to determine the dynamic pressure value at STP Pdyn@sTP. The value of pdyn@sip is a dynamic pressure value corrected to standard temperature and pressure. Accordingly, the dynamic pressure value is normalised for the ambient conditions in which the motor is operating. This allows, for example, a single look-up curve to be defined relating dynamic pressure values to airflow rates or other parameters. The applicable density ratio for a given motor may depend on a type of the motor. For example, the following density ratio formulae (1) to (3) apply, respectively, for constant power motors, AC series motors, and constant speed motors: - i / pi A ( 293 Pdyn@STP — Pdyn I ^101325 / \ro + 273.157 - I f pi A f 293 A Pdyn@STP Pdyn ( \101 325 / \Ta+273.15 / (1) (2) (3) Pdyn@STP Pdyn where pdyn@sTP, pi, and pdyn are in units of kPa, Ta is in units of degrees Celsius, 101.325 is standard pressure in units of kPa, 293 is standard temperature in units of Kelvin and 273.15 is 0 degrees Celsius in units of Kelvin. A determined dynamic pressure value may be mapped to various parameters. For example, the dynamic pressure value may be used as an operating pressure in examples of the methods described above. For example, dynamic pressure values may be mapped to values of inlet restriction and / or filter loading, e.g. in a similar manner to that described above for delta-P values. Additionally, or alternatively, the dynamic pressure value may be mapped to values of airflow rate through the motor. The airflow rate may be used as an operating parameter in example methods described above. The mapping of dynamic pressure values to airflow rate may be determined, for example, by a calibration process. In such a calibration process, the air-moving device may be operated with an airflow rate measuring apparatus, which may comprise a bell mouth, a venturi, or an orifice plate, being used to measure the airflow rate through the device while at the same time measurements are taken which allow dynamic pressure values to be determined which can be corresponded with airflow measurements. Accordingly, when the device is operated after calibration, dynamic pressure values may be determined and mapped to airflow rate values in order to determine the airflow rate through the device in use. Figure 16 shows a flow chart representation of an example method 1600 to control an input power of a motor of an air-moving device, such as a vacuum cleaner. This method may be used with the air moving device, motor assembly or components described with respect to Figures 1,2 and 12-15. The method 1600 comprises, at block 1602, performing a measurement process to determine a first value of an operating parameter of the motor. The operating parameter may be an operating pressure or an airflow rate. In an example, the operating parameter may be continuously measured or sampled, with the first value corresponding to a latest measurement or sample or an average or recent measurements or samples. As previously described, the operating pressure of the motor is an air pressure relating to the motor when the motor is in operation, i.e. 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. Alternatively, the operating pressure is a difference between a first pressure measured when the motor is not running (an ambient pressure) 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, e.g. 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. In some examples described herein, the operating pressure is normalised as previously described, for example the operating pressure may be normalised delta-P. The airflow rate is a rate at which, in use, air, being drawn by the operation of the motor, flows through the motor. Various methods of obtaining operating pressure measurements and airflow rate measurements have been previously described. At block 1604, the method 1600 comprises performing a determination process to determine, based on previous values of the operating parameter of the motor, a baseline value of the operating parameter of the motor. In an example, the baseline value may be dynamic as it tracks changes in the operating parameters, depending on certain conditions as described in more detail below. As previously described, the operating pressure of the motor or the airflow through the motor may be affected by restrictions to the airflow through the air moving device 200. Inlet restrictions may include tool attachment and the operational mode of the air-moving device, such as whether a vacuum cleaner is cleaning a surface or in an idle, non-cleaning or “free air” mode. 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 value 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 flow of airflow into the device 200, such as bristles for cleaning carpet, while other tools may not include such elements. As previously described, the operating pressure may also be affected by restrictions associated with filter loading or the level of loading of a filter which filters particulate matter from the airflow which passes through the motor. For example, 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 a post-motor filter or may take into account a level of loading of a plurality of filters, e.g. a pre-motor filter and a post-motor filter. The level of loading of the filter may define how much dirt has been collected by the filter. Typically, the level of filter loading may increase steadily during use of the device 200 as air passes through the device and dirt is filtered from the air. The baseline value of the operating parameter of the motor is an operating pressure or airflow rate which is set at the measured or first value of the operating pressure after certain stability conditions are meet. The baseline value may change over time, for example as the filter loading steadily increases the baseline value may also steadily increase, until the filter is change and the baseline value returns to a lower value. Similarly, the baseline value may change in response to changes or tool attachments. This simplified approach does not require the predetermined relationships previously described to account for restriction changes, but can alternatively combine with these. In an example, a number of samples of the operating parameter may be determined and a variance calculated. The variance may be a maximum variance of sample values over a moving time period or window. If calculated variance falls within a variance threshold, indicating a stable operating parameter, the baseline value may then be reset or updated to the latest sample value, or an average of the sample values within the window. At block 1606, the method 1600 comprises controlling the input power of the motor, based on the first value and the baseline value of the operating parameter of the motor. The controlling may comprise adjusting the motor input power according to a set profile having switching thresholds relating motor input power values to the first value of the operating parameter, where the profile is dependent on the baseline value. In some examples, different profiles may correspond to different ranges of baseline values. Each of the set profiles may comprise a plurality of discrete power levels or thresholds, with each of the power levels corresponding to a range of values of first values of the operating parameter. The number of discrete power levels or thresholds may be, for example, two or three or more. The switching thresholds for increasing power may be higher than the corresponding switching threshold for reducing power, providing a control hysteresis loop in order to improve control stability as previously described. In other examples, the set profile may be a continuous profile. For example, the power profile for each baseline value may comprise a curve relating a range of values of the first values of the operating parameter. In some examples, the baseline value is updated to accommodate changes in restriction such as tool attachments and filter loading. The baseline value corresponds to a low power setting, which may be appropriate for idle or noncleaning modes where the air moving device is running with free air. Power switching thresholds are calculated based on the baseline value and power may be increased if the measured operating parameter exceeds these thresholds, and subsequently the power is reduced when the measured operating parameter falls below an appropriate switching threshold. This may correspond to a cleaning event such as applying the inlet of the air moving device to a surface such that restriction to the airflow of the air moving device is increased, resulting in increased measured operating parameter value. Similarly, when inlet of the air moving device is removed from the surface, the measured operating parameter value falls which may correspond to a reduction in power level. Thus, the power level of the motor is adapted to changing conditions, for example to increase power when a user applies the inlet to a surface to be cleaned and to reduce power when the user removes the inlet from the surface such that the air moving device is running in free air. These changes in restriction during cleaning activities typically tend to be of short duration, with many changes of measured operating parameter value. Changes in attachments are typically characterised by large longer duration changes in measured operating parameter values. Such change in measured operating parameter tend to be stable, with little variance over a longer period. For example, if a user removes a low restriction tool attachment and fits a high restriction tool attachment with the motor running, the air moving device will be running in free air or idle mode. The measured operating parameter values will change from low values corresponding to the idle or non-cleaning mode without tool attachment, followed by higher values corresponding to the idle mode with the new higher restriction tool attachment. As the operating parameter values will then settle around these new higher values, this condition may be used to reset the baseline condition to the higher value. New switching thresholds may then be calculated for the new baseline, and power increased when a cleaning event occurs, such as applying the inlet to a surface resulting in further restriction of the airflow, and an increase in power level if the new switching threshold is exceeded. Similarly, when the high restriction attachment tool is removed, the measured operating parameter will fall as the air moving device is now running in free air. As the new measured operating parameter values will be below the previous baseline value, the baseline value will be reset to the new lower measured operating parameter value. Changes in filter loading tend to be characterised by slow and incremental changes in operating parameter value over a long time period. In an example, this can be determined by resetting the baseline value whenever the motor starts. When starting the device is likely to be in a non-cleaning or idle mode, corresponding to a lowest power level. At the lowest power level, after a settling period or delay to allow the measured operating parameter value to reach a steady state, the baseline value is reset at the measured operating parameter value. This allows the baseline value to adapt to increases in filter loading over many cycles of use of the air moving device, and also allows the device to adapt to the replacement of the filter which will significantly lower the filter loading and restriction experienced by the device. The baseline value may then be reset at the new lower measured operating parameter value. These different characteristics of changes in measured operating parameter can be used to distinguish between filter loading, tool attachment changes and changes in cleaning modes. Operation of the air moving device can then be automatically controlled appropriately and optimally. In some examples, this can be achieved using a single sensor. Figure 17 shows plots of normalised delta-P (lower) and input power (upper) over time for an air moving device such as a vacuum cleaner according to an example. This shows changes in normalised delta-P corresponding to different events such as a user applying (and removing) an inlet of the device to (from) a surface for cleaning as well as tool attachment changes. Corresponding changes in motor input power are also shown. As previously described, delta-P corresponds to a value of an operating pressure obtained by determining a difference between an ambient pressure measurement, taken when the motor is not running, e.g. before start-up of the air-moving device 200, and a pressure measurement taken during operation of the motor. The delta-P values may be normalised with respect to ambient conditions as previously described, for example with respect to values of build tolerance, ambient pressure, ambient temperature and motor input power. In this example, three different input power levels P1 - P3 are available for the motor, however in other examples different numbers and levels of input power may be used. In the lower plot, a curve 1705 shows the normalised Delta-P over time, with specific times TO - T11 illustrated. The normalised Delta-P is the measured and adjusted operating parameter value used for controlling the input power to the vacuum cleaner. Six switching lines 1710a - 171 Of are shown which correspond respectively to: a switch down threshold (from P2 -> P1) for a first lower baseline (1710a); a switch up threshold (from P1 -> P2) for the first lower baseline (1710b); a first switch down threshold (P2 -> P1) for a second higher baseline (1710c); a first switch up threshold (P1 -> P2) for the second baseline (171 Od); a second switch down threshold (P3 -> P2) for the second higher baseline (1710e); a second switch up threshold (P2 -> P3) for the second higher baseline (171 Of). Different thresholds may be calculated for different baseline normalised delta-P values. In this example, the lower baseline only uses two power levels P1 and P2, switching up from P1 to the higher P2 level when the normalised delta-P exceeds threshold 1710b and switching down from the higher P2 level to the lower P1 level when the normalised delta-P value falls below threshold 1710a. The switch down condition may occur when a user removes an inlet from a surface to be cleaned such that the device is in a non-cleaning or idle mode. In some examples a switch down delay may be introduced to prevent the device from switching down to quickly, such as when the user intends to again enter a cleaning mode by quickly reapplying the inlet of the device to another surface to be cleaned. Sometimes the switch down condition may occur when the user removes a restrictive tool attachment requiring higher pressures and a higher power level even when in idle mode. The switch up condition may occur when a user applies the inlet of the device to a surface to be cleaned such that the device enters a cleaning mode where higher pressures are involved due to airflow restriction, requiring higher power. Sometimes the switch up condition may occur when the user attaches a restrictive tool to the device resulting in higher pressures to counter the increased airflow restriction, requiring a higher power level, even when in a non-cleaning or idle mode. Switching input power levels may result in changes of normalised delta-P, however for simplicity of this initial explanation these are not considered here but indicated with upward sloping sections of the curve 1705. Also for simplicity, input power level switch down events are illustrated as occurring immediately after falling below a corresponding switching threshold, however as noted above a switch down delay may be introduced to improve control stability and usability for a user. The lower plot also shows times TB1, TB2 when the baseline normalised Delta-P value changes. The baseline normalised Delta-P value may be reset when the measured or current normalised Delta-P value (from curve 1705) becomes stable over a predetermined period and is different from the current baseline normalised Delta-P value. For example, if the variance of the measured normalised Delta-P values is below a variance threshold for a predetermined stability period, for example 2.5 seconds, then the baseline normalised Delta-P value may be reset to the current normalised Delta-P, or a derivative of these values over the predetermined stability period, such as the average. In the upper plot, an input power curve 1720 shows the input power level over time, with changes corresponding to normalised Delta-P curve 1705 passing the thresholds 1710a - 171 Of as illustrated. Three fixed input power levels P1 - P3 are used for driving the motor, however different numbers of input power levels may be used. The number of input power levels used for each baseline normalised delta-P value may be the same or different. At time TO, the vacuum cleaner is running at power level P2, with a normalised Delta-P of approximately 3.0 kPa. For the purposes of this illustration, it will be assumed that this operating pressure value has remained stable for a period which corresponds to the vacuum cleaner having a low restriction tool attached and running in a non-cleaning or idle mode. It is further assumed that the baseline normalised Delta-P for the vacuum cleaner at this time is 3.0kPa. At T1, the normalised Delta-P has fallen to approximately 0.8 kPa and as the normalised Delta-P value has fallen below the lower threshold 1710a, the power level is reduced from P2 to P1, as can be seen in the upper plot. The lower normalised Delta-P value remains stable from T1 until time T2, which exceeds the predetermined stability period. In practice, some small variability of the normalised Delta-P may occur, but if the variance of the normalised Delta-P remains low (below a variance threshold) over the predetermined stability period, then the change of normalised Delta-P may be considered stable. When there is a stabilised change in normalised Delta-P, at time TB1, the baseline normalised Delta-P for the vacuum cleaner is reset. In this case, the baseline normalised Delta-P is reset from 3.0kPa to 0.8kPa, which corresponds to the vacuum cleaner free running, with no tool attached and in a non-cleaning or idle mode. The predetermined stability period corresponds to the difference between T1 and TB1, and may for example be 2.5 seconds. However other time periods may alternatively be used and other factors may also be considered such as the current normalised Delta-P and / or power level. In some examples, the predetermined stability period may be different for these different factors. Moving to a new baseline normalised Delta-P value may also result in a change of input power level to the lowest level, corresponding with the vacuum cleaner in an idle or non-cleaning mode with or without tools attached. In this case, the input power level has already dropped to P1 and so no further change in input power is required. However, in some examples where the input power level is higher than the lowest level, for example P2 or P3, the input power level may be forced down to the lowest level P1. At time T2, the normalised Delta-P begins increasing and reaches approximately 2kPa at time T3 a short time later. The normalised Delta-P remains at this level until T4 when it drops back to the previous free running level of 0.8kPa. This corresponds to a cleaning event where the user applies the inlet of the vacuum cleaner to a surface to be cleaned, causing a restriction in the airflow through the device. The normalised Delta-P does not exceed the switching threshold 1710b and so no change of input power occurs. At time T5, the normalised Delta-P rises to approximately 3.0kPa until time T6, when it again falls to 0.8kPa at time T7. Similarly, at time T8, the normalised Delta-P rises to approximately 3.0kPa until time T9, when it again falls to 0.8kPa. These two spikes in normalised Delta-P correspond to cleaning events where the user applies the inlet of the vacuum cleaner to a surface to be cleaned, causing a restriction in the airflow through the device. In these two cases, the normalised Delta-P does exceed the switching threshold 1710b and so there is a change of input power from P1 to P2, when the threshold 1710b is exceeded, and from P2 to P1 when the normalised Delta-P subsequently falls back below the threshold 1710b. These cleaning events may be different from the first cleaning event between T2 and T4, for example the first cleaning event may be cleaning a flat surface such as tiles whereas the second and third cleaning events may be cleaning carpet which results in greater restriction of the airflow and hence requires a higher input power level. At time T10, the normalised Delta-P rises to approximately 3.5kPa until time T11, when it rises to approximately 5kPa at time T11. As the lower normalised Delta-P is rising it again passes the threshold 1710b resulting in an increase in input power level from P1 to P2. The higher normalised Delta-P value of 5kPa also remains stable until time TB2, which exceeds the predetermined stability period. This results in the baseline normalised Delta-P value for the vacuum cleaner again being reset. In this case, the baseline normalised Delta-P is reset from 0.8kPa to 3.5kPa, which corresponds to the vacuum cleaner free running, with a new more restrictive tool attached and in a non-cleaning or idle mode. It is noted that the new baseline normalised Delta-P of 3.5kPa is slightly higher than 3.0kPa associated with time TO. This correspond to a slightly more restrictive tool being attached at time T10, compared with the tool attached at time TO. In some cases, resetting the baseline normalised Delta-P may also result in the input power being reset to the lowest level, as illustrated in this case. However some highly restrictive tool attachments may then force a subsequent change back to a higher power level P2 even when operating in an idle or non-cleaning mode. The change of baseline normalised delta-P value also changes the switching thresholds used from 1710a and 1710b to 1710c - 171 Of. At time T11, the normalised Delta-P value rises to approximately 5kPa and crosses switch up threshold 171 Od, resulting in an increase in power level from P1 to P2. This may correspond to the user applying the newly fitted tool attachment to a surface to be cleaned. At time T12 the normalised delta-P falls back to the new baseline normalised Delta-P of 3.5kPa, falling below the switch down threshold 1710c, resulting in a decrease in input power level from P2 to P1 and corresponding to the device again running in free air without restriction due to cleaning. As noted above, any pressure changes due to changes in input power level are ignored for this part of the discussion. However, appropriate setting of the switching thresholds avoids control instability due to triggering input power level switching due to normalised delta-P crossing a switching threshold simply due to power level changes rather than user activity such as cleaning or changing tool attachments. At time T13, the normalised Delta-P value rises to approximately 7kPa, crossing the first switch up threshold 1710e resulting in the input power level being increased from P1 to P2. Subsequently normalised delta-P also crosses the second switch up threshold 171 Of resulting in a further increase in input power level from P2 to P3. This may correspond with the start of a new cleaning event but this time with a more restrictive surface than the cleaning event between T11 and T12. For example the surface cleaned in the earlier event may have been tiles whilst the surface cleaned from T13 may have been carpet. Although not shown, were normalised delta-P to fall below threshold 1710e the power would reduce from P3 to P2 and then falling below threshold 1710c would result in the power being reduced from P2 to P1. As can be seen, when the normalised Delta-P value passes a threshold, the input power may be increased or decreased. When there is a stabilised change in the normalised Delta-P value, the baseline normalised Delta-P value for the vacuum cleaner may be reset. A change in the baseline normalised Delta-P value may then result in a change of switching thresholds. In an example, a switch up threshold corresponding to 1710b may be set at 300% of the current baseline normalised Delta-P value and the switch down threshold may be set at a lower value, such as 120% of the baseline value. This provides that the vacuum cleaner reverts to the lowest power level P1 when any tools are removed from vacuum cleaner and / or this is running in an idle mode. This is the case even if the previous cleaning operation resulted in some additional filter loading as this is unlikely to increase the lowest recent baseline normalised Delta-P value by more than 20% over a single cleaning operation. More generally, a switch up threshold may be calculated according to baseline * up-gain + up-offset and a switch-down threshold may be calculated according to baseline * down-gain + down-offset. Alternatively a different algorithm may be employed to calculate the switching thresholds. In other examples, different percentages may be employed, and these may be different for different levels of baseline normalised Delta-P value. The thresholds may also be calculated using other parameters than percentage increases, and may utilise algorithms derived from experimentation for example, in order to optimise automatic control of the vacuum cleaner. More than one switch up and switch down threshold may be provided for some or all baseline values. Different baseline values may correspond to different tools, however given changes in filter loading, characteristic baselines for a tool may change over time and may overlap with different tools at different filter loadings. Controlling the input power level in this way avoids the vacuum cleaner running at unnecessarily higher power levels, for example when not cleaning and also when not required for lower restriction or no tools. However, the higher power levels are automatically available when needed for cleaning and / or restrictive tool attachments. This approach optimises battery charge extending the cleaning time of the vacuum cleaner. The life of the device itself is also extended by avoiding unnecessarily running at high power levels. This approach also enables control of the vacuum cleaner to be adapted to changes in filter loading overtime. In an example, the baseline normalised Delta-P value may be set when the vacuum cleaner is started. Assuming that the vacuum cleaner is started in a non-cleaning or free running mode, the loading on the device will be due to the filter and any tool that is attached. Subsequently, when the inlet of the vacuum cleaner is applied to a surface to clean, control of the input power to the motor is then based on the appropriate switching thresholds determined for the current baseline normalised Delta-P value and the measured normalised Delta-P values. The input power levels may then be adjusted as the user engages in cleaning activity as already described. In the event that the user starts the vacuum cleaner in a cleaning mode with the inlet restricted by application to a surface, the baseline normalised Delta-P value will initially be set to a higher level. However, once the user removes the inlet from the surface so that the vacuum cleaner is running in an idle mode, the baseline normalised Delta-P value will then be reset to a lower value consistent with this free running or non-cleaning mode. More appropriate switching thresholds will then be determined and the input power to the motor of the vacuum cleaner will be automatically control in response to changes in restriction due to cleaning activity and / or too attachment changes. In an example, an initial settling time after starting the device may be allowed to lapse before using the measured normalised Delta-P value to reset the baseline value. For example, the settling time may be 2.5 seconds after start-up of the vacuum cleaner. Therefore, as the filter loading increases over time, the baseline normalised Delta-P value associated with the vacuum cleaner in free running or idle mode for a given tool attachment will slowly increase over time as the filter becomes more clogged. When the filter is replaced or cleaned, the filter loading will be reduced resulting in the baseline normalised Delta-P value also being reduced for the vacuum cleaner in free running or idle mode for the same given tool attachment. The baseline normalised Delta-P value may be further reduced when any attached tool is removed so that the vacuum cleaner is running in free air in free running or idle mode. Therefore, the control of the vacuum cleaner will always adapt to the filter loading during the operation of the vacuum cleaner. Figure 18 shows a flow chart representation of an example method 1800 to control an input power of a motor of an air-moving device, such as a vacuum cleaner. This method may be used with the air moving device, motor assembly or components described with respect to Figures 1,2 and 12-15. The method 1800 comprises, at block 1802, performing a start-up settling delay in response to a user starting the device. An example settling delay is 2.5 seconds, however any suitable duration may alternatively be used. The settling delay allows time for the measured operating parameter, such as normalised delta-P, to reach a steady state following start up when the measured value will be at or close to an ambient value. The method comprises, at block 1804, determining an operating parameter value or values of a motor of the air moving device. This may be implemented by measuring an operating pressure or an airflow rate at a sensor of the air-moving device as previously described. In one example a normalised Delta-P is calculated using a second sensor measurement, and then used as the operating parameter. However, alternative operating parameters may be used, such as a normalised airflow rate measurement, a normalised operating pressure measurement without comparing this to a second sensor measurement, or any other suitable metric. A plurality of operating parameter values may be determined over a time period and used to generate an operating parameter value for use in the method, for example by running a moving average on these values. The method comprises, at block 1806, setting an initial baseline operating parameter value. This may be implemented by setting the baseline value as the determined operating parameter value derived from block 1804. As previously described, the baseline operating parameter value will typically correspond to a non-cleaning mode in which the device is running in free air, with or without an attachment tool. This initial baseline will also automatically accommodate changes in filter loading over time, with a heavily loaded or clogged filter corresponding to a higher baseline than a recently replaced or cleaned filter, other influences such as operating mode (cleaning or non-cleaning) and tool attachment remaining equal. Similarly, when a restrictive tool attachment is fitted to the device at start-up, the initial baseline value will be higher than the baseline when a less restrictive or no tool attachment is fitted. The method comprises, at block 1808, calculating switching thresholds for the initial baseline operating parameter value determined in block 1806. This may be calculated as previously described, for example setting power switching thresholds at 200% (switch down) and 300% (switch up) of the baseline operating parameter value. Different calculations based on the baseline operating parameter value may alternatively be used. Example switching thresholds 1710a - 171 Of are described with respect to Figure 17. The switching thresholds correspond to operating parameter values at which the power input to the motor increases and / or decreases. The switching thresholds may be different for different baseline operating parameter values. The method comprises, at block 1810, determining an operating parameter value or values of a motor of the air moving device. This may be implemented in the same way as block 1804, and the operating parameter value determined may be referred to herein as the first value. This first value may be regularly updated by continuous or regularly sampling one or more sensors and from these measurements determining the first value at regular time intervals. As previously described, in one example, the first value or operating parameter value may be normalised Delta-P. At block 1812, the method determines whether there is currently a blanking delay condition in effect, for example by checking a blanking flag controlled by a timer set when a new baseline value is set. Setting a new baseline value is described in more below with respect to Figure 19, however when the baseline value is changed a blanking period may be started during which no other control changes are performed in order to improve control stability. Such other changes may include changing power levels or again changing baseline value. Changing the baseline value may result in changes to switching thresholds, the power level, and transient changes to the operating parameter until a new equilibrium is reached. A suitable blanking period may be 2.5s. If there is a current blanking condition (Y), the method returns to block 1810 and the operating parameter is updated, until the blanking condition expires. If there is no current blanking condition (N), the method moves to block 1814. The method also moves to a parallel control loop B to determine whether the baseline value should be updated. This is described in more detail below with respect to Figure 19. At block 1814, the method determines whether the first value or operating parameter is above a switch up threshold. If the first value is not above the switch up threshold (N), the method moves to block 1818. If the first value is above the threshold (Y), the method moves to block 1816 where the current input power level to the motor is increased to the next highest level. This may correspond to the situations show in Figure 17 following times T5, T8 and T13. The method then returns to block 1810. In some examples, more than one switch up threshold may be employed resulting in moving between multiple input power levels. At block 1818, the method determines whether the first value or operating parameter is below a switch down threshold. If this is not the case (N), the method returns to block 1810. If the first value is below the switch down threshold, the method moves to block 1820. In some examples, more than one switch down threshold may be employed resulting in moving between multiple input power levels. At block 1820, the method determines whether there is a current switch down delay condition in effect, for example by checking a switch down flag controlled by a timer set following a previous switch down event in which the input power level of the motor is reduced. Adding a switch down delay ensures that the device does not switch down too quickly when the user in cleaning, for example between application of an inlet of the device to a surface subsequent brief removal then application to an adjacent or nearby surface. Switching down during this brief period between cleaning operations may be frustrating for a user as the device will lose power and suction before application to the second surface. Incorporating a switch down delay allows the device to maintain a power level suitable for cleaning over an extended period during which the inlet of the device may be temporally removed from a surface prior to application to a surface again. A suitable switch down delay may be 2.5 seconds. If a switch down delay condition is in effect (Y), the method moves to block 1810 where the operating parameter is updated and the again checked against the switch down threshold. If there is no switch down delay in effect (N), the method moves to block 1822. This may correspond to the situations show in Figure 17 following times T1, T6, T9 and T12. Although as previously explained, the example of Figure 17 does not employ switch down delays. At block 1822, the method the current input power level to the motor is decreased to the next lowest level. A switch down delay condition is also set. The method then returns to block 1810. In some examples, more than one switch down threshold may be employed resulting in moving between multiple input power levels. The method 1800 sets the initial baseline operating parameter value and switching thresholds of an air moving device and then controls the input power level to the device based on whether the current determined operating parameter exceeds or falls below respective switch up and switch down thresholds. This approach controls the power of the device based on the relative difference between the sampled or current operating parameter and the baseline operating parameter. This provides a stable control mechanism which automatically changes the power level depending on the demands placed on the device by the user, for example applying an inlet of the device to a surface for cleaning requiring increased power and then removing the inlet from the surface to allow the device to run in free air, requiring less power. Avoiding running the device with unnecessarily high power reduces battery discharge resulting in more operating time for the device between charging. As the baseline is reset each time the device is started, the control of the device accommodates changes in filter loading over multiple operational cycles when the filter is not changed or cleaned. The initial baseline is also appropriate for whatever tool attachment is fitted to the device upon start-up, and the power level control automatically adjusts for this because the switching thresholds are dependent on the baseline value. The baseline may also be updated during an operational cycle between start-up and power down of the device. A method of updating the baseline value is described below with respect to Figure 19. Figure 19 shows a flow chart representation of an example method 1900 of updating the baseline operating parameter value for an air-moving device, such as a vacuum cleaner. This method may be used with the air moving device, motor assembly or components described with respect to Figures 1,2 and 12-15. The method 1900 comprises, at block 1902, calculating a variance of a determined operating parameter such as normalised delta-P. This may be implemented from B in the method 1800 of Figure 18, after block 1812 when it has been determined that there is no blanking condition in place. However, the method 1900 may be employed in conjunction with other input power control methods. The variance corresponds to a stability metric of the operating parameter value over time. The variance may be calculated in different ways, for example the amount of variation of the determined operating parameter value over a stability period. The stability period may be any suitable time period, such as 2.5 seconds for example. Over this period the variation may be 10%, meaning that the determined operating parameter values have varied by 10%, for example between 3kPa and 3.3kPa. Other stability periods or variance calculations may alternatively be used. At block 1904, the method determines whether the calculated variance is within a variance threshold. In one example, the variance threshold may be 5%, however other threshold values or comparisons may alternatively be used. If the calculated variance is within the variance threshold (Y), the method moves to block 1908. If the calculated variance is not within the variance threshold (N) the method may, depending on configuration, return to update the operating parameter, for example by returning to block 1810 in method 1800. In another example, the method may move to block 1906. Examples of the variance threshold being met are illustrated in Figure 17 corresponding to times TB1 and TB2. At TB1, the normalised Delta-P has dropped from 3kPa to 0.8kPa, and has remained stable for the required predetermined stability period. The baseline operating parameter may then be reset to the new settled operating parameter (0.8kPa) as described in more detail below. Similarly at TB2, the normalised Delta-P has increased to 3.5kPa from 0.8kPa, and has remained stable for the required predetermined stability period. The baseline operating parameter may then be reset to the new settled operating parameter (3.5kPa). At block 1906, the method determines whether the determined operating pressure is currently at or below the initial or lowest baseline value during the current operational cycle - between start up and power off of the device. If this situation occurs, that would be equivalent to the device having been started in a cleaning mode and / or with a tool attachment and now not being in a cleaning mode and / or having a lower restriction or no tool attachment. If this is the case, then the baseline may be quickly reset without having to wait for the variance threshold to be met. This provides faster initial stabilisation of the device when started in unusual situations such as with the inlet applied against a surface in an initial cleaning mode. If the determined operating pressure is the lowest (Y), the method moves to block 1908. If the determined operating pressure is not the lowest (N), the method returns to update the operating parameter, for example by returning to block 1810 in method 1800. In some examples, the method may be configured to only move to block 1908 if the determined operating pressure is sufficiently below a previous or initial operating pressure, to avoid unnecessary resetting of the baseline value. A very small difference between the current determined operating parameter and the lowest operating parameter may be due to sensor inaccuracy and so ignoring this may avoid unnecessary control changes which may reduce stability. Similarly, a minimum time period may be introduced for having the current determined operating pressure below the lowest operating pressure. At block 1908, the method resets the baseline operating parameter to the determined operating pressure. As described previously, a blanking period may be provided to prevent changes to the baseline operating parameter from occurring to frequently. For example, this may be implemented using block 1812 from method 1800. Depending on configuration, the method may then move to block 1910 or block 1912. At block 1910, the method reduces the input power level for the motor of the device to the lowest power level - for example P1 in Figure 17. As the baseline operating parameter corresponds to the device running in free air, it may be appropriate to reduce the device to the lowest power level. This may be the case irrespective of the tool attachment. However, in some situations with a very restrictive tool attachment, a higher power level may be required to enable free running or idle without stalling the device. Additional considerations may be incorporated to detect such situations, such as the operating parameter exceeding some threshold whilst being within the variance threshold. Downshifting the power level may prevent the baseline operating parameter from being set too high, for example following a power level switch from P3 down to P2 when the device is fitted with a somewhat restricted tool attachment but is running in free air. Switching down to the lowest power level P1 then allows the baseline to subsequently be reset lower following the blanking period and settling of the device. Similarly, as explained below resetting the baseline may result in changes to the switching thresholds and this may cause the device to switch up power level. Forcing the input power to the lowest level allows the device to settle whilst in the lowest power level before any further resetting of the baseline and / or any further switching of power levels. This improves overall stability and user experience. At block 1912, the method calculates switching thresholds for the newly reset baseline operating parameter. The new switching thresholds will then be used for controlling changes in power level for the device, for example using blocks 1814 - 1822 in method 1800. The new switching thresholds may be different to those used with the previous baseline operating parameter value, or they could be the same. In an example, the switching thresholds may be calculated based on a percentage of the baseline operating parameter value, such as 200% and 300% as previously described. Other methods of calculating the switching thresholds could alternatively be used. In some examples, different baselines may use different numbers of switching thresholds, for example a baseline corresponding to a free running device without any tool attachment may only use two power levels whereas a higher baseline corresponding to a restrictive tool attachment may use three power levels. At block 1914, the method starts a blanking period before returning to determine the operating parameter again, for example by returning to block 1810. In one example the blanking period may be 2.5 seconds, however other periods could alternatively be used. The use of a blanking period prevents the baseline operating parameter value from being reset too quickly in order to provide more stable operational control of the air moving device. The approaches described with respect to Figures 16 to 19 may be considered a relative operating parameter value approach in which a current value of the operating parameter value is compared relative to a baseline operating parameter value in order to determine whether to change the input power level to the motor of the air moving device as well as to determine whether to change the baseline operating parameter value. As previously described, the operating parameter value may be an operating pressure based value such as normalised Delta-P or an airflow rate based value. In some examples, this approach may be implemented using a single operating pressure sensor, such as a single pressure or airflow rate sensor. These approaches allow automatic power control of an air moving device, such as a vacuum cleaner. For example, the motor of the device may automatically switch up power when the user is performing heavier cleaning tasks and switch down power when the device is running in free air. Filter loading is automatically accounted for, in an example by resetting the baseline operating parameter value upon starting the device. This spares the user from having to constantly manually control the device power, and also reduces unnecessarily high power modes which would otherwise reduce the battery charge and hence operation duration of the device. Whilst air moving devices employing these approaches do not also need to include the approaches discussed with respect to Figures 3 to 11B, in some examples, both approaches may advantageously be used together. For example, the different approaches may be used for controlling different power level transitions. In one example, the approach described with respect to Fig. 16 may be used for the initial power level transitions between free air and contact with a surface (e.g. between P1 and P2 in Fig. 17) and the approach described with respect to Fig. 3 may be used for transitions between higher power levels (e.g. between P2 and P3 in Fig. 17), using a specific equivalent diameter. This allows the system to continue to dynamically update the baseline as the filter loads during use and avoids the need to algorithmically account for any estimation of the additional restriction caused by loading the filter. The system simply measures the baseline as the motor turns on and updates the thresholds for switching. This ensures a consistent switching experience for the user between free air and in contact with a cleaning surface, irrespective of the filter loading. The use of the other method (Fig. 3) for switching between P2 (medium) to P3 (max) is that the estimation of the inlet equivalent diameter has a decreasing error as the restriction increases. So this means that for low restrictions (i.e. free air with a large equivalent diameter opening passive tool) the error band is large for the inlet estimation and hence less acceptable to use to control power. As the restriction gets small, we can better estimate the inlet restriction with acceptable error to ensure that above a certain restriction we can consistently switch up to the highest power. This can improve the reliability and consistency of providing maximum power at a fixed equivalent orifice diameter. In some examples using more than one sensor, the use of the method of Fig. 3 may provide greater system control for responding to a greater variety of conditions and user activities. The above embodiments are to be understood as illustrative examples of the invention. Other embodiments are envisaged. It is to be understood that any 5 feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the 10 invention, which is defined in the accompanying claims.

Claims

1. A method to control an input power of a motor of an air-moving device, the method comprising:performing a measurement process to determine a first value of an operating parameter of the motor, wherein the operating parameter is an operating pressure of the motor or an airflow rate through the motor;performing a determination process to determine, based on a previous value of the operating parameter of the motor, a baseline value of the operating parameter of the motor;controlling the input power of the motor, based on the first value of the operating parameter of the motor and the baseline value of the operating parameter of the motor.

2. The method of claim 1, wherein the baseline value of the operating parameter of the motor is determined responsive to a predetermined variance in a plurality of previous values of the operating parameter of the motor.

3. The method of claim 2. wherein the predetermined variance corresponds to a threshold range of previous values of the operating parameter of the motor for a predetermined period.

4. The method of any one of claims 1 to 3, comprising performing a determination process to determine, based on the baseline value of the operating parameter of the motor, one or more threshold values of the operating parameter of the motor; and wherein controlling the input power of the motor is based on the one or more threshold values of the operating parameter of the motor.

5. The method of claim 4, wherein controlling the input power to the motor comprises reducing the input power responsive to the first value falling below asaid threshold value or increasing the input power responsive to the first value exceeding a said threshold value.

6. The method of claim 5, wherein the threshold values comprise an upper threshold value and a lower threshold value and wherein controlling the input power to the motor comprises reducing the input power responsive to the first value falling below the lower threshold value and increasing the input power responsive to the first value exceeding the upper threshold value.

7. The method of claim 5 or 6, comprising preventing the reducing or increasing of input power until a predetermined period after a previous reducing or increasing of input power.

8. The method of any one of claims 1 to 7, wherein controlling the motor is additionally dependent on a current input power of the motor.

9. The method of any one of claims 1 to 7, wherein the first value corresponds to a predetermined time after applying input power to the motor.

10. The method of any one of claims 1 to 8, comprising updating the baseline value of the operating parameter using the determined first value of the operating parameter.

11. The method of any one of claims 1 to 10, comprising:performing a determination process to determine, based on the first value of the operating parameter of the motor and a first pre-determined relationship between values of the operating parameter of the motor and values of an inlet restriction of the air-moving device, a value of the inlet restriction of the air-moving device; andcontrolling, based on the determined value of the inlet restriction, the input power of the motor.

12. The method of claim 11, wherein the operating parameter is the operating pressure and the measurement process comprises determining the first value of the operating parameter of the motor based on:an ambient pressure measurement; anda motor-inlet pressure measurement during operation of the motor.

13. The method of claim 12, wherein the ambient pressure measurement and the motor-inlet pressure measurement are measured at different times by a single pressure sensor.

14. The method of claim 11, wherein the measurement process comprises determining the first value of the operating parameter of the motor based on:a first pressure measurement of a pressure at a first position in a motor assembly in which the motor is located; anda second pressure measurement of a pressure at a second position in the motor assembly;wherein the second position is downstream of the first position.

15. The method of any of claims 11 to 14, wherein the first pre-determined relationship relates values of the operating parameter of the motor and values of one or more further parameters to values of the inlet restriction of the air-moving device, and wherein the determination process comprises:determining the value of the inlet restriction based on one or more respective further parameter values of the one or more parameters.

16. The method of claim 15, wherein the one or more further parameters comprise one or more of:an ambient pressure;an ambient temperature;a motor input power; anda build tolerance of the air-moving device.

17. The method of claim 15 or claim 16, wherein the determination process comprises:determining a first normalised value of the operating parameter by normalising the first value of the operating parameter by use of one or more respective values of the one or more further parameters; anddetermining the value of the inlet restriction of the air-moving device based on the normalised operating parameter.

18. The method of any of claims 15 to 17, wherein the one or more further parameters comprise a value of a filter loading of a filter of the air-moving device.

19. The method of claim 18, wherein the determination process comprises determining the value of the filter loading.

20. The method of claim 19, wherein the determining the value of the filter loading comprises:performing a second measurement process comprising determining a first value of a second operating parameter of the air-moving device, the first value being a value of the second operating parameter when the air-moving device is operating with a first inlet restriction condition; andperforming a second determination process to determine the value of the filter loading, the determination process comprising determining, based on the first value of the second operating parameter and a second pre-determined relationship relating, for the air-moving device when operating with the first inlet restriction condition, values of the second operating parameter to values of the filter loading, the value of the filter loading.

21. The method of claim 20, wherein the second operating parameter is: the operating pressure of the motor of the air-moving device;a speed of the motor of the air-moving device; oran airflow rate through the motor of the air-moving device.

22. The method of claim 20 or claim 21, wherein the determining the first value of the second operating parameter comprises:determining a plurality of values of the second operating parameter;determining a distribution of the plurality of values of the second operating parameter;determining a first property of the distribution; anddetermining, based on the first property of the distribution, the first value of the second operating parameter.

23. The method of claim 22, wherein the first property of the distribution is a minimum value in the distribution.

24. The method of any claims 11 to 23, wherein the controlling comprises adjusting the motor input power according to a set profile relating motor input power values to inlet restriction values.

25. The method of claim 24, wherein the set profile is a continuous profile.

26. The method of claim 24, wherein the set profile comprises a plurality of discrete power levels, each of the power levels corresponding to a respective range of values of inlet restriction.

27. The method of any of claims 11 to 26, comprising, during operation of the air-moving device, performing the measurement process, the determination process and the controlling a plurality of times.

28. A set of machine-readable instructions which when executed by a processor of an air-moving device cause the air-moving device to perform a method according to any of claim 1 to claim 27.5 29. An air-moving device comprising:a processor; anda 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 27.1030. The air-moving device of claim 29, wherein the air-moving device is a vacuum cleaner.15

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