Estimating an ambient temperature relating to an environment surrounding an air-moving device

Abstract

A method of estimating an ambient temperature relating to an environment surrounding an air-moving device, such as a vacuum cleaner, is described. The method comprises: determining, using a sensor con

Description

BACKGROUND Air-moving devices, such as vacuum cleaners, may employ one or more control methods. An example control method may improve user experience by automatically optimising a balance of battery’ run-time and suction power. For the robust, consistent and reliable operation of such control methods, ambient, environmental, and operational factors may be taken into account. One such factor is the ambient temperature, that is, the temperature of the environment surrounding the air-moving device. The ambient temperature may, for example, have an impact on a speed of a motor of the air-moving device when the motor is being driven with a given power, as well as on pressures arising within the device during its operation. SUMMARY According to a first aspect of the invention, there is provided a method of estimating an ambient temperature relating to an environment surrounding an air-moving device, the method comprising: determining, using a temperature sensor configured to measure a temperature of a thermal environment of the air-moving device: a first value of the temperature of the thermal environment at a switch-off time of a motor of the device; and a second value of the temperature of the thermal environment at a second time, wherein the second time is at an end of a time interval beginning at a start time, and wherein the start time is a time at which the temperature of the thermal environment has reduced to the first value following an increase in temperature after the switch-off time; and performing a determination process to determine, based on the first value, the second value, the time interval and a pre-determined cooling characteristic of the thermal environment, an estimate of the ambient temperature. When the motor of the air-moving device is in operation, heat may be generated, for example, due to friction from air flowing through the device, mechanical movement of the motor, and resistive heating from electrical components of the device. Once the motor is switched off, these sources of heating may be removed or reduced, and the device may begin to cool towards a temperature of the immediate surroundings of the air-moving device, which may be referred to as the ambient temperature. The method may allow for the ambient temperature to be estimated based on measurements made by a temperature sensor configured to measure a temperature of a thermal environment of the device which may be different to the ambient temperature. This may, for example, allow for a temperature sensor which is used to monitor a temperature used for other control aspects of the device, for example, a temperature relating to the motor of the device, to be used to estimate the ambient temperature on start-up of the motor of the device or when the motor is not in operation. Accordingly, the method may allow the ambient temperature to be estimated without the use of an additional temperature sensor. This may provide for cost and weight savings and may, for example, reduce the complexity involved in assembling the device. The method may allow for a robust and accurate estimate of the ambient temperature to be determined. For example, by determining the start time as the time at which the temperature of the thermal environment returns to the first value, the estimate of the ambient temperature can take into account the effect of thermal soaking of the thermal environment which may result in an initial increase in the temperature of the thermal environment following switchoff of the device. Accordingly, the estimate of the ambient temperature may be more accurate and robust in circumstances where such thermal soaking occurs. The pre-determined cooling characteristic may be a pre-determined cooling time constant of the thermal environment. This may allow for the estimate of the ambient temperature to be determined based on an observed rate of cooling of the thermal environment and based on a pre-determined cooling constant which characterises the thermal environment. The cooling constant may, for example, be determined by measurement of the rate of cooling of the thermal environment under known conditions. Alternatively, the cooling constant may be determined by a simulation of cooling of the thermal environment. As another alternative, the cooling constant may be determined analytically, for example, by use of an analytical expression including values of one or more parameters characterising the thermal behaviour of the thermal environment, for example, specific heat capacity, mass, heat transfer coefficient and heat transfer surface area. The determination process may comprise determining the estimate of the ambient temperature by: performing a cooling-curve calculation to determine the estimate of the ambient temperature using the first value, the second value, the time interval and the predetermined cooling constant. This may allow for the ambient temperature to be estimated based on an expression relating an observed rate of cooling of the thermal environment to the ambient temperature. The cooling-curve calculation may comprise determining the estimate of the ambient temperature based on an expression representing an exponential cooling of the thermal environment. This may allow estimate of the ambient temperature to be determined by use of an expression which is based on a cooling curve for the thermal environment which represents an exponential decay of the temperature of die thermal environment towards the ambient temperature. By use of the pre-determined cooling constant, the observed rate of cooling can be related, using this exponential relationship, to the ambient temperature towards which the temperature of the thermal environment is decaying. The cooling-curve calculation may take the form: _no I ambient t 1 - where Tambient is the estimate of the ambient temperature, T (t) is the second value of the temperature of the thermal environment, To is the first value of the temperature of the thermal environment, t is the time interval between the start time and the second time, and t is the pre-determined cooling constant for the thermal environment. This equation may allow the estimate of the ambient temperature to be calculated based on the observed rate of cooling of the thermal environment from the first value to the second value and the pre-determined cooling constant for the thermal environment. The second time may be a time at which the motor of the air-moving device is switched on for a first time following the switch-off time. By determining the ambient temperature at a second time when the motor is switched on following a period of being switched off, an updated estimate of the ambient temperature can be provided at the time when the motor is switched back on. At the time at which the air-moving device is switched back on following the switch-off time, the temperature of the thermal environment of the device may remain at a temperature above the ambient temperature since sufficient time may not have passed since the switch-off time to allow the thermal environment to cool to the ambient temperature. The method may provide an estimate of the ambient temperature which can be used an input to a control system configured to control various aspects of the operation of the device, for example, the supply of power to drive the motor. The method may comprise: monitoring, using the temperature sensor, the temperature of the thermal environment following the switch-off time; and based on the monitoring, determining the start time as a time at which the temperature of the thermal environment reduces to the first value following the increase in temperature after the switch-off time. The time at which the temperature of the thermal environment returns to the first value following the increase in temperature after the switch-off time may be determined by monitoring die temperature of the thermal environment by use of the temperature sensor. This may provide an accurately determined start time to form an input to the determination process and therefore may allow for the determination process to provide an accurate and robust estimate of the ambient temperature. The temperature sensor may be configured to, when the motor is operating, measure a temperature relating to the motor of the air-moving device. The temperature sensor may, accordingly, provide a dual function of measuring the temperature of components of the motor assembly and / or air flowing through the motor when the motor is in operation and of allowing the estimate of the ambient temperature to be determined on start-up of the motor or during a period when the motor is switched off. The thermal environment may comprise an enclosure inside the air-moving device containing the temperature sensor. Accordingly, the estimate of the ambient temperature may be determined based on temperature measurements from an enclosure inside the device. These temperature measurements may be used, in addition to providing an input for determining the estimate of the ambient temperature, for monitoring and / or controlling various aspects of the operation of the device. The enclosure may be a housing of the motor of the air-moving device. For example, the temperature sensor may for example, be located inside the housing of the motor and configured to measure the temperature of certain components of the motor, or of air flowing through the motor, when the motor is in operation. According to a second aspect of the present invention, there is provided a set of machine-readable instructions which when executed by one or more processors cause the method according to the first aspect of the present invention to be performed. According to a third aspect of the present invention, there is provided an air-moving device comprising: a temperature sensor configured to measure a temperature of a thermal environment of the air-moving device; 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 aspect of the present invention. The air-moving device may be a vacuum cleaner. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram showing a perspective view of an example air-movmg device. Figure 2 is a schematic diagram showing a highly simplified cross-sectional representation of a motor assembly of the air-moving device of Figure 1. Figure 3 is a graph showing a first example temperature profile of athermal environment of the air-moving device. Figure 4 is a graph showing a second example temperature profile of athermal environment of the air-moving device. Figure 5 is flowchart diagram showing a first example method of estimating an ambient temperature of an air-moving device. Figure 6 is a flowchart diagram showing a second example method of estimating an ambient temperature of an air-moving device. Figure 7 is a schematic diagram showing aspects of an example usage session of an airmoving device. DETAILED DESCRIPTION Figure 1 shows an air-moving device 100. The air-moving device 100, in this example, is a vacuum cleaner. Hie vacuum cleaner 100 comprises an inlet tube 102 with a tool 104 attached to a distal end of the inlet tube 102. The tool 104 is for engaging with a surface to be cleaned by the vacuum cleaner 100 and comprises an air inlet (not shown) to the vacuum cleaner 100. The tool 104 may be active, comprising one or more mechanically operated components, for example, a rotating brush bar, to assist with cleaning tasks. Alternatively, the tool 104, 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 some examples, the inlet tube 102 or a portion thereof may be removable. In such examples, a tool, such as a passive tool, may be attached to the vacuum cleaner 100 when the inlet tube 102 or the portion thereof is removed. The vacuum cleaner 100 also comprises a dirt-separating chamber 106, which may, for example, be a cyclone chamber. The vacuum cleaner 100 comprises an outer housing 120 which contains a motor assembly 200 (see Figure 2). The vacuum cleaner 100 further comprises a processor 108 and a storage 110 for storing machine-readable instructions for execution by the processor 108 to control operation of components of the vacuum cleaner 100. In examples, the machine-readable instructions when executed may cause the processor 108 to carry out any of the example methods described herein or aspects of such methods. Figure 2 shows an example schematic representation of the motor assembly 200 of the vacuum cleaner 100. As mentioned above in the description of Figure 1, in use, the motor assembly 200 is contained within the housing 120 of the device 100. The outer housing 120 is omitted from Figure 2 for clarity. The motor assembly 200 comprises a set of coils 202, a shaft 204 with magnets (not shown) mounted thereon, bearings 206 and an impeller 208. The motor assembly 200 comprises motor air inlets 210, and air outlets / a diffuser 212. The motor assembly comprises a circuit board 214 on which are mounted sensors including a temperature sensor 216 and a pressure sensor 218. The motor assembly 200 comprises a motor housing 224 in which the other components are housed. The motor assembly 200 further comprises a pre-motor filter 226 for filtering air which is drawn into the motor in use. With reference to Figures 1 and 2, in use, the motor of the motor assembly 200 draws air through the air inlet of the device 100, and along an airflow path 128 which flows through the device 100 and exits the device through an exhaust 112. After being drawn into the device 100 through an air inlet (not shown) in the tool 104, the airflow path 128 continues through the inlet tube 102, then through the dirt-separating chamber 106, then through the motor assembly 200 and finally exits the device 100 through the exhaust 112. Returning to Figure 2, when the motor is in use in the device 100, an electric current is passed through the coils 202, in a manner which causes a varying magnetic field to be generated. This vaiying magnetic field acts on the magnets 206 on the shaft 204 to cause the shaft 204 to rotate about its longitudinal axis. This in turn rotates the impeller 208. Air, driven by the impeller 208, is drawn into the air-moving device 100 and along the airflow path 128. The airflow path 128 enters the motor assembly 200, passing through the pre-motor filter 226, which removes particulate matter from the air, and into the motor housing 224 through the air inlets 210 (shown, in Figure 2, as gaps in the motor housing 224). Hie airflow path 128 continues through the motor to the impeller 208 and, after passing over the impeller 208, exits the motor assembly 200 through the air outlets 212, and thereafter exits the device 100 through the exhaust 112. The device 100 may be configured to implement one or more control methods, for example, by the processor 108 executing machine-readable instructions stored on the storage 110. Such control methods may be used to control or monitor certain aspects of the operation of the device 100. For example, a control method may be implemented to control a power provided to drive the motor of the device 100. As another example, a control method may be implemented to monitor the device 100 to detect certain conditions, such as when a filter of the device 100 should be washed or changed. In some examples, based on such monitoring, a control method may include providing alerts to a user of the device 100. Various parameters of tire device 100 may be monitored, for example, for use as inputs to one or more control methods, such as those examples described above. Parameters which may be monitored may, for example, include a speed of rotation of the motor of the device 100, an operating pressure inside the motor assembly 200, for example, when the motor is in operation or when the motor is not in operation, and an operating temperature inside the motor assembly 200 again, for example, when the motor is in operation or when the motor is not in operation. Various sensors may be used to measure such parameters. For example, a motor-speed sensor (not shown) may be configured to measure a speed of rotation of the motor. The temperature sensor 216 and the pressure sensor 218 may be used to measure, respectively, a temperature and a pressure inside the motor assembly 200. One particular parameter which may be monitored is the ambient temperature, that is, the temperature of the environment surrounding the device 100. A value for the ambient temperature in which the device 100 is operating may, for example, be used an input to respective control methods for controlling the power supplied to the motor and for determining when a filter of the device 100 should be washed or changed. In the example air-moving device of Figures 1 and 2, an estimate of the ambient temperature can be determined based on temperature values measured by the temperature sensor 216 of a thermal environment of the device 100. The thermal environment includes the temperature sensor 216 and the immediate surroundings of the temperature sensor 216 which can be treated, for the purposes of modelling the thermal behaviour of the device 100, as forming a thermal system arranged such that it can exchange heat with the ambient environment. As discussed above, the temperature sensor 216 is inside of the motor assembly 200, which, in use, is enclosed in the outer housing 120 of the device 100. In this example, therefore, the thermal environment includes the temperature sensor 216 itself and may include further components of the device 100 which are in direct or indirect thermal contact with the temperature sensor 216. For example, the thermal environment may include the circuit board 214 to which the temperature sensor 216 is mounted and air which surrounds the temperature sensor 216. Figure 3 includes a graph showing a simplified schematic representation of a first example temperature profile of the thermal environment of the device 100. In broad overview, Figure 3 shows a temperature profile of the thermal environment of the device 100 over a period beginning with the motor of the device 100 in operation in which the motor is switched off and then switched back on again after a period of cooling. For example, the temperature profile may correspond with a scenario where a user is initially using the vacuum cleaner 100 to clean a surface, stops cleaning the surface for a period by switching of the motor, and after a period switches the motor back on again to resume cleaning. In Figure 3, prior to a time tswitch_o^, the motor of the device 100 is in operation. While the motor is in operation during this period, the thennal environment is heated due to the operation of the motor. For example, the thermal environment may be heated due to friction from movement of the motor and / or due to resistive losses from components configured to drive the motor. The temperature value measured by the temperature sensor 216 when the motor is in operation is therefore greater than the ambient temperature Tamhient. In the simplified temperature profile of Figure 3, the temperature in the period immediately prior to tSWitch_Of f is a constant temperature To. In some examples, the temperature during this period may remain substantially constant while the motor is in operation since, although heat is being generated by the operation of the motor, the airflow through the motor carries away heat such that a substantially steady state may be maintained. In the example of Figure 3, prior to the period shown in the figure, the temperature may have increased from a lower value before reaching a steady state at the temperature To. However, it should be noted that, in other examples, the temperature in the period immediately prior to tswitch_0^ may not be constant. For example, the temperature may still be increasing when the motor is switched off at tswitch-off- At the motor of the device 100 is switched off. Hie temperature of the thermal environment at tswitch_0^, as measured by the temperature sensor 216, is To. With the motor of the device 100 being switched off, the source of heating which was present when the motor was in operation is removed. Beginning at tswitch_Off the temperature of the thermal environment decays towards the ambient temperature Tambient as heat from the thermal environment is lost to the ambient environment. The temperature of the thermal environment following switch-off of the motor may be modelled as an exponential decay towards an asymptote at the ambient temperature Tambient. The thermal environment continues to cool towards the ambient temperature Tambient over a time interval At. The time interval At ends at a time at which the motor is switched back on and heating of the thermal environment begins again. Hie profile which the temperature of the thermal environment could have been expected to take if the motor were not switched back on at is represented in Figure 3 by the dashed line which represents a continued exponential decay towards Tambient. The temperature of the thermal environment during the time interval At can be expressed as: T(t) T’am&ient + (To ^ambient) ' ® T Equation 1 where T(t) is the temperature at a time t. defined such that tSwitch^off = 0; is the temperature of the thennal environment at tswitch_0^; and t is a cooling constant for the thermal environment. Equation 1 can be rearranged to provide the following expression for the ambient temperature Tamhient: t ^ambient = T Equation 2 1-e t Figure 4 is a graph showing a simplified schematic representation of a second example temperature profile of the thermal environment of the device 100. This model accounts for an effect which may occur in certain examples in which tire temperature measured by the temperature sensor 216 increases following the switch-off time. Such an effect may occur, for example, at high motor powers at which large amounts of heat are generated. In such cases, while the motor is in operation, the heat may be effectively carried away from the thennal environment by the higher airflow which is produced by the high motor power. Accordingly, as in the example of Figure 3, the temperature while the motor is in operation prior to the time tswltch^Off may remain substantially constant at T^, although TQ in this example may be higher than in the example of Figure 3 due to the increased heating from the higher motor power. In other examples, as discussed above in relation to Figure 3, the temperature immediately prior to switch-off may not be constant. For example, the temperature may still be increasing at the switch-off time due to the thermal environment not having yet reached a steady-state temperature. As in the example of Figure 3, the motor is switched off at a time tswuch-off, which time heating of the thennal environment by the operation of the motor stops. At the same time, the airflow generated by the operation of the motor stops such that heat is no longer effectively removed from the motor assembly 200. In the example shown in Figure 4, this results in thermal soaking of the temperature sensor 216 which may occur, for example, via heat being conducted from the circuit board 214 into the temperature sensor 216. Accordingly, as shown in Figure 4, the temperature of the thermal environment increases for a period following switch-off of the motor. A short time after the switch-off time tswitch_ojf, the thermal soaking effect begins to abate, such that, at a particular time after the switch-off time tswitch-off^ ^1C temperature of the thermal environment reaches a maximum temperature. The temperature then begins to decrease. At a time treturn, the temperature of the thennal environment has returned to the temperature at switch-off To. In the time interval At between the time treturn and a time tt at which the motor is switched back on, the same expression, Equation 1, set out above in relation to Figure 3 may be used to represent the temperature of the thermal environment as it exponentially cools towards the ambient temperature Tambient. As in the example of Figure 3, the thermal environment continues to cool until a time tA at which the motor is switched back on at which point the temperature of the thermal environment begins to increase once more. Figure 5 is a flowchart diagram showing an example method 500 of estimating an ambient temperature relating to an environment surrounding the air-moving device 100. By way of example with reference to Figure 4, the method 500 may allow an estimate of the ambient temperature Tamhient to be determined based on temperature measurements measured by the temperature sensor 216. The method 500 comprises, at block 502, determining, using the temperature sensor 216 configured to measure a temperature of the thermal environment of the air-moving device 100: a first value of the temperature of the thermal environment at a switch-off time of a motor of the device 100. By way of example with reference to Figure 4, block 502 may include measuring, using the temperature sensor 216, the temperature To of the thermal environment at the switch-off time tswitch-off of the motor of the device 100. At block 504, the method 500 comprises determining, using the temperature sensor 216, a second value of the temperature of tire thermal environment at a second time. The second time is at an end of a time interval beginning at a start time, and the start time is a time at which the temperature of the thermal environment has reduced to the first value following an increase in temperature after the switch-off time. By way of example with reference to Figure 4, the start time may be determined by measuring, using the temperature sensor 216, the time at which the temperature of the thermal environment returns to the temperature To following the switch-off time tswitch_Off. This time is denoted treturn in Figure 4. For example, the temperature of the thermal environment as measured by the temperature sensor 216 may be constantly monitored, for example, at a particular frequency, such as 1Hz. A time at which the motor is switched-off at tswitch_Off may be recorded. Hie temperature measured by the temperature sensor 216 may be monitored following the switch-off time tswitch_Off, and the time treturn at which the temperature returns to the switch-off temperature To following tswitch_Off may be recorded and set as the start time. Block 504 may then include measuring, using the temperature sensor 216, a temperature of the thermal environment at a second time after the start time. The second time may be any time before or at the time t\ when the motor is switched back on. At block 506, the method 500 comprises performing a determination process to determine, based on the first value, the second value, the time interval and a pre-determined cooling characteristic of the thermal environment, an estimate of the ambient temperature. By way of example with reference to Figure 4, block 506 may include applying the expression set out above for the ambient temperature to determine the estimate of the ambient temperature. That is, Equation 2, above, may be used to determine an estimate of the ambient temperature Tambient. The terms T(t) and To in Equation 2 may be measured by the temperature sensor 216, as described above, where t is the time interval which has elapsed, at the time when the estimate of the ambient temperature is being made, since the start time. The cooling constant t of the thermal environment is pre-determined by a suitable method. For example, the cooling constant r may be measured by measuring a temperature profile of the thermal environment when cooling under conditions where the ambient temperature is known. Tire cooling constant t may then be determined by fitting a curve to the temperature profile, for example, assuming an exponential decay profile in accordance with the expressions set out above. Alternatively, the cooling constant r may be determined analytically. For example, the cooling constant t may be determined according to the following expression: r = — Equation where c is the specific heat capacity of the thermal environment in units of J / (K*kg), m is the mass of the thermal environment in units of kg, h is the heat transfer coefficient of the thermal environment in units of W / (m2*K), and A is tire heat transfer surface area between the thermal environment and the ambient environment in units of nf. As described above with reference to Figures 3 and 4, T(t) is a temperature of the thermal environment as measured by the temperature sensor 216 at a time t in the time interval At over which the thermal environment is cooling towards the ambient temperature. In one particular example, T(t) is a temperature which is the temperature of the thermal environment at the time t1 at which the motor of the device 100 is switched back on following the switch-off time tswitch_Off. This may provide an estimate of the ambient temperature at the switch-on time which may, for example, be used in a method for controlling the power to supply to the motor and / or for one or more methods monitoring the operation of the device 100 while the motor is running. The ambient temperature may, additionally, or alternatively, be estimated at any given time in the time interval At before the motor is switched back on by inputting into the above expression a measured value for T(t) at the given time t. For example, the ambient temperature may be estimated at a given pre-determined interval following the start time, rather than, or in addition to, at a time at which the motor is switched-on. For example, the ambient temperature may be estimated after one cooling constant t has elapsed following the start time. According to the following expression, the temperature of the thermal environment after one cooling constant can be estimated to have dropped by 63.2% of the difference between Tb and Tambient. Tambient = % „,,,., Equation 4 U.O5Z By measuring the temperature of the thermal environment at one cooling constant after the start time, Equation 4 can be used to determine an estimate of the ambient temperature T'ambient- this way, the estimate may be determined in a computationally inexpensive manner since none of tire terms in the expression include the computation of an exponential function. At the time when an estimate of the ambient temperature is to be made, the temperature of the thermal environment may remain above the ambient temperature, for example, due to the heat capacity of the temperature sensor 216 itself, the circuit board 214 and the surrounding enclosure. If, for example, such a temperature measurement was used directly as an estimate of the ambient temperature, then this would provide an incorrect estimate. Inputting such an incorrect estimate into a control method, for example, may result in a degradation in the operation of the control method. Example methods described herein may allow for an accurate estimate of the ambient temperature to be provided taking into account factors such as those described above. By using values determined by a temperature sensor in a thermal environment of the device to estimate the ambient temperature, the estimate of the ambient temperature may be obtained without the need for an additional temperature sensor. This may provide for cost and weight savings and reduced complexity in assembling the device 100. As an example, the temperature sensor 216 in the example device 100 may be used to obtain estimates of the ambient temperature when the motor is not in operation, for example, on start-up of the motor, while also being used when the motor is in operation to provide temperature measurements which may be relevant to the operation of the motor. For example, the temperature sensor 216 may be used to monitor the temperature of components of the motor or of air flowing into the motor when the motor is in operation. Such measurements may, for example, be used to allow potential overheating in the motor assembly 200 to be detected. Moreover, the method 500 may allow for an accurate and robust estimate of the ambient temperature to be obtained even in the case where the temperature of the thermal environment increases following the switch-off time of the motor, for example, due to thermal soaking of the type described above. By performing the cooling calculation with a start point for the modelled exponential cooling of thermal environment being set as a time at which the temperature has returned to the switch-off temperature, the effect of thermal soaking is accounted for in the calculation of the ambient temperature. For example, if the cooling calculation was instead performed assuming that the thermal environment begins to cool exponentially from the switch-off time, then an incorrect result for the ambient temperature may be obtained in the case where thermal soaking of the type described above occurs. Figure 6 is a flowchart showing another example method 600 of determining an estimate of the ambient temperature relating to the environment of the air-moving device 100. The method 600 comprises, at block 602, determining a first value of a temperature of the thermal environment of the air-moving device 100 at a switch-off time of a motor of the device 100. Block 602 may comprise any of the features described above with reference to block 502. By way of example with reference to Figure 3 or 4, block 602 may include measuring, using the temperature sensor 216, the temperature To of the thermal environment at the switch-off time tSWitch^off °f the motor of the device 100. At block 604, the method 600 comprises determining a second value of the temperature of the thermal environment at a second time at an end of a time interval beginning at a start time, the start time being at or later than the switch-off time. At block 606, the method 600 comprises selecting a determination process from a plurality of determination processes for determining an estimate of the ambient temperature at the second time. The selecting is based on one or more pre-determined criteria relating to the first value and the second value and / or the time interval. Hie plurality of determination processes comprises a first determination process comprising determining the estimate of the ambient temperature at the second time based on the first value, the second value, the time interval and a pre-determined cooling characteristic of the thermal environment. At block 608, the method 600 comprises performing the selected determination process to determine the estimate of the ambient temperature. The first determination process may have any of the features described above with reference to Figure 5. By way of example, with reference to Figure 3 and Figure 4, the first determination process may include determining an estimate of the ambient temperature Tambient >n a manner as described above based on Equation 2. The first determination process may include setting the start time. For example, if the temperature of the thermal environment is determined to be rising following the switch-off time tswitch_Off, as in the example shown in Figure 4, the start time may be set as the time ^return at which the temperature has returned to the switch-off temperature To. That is, thc cooling-curve calculation may be performed assuming an exponential decrease in the temperature of the thermal environment beginning from the time treturn, as in the example method 500. Alternatively, if the temperature of the thermal environment is determined to be falling following the switch-off time tswuCh^off^ as 111 the example shown in Figure 3, then the switch-off time tSWitch-off itself may be set as the start time at block 504. That is, the cooling-curve calculation may be performed assuming an exponential decrease in temperature of the thermal environment beginning at the switch-off time tswitch_0^. The one or more pre-determined criteria on which the selecting of a determination process is based may include a first pre-determined criterion that a magnitude of a decrease in temperature from the first value to the second value is greater than or equal to a predetermined amount. For example, in the examples of Figures 3 and Figure 4, at a time when the ambient temperature Tambient is to be determined, for the first pre-determined criterion to be satisfied, a minimum temperature drop from the switch-off temperature To must have occurred. For example, if the ambient temperature is to be estimated at the switch-on time t15 then, for the first pre-determined criterion to be satisfied, the temperature drop Tdrop over the time interval At must be greater than a pre-determined minimum temperature drop. By way of example, the pre-determined minimum temperature drop may be a proportion of the temperature value To, for example, as measured in degrees Celsius. In one example, the predetermined minimum temperature drop may be around 10% of the value of To in degrees Celsius. For example, if To is 50°C then the minimum temperature drop may be around 5°C. The one or more pre-determined criteria on which the selecting of a determination process is based may include a second pre-determined criterion that the time interval is less than a pre-determined time interval. The pre-determined time interval may be a time interval over which the temperature of the thermal environment would be expected to reduce to substantially the ambient temperature. For example, the second pre-determined criterion may be satisfied if, at the time an estimate of the ambient temperature is to be made, the time which has elapsed since the start time is such that the temperature of the thermal environment can be expected to remain above the ambient temperature. The pre-determined time interval may be defined in terms of a multiple of the cooling constant r. For example, the predetermined time interval may be equal to five or six times the cooling constant T. As an example, the cooling constant r may be on the order of tens of seconds to minutes, for example, from around 30 seconds to around 5 minutes. The pre-determined minimum temperature drop may occur over a time period which depends on the value of the cooling constant t. For example, a larger value for the cooling constant T will mean that a given minimum temperature drop will take longer to occur than if the cooling constant T had a smaller value. The time for the minimum temperature drop to occur may also depend on the ambient temperature Tambient towards which the thermal environment is cooling. For example, due to the exponential nature of the temperature decay, a larger difference between the switch-off temperature To and the ambient temperature Tambient result in a given minimum temperature drop occurring more quickly. Due to the exponential nature of the temperature decay, the rate at which the temperature drops will be greatest at the beginning of the cooling. Accordingly, the minimum temperature drop may occur relatively quickly. For example, depending on the difference between the switch-off temperature and the ambient temperature, the minimum temperature drop may occur over a time frame which is short compared with the cooling constant r. Purely as an example, when the cooling constant t is on the order of minutes, the minimum temperature drop may take on the order of tens of seconds to occur. In one example, the first determination process is selected if both the first pre-determined criterion and the second pre-determined criterion are satisfied. For example, with reference to Figures 3 and 4, if, when estimating the ambient temperature Tambient at the switch-on time firstly, the minimum temperature drop has occurred, and, secondly, the time interval At is less than the pre-determined time interval, then the first determination process may be applied to estimate the ambient temperature Tambient. As described above, the first determination process may include performing a cooling curve calculation based on Equation 2. By applying tire cooling calculation of Equation 2 on the condition that the minimum temperature drop has occurred, the reliability of the results of the cooling calculation may be improved. By applying the cooling calculation of Equation 2, further, on the condition that the time interval is less than a pre-determined time interval, the cooling curve calculation may be applied when insufficient time has passed for the thermal environment to have decayed to substantially the ambient temperature Tambient. If, on the other hand, the time which has elapsed is greater than the pre-determined time interval, then a second determination process may be selected. For example, since the predetermined time interval may be a time interval over which the temperature of the thermal environment can be expected to have cooled to the ambient temperature, for example around five or six times the cooling constant t, if the pre-determined time interval has elapsed, then the value measured by the temperature sensor 216 may be used directly as an estimate of the ambient temperature. If the first pre-determined criterion is not satisfied and the second pre-determined criterion is satisfied, then a third determination process may be selected. For example, if the minimum temperature drop has not occurred and the time interval is less than the pre-determined time interval, then the third determination process may be selected. In examples, the third determination process may include determining the estimate of the ambient temperature based on an estimate of the ambient temperature obtained at an earlier time. This may allow a previous estimate of the ambient temperature to be used to estimate the ambient temperature at a time when the minimum temperature drop for applying the cooling curve calculation has not occurred and the time elapsed since the start time is not long enough for the temperature of the thermal environment to have cooled to the ambient temperature. Figure 7 is a schematic diagram illustrating aspects of operation of the device 100 when performing an example of the method 600 to estimate the ambient temperature. Namely, Figure 7 represents a usage session of the device 100, showing, with time increasing in the horizontal direction from left to right, periods during which the motor of the device 100 is operating (denoted “ON”) and periods during which the motor of the device 100 is not operating (denoted “OFF”). In this example, an estimate of the ambient temperature is obtained each time the motor of the device 100 is switched on. Figure 7 shows a pre-determined time interval t0^_min which is used as part of the process of selecting which determination process is to be used to estimate the ambient temperature. Tn this example, is a time interval for the thermal environment to cool to the ambient temperature from the switch-off temperature. As in examples described above, tOff_min, may, for example, be around five times the cooling constant for the thermal environment. It should be noted that in the example of Figure 7, for the sake of simplicity, an example is considered where the temperature profile of the thermal environment upon switch-off follows a profile such as that shown in Figure 3, such that the start time is always equal to the switch-off time tswttCh^off- However, the principles described with respect to Figure 7 also apply in examples such as is shown in Figure 4, wherein the temperature of the thermal environment increases following the switch-off time and the start time is set as a time treturn later than the switch-off time. In Figure 7. prior to a time tA, the motor of the device 100 is switched off. At time tA, the motor is switched on. Figure 7 shows that at the time tA, the motor has been switched off for longer than the pre-determined time interval The temperature measured by the temperature sensor 216 at tA can therefore be expected to be substantially equal to the ambient temperature Tambient. Accordingly, the estimate of the ambient temperature at tA is obtained by directly using the value measured by the temperature sensor 216 at tA as an estimate of the ambient temperature. At time tB, the motor is switched off again. The motor remains switched off until tc. at which time it is switched back on again. At tc, as is the case at tA, it can be seen that the motor has been switched off for longer than the pre-determined time interval t0^_min. Accordingly, the estimate of the ambient temperature at time tc is also determined by directly using the measured temperature value as the estimate of the ambient temperature. At time tD, the motor is switched off again. The motor remains switched off until time tE at which time it is switched back on again. The time interval between time tE and tD is much shorter than the minimum time interval to^_min. Moreover, the time interval tD to tE is short enough that the minimum temperature drop for applying a cooling curve calculation using Equation 2 has not occurred. As such, to estimate the ambient temperature at tB, a previous estimate of the ambient temperature is used. In this example, the most recent estimate of the ambient temperature is used, which in this case is the estimate of the ambient temperature obtained at time tc. At time tF, the motor is switched off again. The motor remains off until time tG. The time interval tF to tG is less than the pre-determined time interval l,>rr min - Moreover, the time interval tF to tG is long enough for the minimum temperature drop to occur. Accordingly, to estimate the ambient temperature at time tG. a cooling curve calculation according to Equation 2 is performed. By the above-described method, a determination process can be selected in a manner which allows the ambient temperature to be accurately and efficiently computed in a range of circumstances. For example, during periods where the motor has been switched off for a long time, directly using the value measured by the temperature sensor may provide an efficient and accurate means for estimating the ambient temperature. During periods where the value measured by the temperature sensor can be expected to be greater than the ambient temperature, a previous estimate of the ambient temperature can be used if the minimum temperature drop has not occurred, which may provide a quick and efficient estimate of the ambient temperature, or the cooling curve calculation can be performed if the minimum temperature drop has occurred, such that a reliable and up to date estimate of the ambient temperature may be obtained by the calculation. Although in examples described above, the method of determining an estimate of the ambient temperature is performed by the air-moving device, in other examples, at least some aspects of the method may be performed by another device. For example, the air-moving device may be in communication with another device, such as a cloud-computing device or a smartphone, which may be provided with temperature measurements obtained by the temperature sensor of the device and which may perform certain aspects of the method for determining the estimate of the ambient temperature based on the provided temperature measurements. The above embodiments are to be understood as illustrative examples of the invention. Other embodiments are envisaged. It is to be understood that any 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 invention, which is defined in the accompanying claims.

Claims

1. A method of estimating an ambient temperature relating to an environment surrounding an air-moving device, the method comprising:determining, using a temperature sensor configured to measure a temperature of a thermal environment of the air-moving device:a first value of the temperature of the thermal environment at a switch-off time of a motor of the device; anda second value of the temperature of the thermal environment at a second time, wherein the second time is at an end of a time interval beginning at a start time, and wherein the start time is a time at which the temperature of the thermal environment has reduced to the first value following an increase in temperature after the switch-off time; andperforming a determination process to determine, based on the first value, the second value, the time interval and a pre-determined cooling characteristic of the thermal environment, an estimate of the ambient temperature.

2. The method of claim 1, wherein the pre-determined cooling characteristic is a predetermined cooling time constant of the thermal environment.

3. The method of claim 2, wherein the determination process comprises determining the estimate of the ambient temperature by:performing a cooling-curve calculation to determine the estimate of the ambient temperature using the first value, the second value, the time interval and the pre-determined cooling constant.

4. The method of claim 3, wherein the cooling-curve calculation comprises determining the estimate of the ambient temperature based on an expression representing an exponential cooling of the thermal environment.

5. The method of claim 4, wherein the cooling-curve calculation takes the form:_T(t)-T0-e4 ‘ambient ~ t1-e^where Tambient is the estimate of the ambient temperature, T (t) is the second value of the temperature of the thermal environment, To is the first value of the temperature of the thermal environment, t is the time interval between the start time and the second time, and t is the pre-determined cooling constant for the thermal environment.

6. The method of any preceding claim, wherein the second time is a time at which the motor of the air-moving device is switched on for a first time following the switch-off time.

7. The method of any preceding claim, comprising:monitoring, using the temperature sensor, the temperature of the thermal environment following the switch-off time; andbased on the monitoring, determining the start time as a time at which the temperature of the thermal environment reduces to the first value following the increase in temperature after the switch-off time.

8. The method of any preceding claim, wherein the temperature sensor is configured to, when the motor is operating, measure a temperature relating to the motor of tire air-moving device.

9. The method of any preceding claim, wherein the thermal environment comprises an enclosure inside the air-moving device containing the temperature sensor.

10. The method of claim 9, wherein the enclosure is a housing of the motor of the airmoving device.

11. A set of machine-readable instructions which when executed by one or more processors cause the method according to any of claim 1 to claim 10 to be performed.

12. An air-moving device comprising:a temperature sensor configured to measure a temperature of a thermal environment of the air-moving device;a processor; anda storage comprising a set of machine-readable instructions which when executed by 5 the processor cause the processor to perform a method according to any of claim 1 to claim 10.

13. The air-moving device of claim 12, wherein the air-moving device is a vacuum cleaner.10

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