Device with resistive heater and light sensor

Abstract

A device, such as a hairdryer or a space heater, includes an air path comprising an inlet and an outlet. A resistive heating element is disposed in the air path for heating air passing through. An airflow generator moves air in through the inlet, along the air path past the resistive heating element, and out through the outlet. A light sensor disposed so as to receive light from the resistive heating element, the light sensor being configured to sense a range of wavelengths that is a proper subset of the visible light spectrum, at least a portion of the range of wavelengths comprising wavelengths indicative of the resistive heating element overheating. The device is configured such that the light sensor sensing light from the resistive heating element within the range of wavelengths causes power to the resistive heating element to be reduced or isolated.

Description

[0001] DEVICE WITH RESISTIVE HEATER AND LIGHT SENSOR

[0002] BACKGROUND

[0003] Devices that heat air passing through an air path using an electrical resistive heating element are known. A drive current is passed through the heating element, which heats up and transfers heat to air passing it within the air path. An impeller can be used draw air through the air path. Examples of such devices include hair styling apparatuses, such as hairdryers, and space heaters.

[0004] SUMMARY

[0005] In accordance with an embodiment of the invention, there is provided a device comprising: an air path comprising an inlet and an outlet; a resistive heating element disposed in the air path for heating air passing through the air path; an airflow generator for moving air in through the inlet, along the air path past the resistive heating element, and out through the outlet; and a light sensor disposed so as to receive light from the resistive heating element, the light sensor being configured to sense a range of wavelengths that is a proper subset of the visible light spectrum, at least a portion of the range of wavelengths comprising wavelengths indicative of the resistive heating element overheating; the device being configured such that the light sensor sensing light from the resistive heating element within the range of wavelengths causes power to the resistive heating element to be reduced or isolated.

[0006] By sensing such a range of wavelengths, the device may be able to quickly reduce or isolate power to the resistive heating element in the event it overheats.

[0007] The device may comprise a controller, the controller being coupled for receiving a light signal from the light sensor and for controlling the resistive heating element, the controller being configured to: receive the light signal from the light sensor, the light signal being indicative of a wavelength(s) and / or an intensity of light being generated by the resistive heating element; determine, in dependence on the wavelength(s) and / or the intensity indicated by the light signal, that the resistive heating element is overheating; and responsive to determining that the resistive heating element is overheating, reduce power to, or isolate, the resistive heating element. The use of a controller may offer increased flexibility and / or improved performance as regards reducing power to the resistive heating element in the event it overheats.

[0008] The device may further comprise a temperature sensor, the temperature sensor having a slower response time than the light sensor and being coupled to provide a temperature signal to the controller, the controller being configured to: receive the temperature signal from the temperature sensor, the temperature signal being indicative of a temperature of at least a portion of the resistive heating element; determine, in dependence on the temperature signal, that the resistive heating element is overheating; and responsive to determining that the resistive heating element is overheating, reducing, or isolating power to the resistive heating element.

[0009] Using a light sensor and a temperature sensor may improve responsiveness to different types of resistive heating element overheating modes.

[0010] The controller may be configured to reduce or isolate power to the resistive heating element responsive to determining, in dependence on the light signal, that at least a portion of the resistive heating element is overheating, irrespective of whether the temperature signal is indicative of the resistive heating element overheating. This may assist in the detection of hot spots on the resistive heating element.

[0011] Optionally, when the controller reduces or isolates power to the resistive heating element responsive to determining, in dependence on the light signal, that at least a portion of the resistive heating element was overheating, the controller causes storage of an indication that the resistive heating element was reduced or isolated responsive to the light signal. Storing the indication in this way may allow for improved troubleshooting and / or guidance regarding maintenance and / or repair of the device.

[0012] Optionally, when the controller reduces or isolates power to the resistive heating element responsive to determining, in dependence on the light signal, that at least a portion of the resistive heating element was overheating, the controller may: cause the device to enter a resistive heating element cleaning mode in which the airflow generator controls the airflow through the air path so as to attempt to dislodge any blockage that might have caused a local hotspot on the resistive heating element leading to the light signal being indicative of the resistive heating element overheating; output, to the user, an indication that the user should place the device into a resistive heating element cleaning mode; output, to the user, an indication that power to the resistive heating element was reduced or isolated responsive to the light signal; output, to the user, an indication of a fault condition; and / or output, to the user, an indication that the device requires repair or servicing.

[0013] Each of these options offers different advantages in the event the resistive heating element overheats.

[0014] The range of wavelengths may have a lower cut-off wavelength of 550 nm. This may exclude wavelengths that are less likely to be indicative of the resistive heating element overheating.

[0015] The range of wavelengths may have an upper cut-off wavelength of 780 nm. this may exclude wavelengths that are less likely to be indicative of the resistive heating element overheating.

[0016] A detection peak of the range of wavelengths may be centred at between 550 nm and 650 nm. A peak within this range may provide improved sensitivity to wavelengths that are more likely to be indicative of the resistive heating element overheating.

[0017] The device may comprise a light guide comprising at least one reflective surface, the light guide being configured and positioned to communicate light from the resistive heating element to the light sensor. This may allow the light sensor to be positioned further from the resistive heating element, which may reduce the temperature experienced by the light sensor when the device is in operation.

[0018] The light guide may comprise at least two mirrors. This may provide improved, or at least alternative, options for the path taken by light from the resistive heating element to the light sensor.

[0019] The light guide may be elongate in a direction generally between the resistive heating element and the light sensor.

[0020] The device may be a hair-styling appliance or a heater. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a schematic view of a device in the form of a hairdryer, according to an embodiment of the invention;

[0022] Figure 2 is a graph showing a sensitivity curve for a light sensor forming part of the device of Figure 1;

[0023] Figure 3 is a schematic view of the hairdryer of Figure 1, including two attachments.

[0024] Figures 4 and 5 show the sensitivity curve of Figure 2, marked with emission wavelengths for different heating element temperatures;

[0025] Figure 6 is a graph showing output power of a drive circuit of the hairdryer of Figure 1, responsive to a temperature signal output by a temperature sensor of the hairdryer of Figure 1 ; and

[0026] Figures 7 to 11 are schematic views showing various examples of light guides that can be used with embodiments of the invention.

[0027] DETAILED DESCRIPTION

[0028] Figure 1 shows a device in the form of a hairdryer 100. Hairdryer 100 is illustrated schematically, and the form, size and relative positions of the components and features of a hairdryer manufactured in accordance with the illustrated schematic may vary from what is shown in Figure 1.

[0029] Hairdryer 100 comprises a housing 102, within which is defined an air path 104. Air path 104 includes an inlet 106 at a first end, inlet 106 including an air filter 108 for reducing the amount of dust, lint etc. entering the air path 104. Air filter 108 can take the form of, for example, a mesh filter, multiple small holes formed in the housing, or a separate insert. Air path 104 also includes an outlet 110, through which air passes when hairdryer 100 is in operation.

[0030] A resistive heating element 112 is disposed in air path 104. Heating element 112 can take the form of, for example, a metal heater, such as an open wire heater. The wire can formed of any suitable material, such as NiCr, for example. The wire can be straight, crimped, coiled, or can take any other form or profile.

[0031] An airflow generator 114 is also disposed in air path 104, upstream of heating element 112. Airflow generator 114 includes an electric motor 116, and a rotary impeller 118 connected to be driven by electric motor 116 so as to generate airflow within the air path 104, from inlet 106 to outlet 110 past heating element 112. Hairdryer 100 includes a handle 120 connected to, and extending downwards from, housing 102. Handle 120 includes a user interface 122 on its outer surface for allowing a user to control operation of hairdryer 100, including heat and airspeed settings for example, as described in more detail below. User interface 122 can also include visual, audio, and / or haptic communication elements (not shown) for providing the user with feedback about operation of the hairdryer. Details of user interface 122 are not shown, but it will be understood that any suitable combination of one or more buttons, sliders, knobs, screens, touchscreens, and lights, etc., can be provided depending upon the implementation.

[0032] Handle 122 is at least partly hollow, and contains within it a power supply 124 that accepts mains power from a power cable 126 that enters through the lower end of handle 122. Power cable 126 terminates in a plug (not shown) that can be inserted into a mains power outlet (not shown). Power supply 124 includes an AC-to-DC converter (not shown) for converting mains AC power to one or more DC voltages that are used to power other components within hairdryer 100.

[0033] In other implementations, the device (e.g., hairdryer or heater) can be battery powered, in which case a rechargeable or other battery can power the controller, airflow generator, heating element, and other components of the device.

[0034] A controller 128 for controlling the various features and functions of hairdryer 100 is disposed within handle 122 and is connected to be powered by power supply 124. Controller 128 includes a processor 130, and memory 132 for storing instructions operable by processor 130 as described in more detail below, along with other functional elements such as I / O circuits for sending and receiving control signals and information from sensors within hairdryer 100, including those described in more detail below. Although hairdryer 100 shows only a single controller 116, it will be appreciated that implementation of the features and components of hairdryer 100 can be distributed across two or more processors, located within housing 102, handle 120, or externally such as in a connected enclosure, or across any combination of such locations. Additional supporting circuitry, such as some sensors, communications, and power, are omitted for clarity. An example of a suitable MCU is the ARM Cortex-M0+.

[0035] Controller 128 is connected to control a heater drive circuit 134 disposed within handle 120. Heater drive circuit 134 can include, for example, one or more current control devices such as TRIACs (not shown) that controller 128 can control to modulate the power output of heating element 112. Drive circuit 134 is connected to receive mains power from power cable 126. Drive circuit 134 can be operable by controller 128 in a simple on / off fashion, or can be modulated across a range of power outputs to change the temperature of air exiting outlet 110, including maintaining a constant temperature at different rates of air output, for example.

[0036] Controller 128 is also connected to control a motor controller 136 disposed within handle 120. Motor controller 136 receives control signals from controller 128 as described in more detail below, and outputs a motor drive signal to motor 116. Optionally, the motor drive signal can be connected to the motor via an electromagnetic compatibility (EMC) fdter (not shown). Such an EMC filter filters out harmonics generated by motor 116, in use.

[0037] User interface 122 allows a user to set a target temperature and a fan speed, for example. A target temperature and a fan speed may each be selectable from a relatively small number of options (e.g., high, medium, and low settings for each of target temperature and fan speed). Alternatively, either or both of the target temperature and fan speed may be selected in a more granular way. For example, target temperature may be chosen as a specific temperature in degrees C or F, to a resolution of, say, 5°, 10° or 20°. Similarly, fan speed may be chosen as a specific airflow rate, such as in litres per second. The target temperature and fan speed are stored within memory 132. They may be stored persistently or reset to default values at each start-up of hairdryer 100. In yet other implementations, the fan speed and / or temperature or heating element output can be fixed.

[0038] A light sensor 138 is disposed within hairdryer 100 so as to receive light from heating element 118. In the illustrated example, light sensor 138 is positioned on a mount 140 that is positioned within housing 102 to support airflow generator 114. In this implementation, light sensor 138 is positioned downstream of motor 116, facing in generally the same direction as the airflow within air path 104. In other implementations, light sensor 138 can be positioned downstream of heating element 118 looking back upstream at heating element 118, or anywhere else that allows light sensor 138 to “see” some or all of heating element 118.

[0039] Light sensor 138 is also positioned generally centrally relative to the cross-sectional area of housing 102, enabling it to “see” substantially all of the downstream heating element 112, as indicated by dashed lines 160. Light sensor 138 can be configured to “see” the heating element as a result of its shape, configuration, and location.

[0040] Alternatively, or in addition, one or more optical elements, such as a light duct or a lens (such as a fisheye lens) can collect light and direct it to light sensor 138. For example, as shown in Figure 11, a light guide 170 can extend from the light sensor 138 into and at least partly through an opening through the heating element 112. Light guide 170 can be formed from borosilicate glass or any other suitably heat-resistant material that is transparent to the wavelength(s) of interest. Light 176 captured by light guide 170 is directed to light sensor 138. Light guide 170 can rely on total internal reflection, and / or can have a mirrored coating to ensure light does not escape on the way to the light sensor 138. The use of such optical elements may reduce the need for the sensor itself to be capable of sensing over a wide angular extent.

[0041] Light sensor 138 is configured to sense a range of wavelengths that is a proper subset of the visible light spectrum, at least a portion of the range of wavelengths comprising wavelengths indicative of the resistive heating element overheating.

[0042] Light sensor 138 can be configured to sense such a range of wavelengths in any suitable way. In one implementation, a photodiode having sensitivity only in the desired range of wavelengths can be used. One example is an Osram® SFH 2270R photodiode with a peak detection at 560 nm, and wavelengths detection range extending between 480 and 650 nm. This provides good sensitivity in the orange / red region, which is a colour associated with the resistive heating element overheating. Other ranges of wavelengths, optionally centred on a different peak (if there is a peak), can be used depending upon the specific wavelengths that are expected to be generated by the heating element if it overheats. Figure 3 shows a sensitivity curve for light sensor 138.

[0043] The range of wavelengths can optionally have a lower cut-off wavelength of 550 nm. That is, the light sensor (including any light filter(s)) can be relatively insensitive to wavelengths shorter than 500 nm. Filtering out shorter wavelengths may help reduce false positives, such as might happen from sunlight or artificial lighting leaking into hairdryer 100 when it is in use.

[0044] The range of wavelengths can optionally have an upper cut-off wavelength of 780 nm. That is, the light sensor can be relatively insensitive to wavelengths longer than 1200 nm. Filtering out longer wavelengths may help reduce false positives as a result of temperature variations that happen during ordinary operation of hairdryer 100.

[0045] A detection peak of the range of wavelengths can optionally be centred at between 550 nm and 650 nm. The light sensor peak can be selected to be near (e.g., within + / -10%, or + / - 5%) a temperature that will be more likely to be reached by hotspots than the heating element as a whole, for the reasons discussed below. Other light sensor types can also be used. For example, the light sensor can comprise one or more light dependent resistors, photodiodes, or phototransistors.

[0046] Depending on the device type and / or implementation, the light sensor can be configured to sense the range of wavelengths as a result of its inherent characteristics. Alternatively, one or more filters, such as a colour filter, can be used to filter light that is incident on the device, in order to effectively limit the range of wavelengths to which it responds. For example, a photodiode that is sensitive to the full visible spectrum can be paired with an orange / red filter that transmits wavelengths indicative of the resistive heating element overheating while absorbing and / or reflecting at least part of the visible spectrum.

[0047] Any filter(s) used can be included in the package of the device itself (i.e., covering the diode), or can take the form of a separate filter(s) positioned between the heating element and the device so as to filter light passing through it. The optional light guides, lenses, reflectors, and other optical components described below can incorporate one of more such filter(s), in which case they can be considered part of the light sensor.

[0048] Such filters can be coloured (e.g., with dyes or other colourants that cause the required filter characteristics), and / or can include interferometric or polarising characteristics.

[0049] The hairdryer 100 can optionally include support circuitry (not shown) that is connected to receive signals from the light sensor 138 and process them as required. For example, such support circuitry can include one or more amplifiers, filters (including high pass, low pass, band-pass, band-stop, and / or noise filters, for example) that convert the raw signal from light sensor 138 into a format that is more suitable for use by controller 128. Such a format can include, for example, and analogue value (such as a voltage) or digital value (such as a number) that can be received and processed by controller 128, optionally including further amplification and filtering, etc.

[0050] Alternatively, the controller 128 can receive the raw signal from light sensor 138, sample it, and manipulate it (if necessary) in the digital domain.

[0051] In use, light sensor 138 outputs a light signal responsive the intensity and wavelength(s) of light incident on it. The light signal is supplied (optionally via support circuitry) to controller 128. Where light sensor 138 is a photodiode, the light signal can be a voltage representative of the current presently passing through light sensor 138, the current being representative of the intensity and / or wavelength of light incident on sensor 138.

[0052] In use, hairdryer 100 can be used by a user to dry hair. The user interacts with user interface 122 to select a heat setting and airflow rate. Controller 128 turns on airflow generator 114 and heating element 112, and controls them so that they output air at the correct temperature and speed.

[0053] While this is happening, there may be some ambient light incident on light sensor 138, as a result of light leakage in through outlet 110 for example. However, because light sensor 138 is configured to sense a range of wavelengths indicative of the resistive heating element overheating, it does not respond to most ambient light. There may be a small amount of light falling on light sensor 138 that falls within the range of wavelengths that light sensor 138 is configured to sense. However, the position and sensitivity of light sensor 138, along with the interaction of any filter(s) (not shown) and further processing by any support circuitry (not shown), can be configured to reduce false positives.

[0054] In addition, hairdryer 100 can optionally be configured to operate only when an attachment is attached to outlet 110. For example, Figure 3 shows a hairdryer 100 that is configured to accept different attachments. When nothing is attached to outlet 110, processor 128 prevents operation of hairdryer 100 (and in particular, heating element 112 and airflow generator 114). When an attachment, such as a diffuser 162 or a concentrator 164, is attached to outlet 110 as indicated by arrow 166, a microswitch, reed switch, or other sensor (not shown) indicates to processor 128 that a compatible attachment has been attached to hairdryer 100. Processor 128 then allows operation of hairdryer 100.

[0055] Attachments such as diffuser 162 and concentrator 164 can reduce the amount of ambient light that enters outlet 110. Also, even light that does enter through an attachment has to travel further than if it were able to directly enter outlet 110. This may further help reduce false positives due to ambient light falling onto light sensor 138.

[0056] As heating element 112 heats up, the wavelength of the radiation it emits changes, in accordance with black body radiation principles known to the skilled person. During ordinary operation when it is not overheating, the surface temperature of heating element 112 is typically in the range of around 200 to 600 °C. It will be appreciated that this range is a guide, and that other temperatures and wavelengths may apply depending upon the implementation. Also, the temperature of the heating element can be different depending on factors such as the amount of local airflow it experiences and proximity to heat absorbing or reflecting elements within the airflow path. At these temperatures during ordinary operation, heating element 112 generally emits radiation in the infrared band, and emits substantially no radiation with the range of wavelengths that light sensor 138 is configured to sense.

[0057] Overheating involves an increase in emission of visible light. As heating element 112 increases in temperature as overheating begins, the wavelength of the emitted radiation shortens. The infrared radiation becomes red and then orange / red as the heating element 112 continues to overheat. It is this orange / red colour to which light sensor 138 is particularly sensitive in the described implementation.

[0058] As a result of the wavelength of the radiated light reaching the range of wavelengths to which light sensor 138 is sensitive, light sensor 138 (being a photo diode) begins to conduct current (beyond the limited leakage current that will flow when the light sensor is not “on”). The current passes through a resistor (not shown), causing a voltage to be generated. This voltage is sensed by controller 128.

[0059] When the voltage sensed by controller 128 rises to a predetermined level, the controller determines that the heating element 112 is overheating, and causes the drive current to heating element 112 to be stopped. Since the drive current is needed to keep the heating element 112 hot as heat is being extracted by air drawn over it by airflow generator 114, the heating element 112 will quickly cool once drive current is stopped. In other implementations, the drive current can be reduced rather than stopped.

[0060] Optionally, the airflow generator can continue to operate to remove some of the excess heat from the overheating heating element 112. Alternatively, the airflow generator 114 can be operated for a short time and then shut down, or immediately shut down upon detection of overheating.

[0061] It will be appreciated that, depending upon the implementation, the overheating can be determined as a result of the wavelength(s) sensed by light sensor 138, the intensity, or a combination of both.

[0062] For example, where light sensor 138 has a peak in its wavelength response, wavelengths at or near that peak can generate a similar strength light signal to wavelengths further from the peak but at a higher intensity.

[0063] An example of this relationship will now be described with reference to Figures 4 and 5. If all or most of the heating element is overheating (due to it being overdriven as a result of a failure elsewhere within hairdryer 100, for example), then a relatively large surface area is emitting radiation at a similar average wavelength. As the heating element 112 gets hotter and the average wavelength gets shorter and eventually falls into the wavelength range of light sensor 138, the relatively high power output due to the large emission area generates a light signal sufficient to cause the shutdown or power reduction described above. However, because of the relatively high power, the light signal may cause controller 128 to shut heating element 112 down well before the average wavelength it emits reaches the peak sensitivity of light sensor 138. With reference to Figure 4, for example, overheating causing an average emission wavelength of, say 600 nm (see vertical line 168) over substantially all of the heating element’s surface generates a light signal sufficient to cause the shutdown or power restriction described above.

[0064] Another form of overheating is a hotspot, in which a relatively small area of heating element 112 gets much hotter than the rest of the heating element. This can be caused by, for example, a local short within heating element 112, or a piece of detritus blocking local airflow. Due to the relatively small size of such hotspots, they may not initially output enough power to cause shutdown or power restriction as the average wavelength shortens and falls into the wavelength range of the sensor. However, as the temperature of the hotspot increases and the emitted wavelength reduces and moves towards the sensor’s sensitivity peak, the light signal will increase. Even though the average power of a hotspot may be lower than when the heating element as a whole is overheating, the additional sensitivity of the light sensor to the higher temperature reached, and shorter average wavelength output, by the hotspot means the light sensor still generates a light signal sufficient to cause the shutdown or power restriction described above. With reference to Figure 5, for example, a hotspot emitting radiation at an average wavelength of 600 nm (as in Figure 4) is not sufficient to cause shutdown or power restriction, due to the relatively low power output of the hotspot. However, at 570 nm (see vertical line 172), the greater sensitivity of the light sensor 138 generates a light signal sufficient to cause the shutdown or power restriction described above despite the lower power output of the hotspot.

[0065] The particular temperatures and wavelengths that will cause shutdown or power reduction, and the matching of the light sensor’s characteristics to general heating element overheating versus hotspot overheating will be determined based on the implementation. For example, the various temperature / wavelength values and ranges can be selected by modelling, experimentation, or both, for example. Returning to Figure 1, hairdryer 100 can optionally include a temperature sensor. For example, a temperature sensor in the form of a thermistor 142 can optionally be provided in air path 104. Thermistor 142 is coupled to provide a temperature signal to controller 128. Any other form of temperature sensor can be used to suit the particular implementation requirements.

[0066] Thermistor 142 has a slower response time than light sensor 138. This is typically the result of lag introduced by the mass of thermistor 142, for example, although other reasons for the slower response time are possible depending on the temperature sensor type and any supporting circuitry (not shown) .

[0067] Thermistor 142 can optionally be used by hairdryer 100 as part of its temperature control mechanism. Since it is positioned downstream of heating element 112, it senses the temperature of the air shortly before it exits outlet 110. The temperature signal provided to controller 128 by thermistor 142 can be used as feedback such that controller 128 can adjust the output of drive circuit 134 and / or motor drive controller 136, thereby to maintain the air temperature at a suitable level.

[0068] Thermistor 142 can also be used to identify an overheating situation. Although it does not directly sense the temperature of heating element 112, the temperature signal generated by thermistor 142 is at least indirectly indicative of that temperature. For example, in a situation where the airflow has been restricted, for example by a blockage somewhere in the air path 104, the reduced airflow for the current heat output of heating element 112 will cause a temperature rise at thermistor 142. Controller 128 can reduce the power output of heating element 112 by controlling drive circuit 134, and / or attempt to increase airflow by controlling motor controller 136. However, if this is not effective, the controller 128 can determine that the temperature signal has remained too high for too long, indicating that the resistive heating element is overheating. In response, controller 128 can reduce or isolate power to the resistive heating element.

[0069] Figure 6 is a graph 144 showing how controller 128 controls the output power of drive circuit 134 responsive to the temperature signal output by thermistor 142. Before hairdryer 100 is first turned on by way of user interface 122 at time 200, thermistor 142 is at room temperature. At time 200, controller 128 causes drive circuit 134 to output maximum power to heating element 112, while also causing motor controller 136 to drive motor 116 and impeller 118 in accordance with the currently selected airflow speed. After a brief lag, the temperature signal from thermistor 142 settles into a consistently rising air temperature. As the temperature of the air as determined by thermistor 142 rises, maximum power is maintained until the air temperature approaches the target temperature, which in graph 144 is 140 °C. Shortly before (to account for some expected overshoot) reaching this temperature, at time 202, power is reduced, resulting in the downward sloping power line. Power continues to be reduced as the temperature approaches the target temperature. Once the temperature reaches the target temperature at time 204, power is maintained at that level.

[0070] If the air temperature exceeds the target temperature, for example by some threshold, power is reduced to cause the air temperature to reduce towards the target air temperature. If the power reduction is ineffective, the air temperature may continue to rise. For example, in graph 144, the threshold temperature for determining an overheating event is 150 °C.

[0071] Heating element 112 begins to overheat, causing the temperature indicated by thermistor 128 to rise at time 206. Controller 128 responds by reducing power, but in this example, this is not sufficient to stop the heat increasing, as shown by the continued increase of temperature after time 206.

[0072] Eventually, at time 208, the temperature indicated by thermistor reaches 150 °C. Processor 128 determines that an overheating event has occurred, and shuts down power to heating element 112.

[0073] Optionally, the determination about whether to significantly reduce or isolate power to heating element 112 based on the temperature signal can be based on changes to the temperature signal over time. For example, the temperature signal can be averaged or otherwise low-pass filtered to avoid brief temperature spikes causing a false positive.

[0074] Controller 128 can determine that at least a portion of resistive heating element 112 is overheating based on either or both of the light signal and the temperature signal. This provides improved safety. Not only are there two different sensors operating to determine whether the heating element is overheating, they each have different and complementary advantages. For example, thermistor 142 is relatively slow to respond, but can detect a relatively small amount of overheating. In contrast, light sensor 138 is relatively fast to respond, but at least in some circumstances requires a greater temperature difference above nominal operating temperature in order to cause the reduction or isolation of power. Thermistor 142 may be slow to react to hotspots, especially if they are small, because hotspots may not increase the temperature of the air enough to indicate that overheating is taking place, whereas light sensor 138 may identify hotspots more easily. In one implementation, the light sensor is configured or optimised to identify hotspots while the temperature sensor is configured or optimised to identify overheating of the heating element as a whole.

[0075] A potential advantage of using a sensor that senses overheating based on wavelength is that the same sensor can be used with different heating element materials having different characteristics. For example, a nickel-chromium based heating element will start to emit visible light at a different temperature compared with an aluminium-based heating element. However, in both cases, the emission of visible light is indicative of overheating, and so the same sensor can be used with both heating element types.

[0076] Optionally, controller 128 can store an indication of what caused power to the resistive heating element to be reduced or isolated. For example, when the power to the resistive heating element was reduced or isolated as a result of the light signal being indicative of overheating, the controller can cause this information to be stored in (non-volatile) memory 132. This information can subsequently be used in a number of ways.

[0077] For example, controller 128 can cause hairdryer 100 to enter a resistive heating element cleaning mode. In this mode, airflow generator 114 operates to control the airflow through air path 104 so as to attempt to dislodge potential blockages. Such blockages can reduce airflow, including bulk airflow over heating element 112 (which can lead to general overheating of element 112) and / or localised airflow over a small area of heating up in 112 (which can lead to a hotspot). A resistive heating element cleaning mode can include, for example, controlling motor 116 to change speeds quickly, to reverse its direction of rotation, and to gradually speed up and slow down over a wide range of speeds.

[0078] Alternatively, or in addition, controller 128 can output, via user interface 122 for example, an indication that the user should place the device into a resistive heating element cleaning mode such as that described above. This gives the user control over when such a cleaning mode takes place.

[0079] Alternatively, or in addition, controller 128 can output, via user interface 122 for example, an indication that power to the resistive heating element was reduced or isolated responsive to the light signal. Alternatively, in addition, controller 128 can output, via user interface 122 for example, an indication of a fault condition, and / or an indication that the device requires repair or servicing. These options may alert the user to the desirability of undertaking maintenance or repair, whether by the user where appropriate, or by a suitable service centre.

[0080] While the light sensor 138 of hairdryer 100 is mounted in a position to directly sense the temperature of heating element 112, in other implementations, the light sensor can be positioned more distant from the heating element. In that case, the device can comprise a light guide comprising at least one reflective surface, the light guide being configured and positioned to communicate light from the resistive heating element to the light sensor.

[0081] The light guide can take several forms dependent upon the implementation requirements. Also, the word “guide” in this context merely means a space through which light is directed, and can include air or another transparent medium such as glass or a polymer. The light guide need not be bounded by, for example, walls or other structures.

[0082] Optionally, the light guide can be configured to filter certain wavelengths of light. For example, the light guide can take the form of a bandpass filter. Optionally, the bandpass filter is configured to pass at least some of the range of wavelengths described above, in which case the light guide can be considered part of the light sensor.

[0083] Similarly, any reflective surface within the light guide can be configured to more strongly reflect certain wavelengths of light. For example, an interferometric reflector can be configured to more strongly reflect certain wavelengths of light. The reflective surface can be configured to pass at least some of the range of wavelengths described above, in which case the reflective surface can be considered part of the light sensor.

[0084] Turning to Figures 7 to 10, there are shown detailed schematic views of components of various devices. The devices can be hairdryers, heaters, or any other device that heats air with a resistive heating element.

[0085] Figure 7 shows heating element 112, with light sensor 138 positioned remotely from it. In this context, “remotely” means that the light sensor is too far from the heater to sufficiently sense its emissions, or positioned so as not to have a line-of-sight view of heating element 112. A light guide 145 extends from a point adjacent heating element 112 to a point adjacent light sensor 138. Light guide 145 takes the form of a slab or cylinder of a heat-tolerant material such as glass (e.g., borosilicate glass) or a suitable polymer. A first end 146 of light guide 145 is angled at around 45°, and a second end 148 of light guide 145 is similarly angled at around 45°. At least the angled portions of first and second ends 146 and 148 are reflective and configured in a periscope arrangement, such that light 150 from heating element 112 entering light guide 145 laterally at first end 146 is turned along light guide 145. When light 150 hits second end 148, it is turned again and exits light guide 145 laterally. Light 150 exiting light guide 145 at second end 148 falls onto light sensor 138, which behaves as described above.

[0086] Figure 8 shows an alternative version of the arrangement of Figure 7, in which the light guide comprises a first mirror 152 positioned adjacent heating element 112 and a second mirror 154 positioned adjacent light sensor 138, and air space between first mirror 152 and second mirror 154. First and second mirrors 154 are angled in the same fashion as the first and second ends 146 and 148 of light guide 145. Operation of the light guide in Figure 8 is similar to that shown in Figure 7. One difference is that the arrangement of Figure 8 may have lower losses due to the light not having to pass through a medium such as glass.

[0087] Figure 9 shows an alternative version of the arrangement of Figures 7 and 8, in which the light guide just the first mirror 152 and the air path between it and light sensor 138. In this case, first mirror 152 reflects light 150 directly to light sensor 138.

[0088] In each of Figures 7 to 9, the light guide is elongate in a direction generally between the resistive heating element and the light sensor. It will be appreciated that more complex paths are also possible, and that the angles of reflective surfaces need not be 45°, for example. The light guide can also include several light guide elements.

[0089] Figure 10 shows a further version of a light guide 145. In this case, light guide 145 takes the form of a bundle of optic fibers extending from a first end 156 near the heating element 112 to a second end 158 near the light sensor 138. In this case, the air-fiber interface is the reflective surface, as a result of the total internal reflection that happens with such fibers. Any suitable number of optic fibers can be used.

[0090] Optionally, one or more collimating lenses or reflectors can be employed to focus more of the emissions from heating element 112 into first end 156, thereby increasing the amount of light from heating element 112 that reaches light sensor 138. An example is collimating lens 174 shown in Figure 10, which focuses light from heating element 112 into a smaller area so more of it can enter the end of the optic fibers of the light guide.

[0091] An advantage of using optic fibers is that they can bent into (within reason) arbitrary shapes, allowing them to move light through a potentially quite complex route between heating element 112 and light sensor 138.

[0092] It will be understood that different types, combinations and configurations of light guide can be employed. For example, a greater or larger number of reflective surfaces can be used. An outer surface of the light guide (such as light guide 145) can be coated and / or treated such that light is internally reflected within the light guide, which may increase the amount of light reaching light sensor. Elements having a focusing ability can be included. For example, one or more lenses and / or curved mirrors can be used manipulate light from the heating element. For example, a collimating lens or mirror can be used to focus light from some of all of the heating element into a smaller spot, enabling the light sensor to sense light from more of the heating element’s surface.

[0093] Although various hair-styling devices in the form of hairdryers have been described, and there are certain advantages that arise from this use, it will be appreciated that the invention also has application in any other heated air device. Another example of such a device is a heater, such as a domestic space heater.

[0094] Although the invention has been described with reference a number of specific embodiments and implementations, the skilled person will appreciate that the invention may be embodied in many other forms.

Claims

CLAIMS1. A device comprising: an air path comprising an inlet and an outlet; a resistive heating element disposed in the air path for heating air passing through the air path; an airflow generator for moving air in through the inlet, along the air path past the resistive heating element, and out through the outlet; and a light sensor disposed so as to receive light from the resistive heating element, the light sensor being configured to sense a range of wavelengths that is a proper subset of the visible light spectrum, at least a portion of the range of wavelengths comprising wavelengths indicative of the resistive heating element overheating; the device being configured such that the light sensor sensing light from the resistive heating element within the range of wavelengths causes power to the resistive heating element to be reduced or isolated.

2. The device of claim 1, further comprising a controller, the controller being coupled for receiving a light signal from the light sensor and for controlling the resistive heating element, the controller being configured to: receive the light signal from the light sensor, the light signal being indicative of a wavelength(s) and / or an intensity of light being generated by the resistive heating element; determining, in dependence on the wavelength(s) and / or the intensity indicated by the light signal, that the resistive heating element is overheating; and responsive to determining that the resistive heating element is overheating, reducing power to, or isolating, the resistive heating element.

3. The device of claim 2, further comprising a temperature sensor, the temperature sensor having a slower response time than the light sensor and being coupled to provide a temperature signal to the controller, the controller being configured to: receive the temperature signal from the temperature sensor, the temperature signal being indicative of a temperature of at least a portion of the resistive heating element; determine, in dependence on the temperature signal, that the resistive heating element is overheating; and responsive to determining that the resistive heating element is overheating, reduce or isolate power to the resistive heating element.

4. The device of claim 3, wherein the controller is configured to reduce or isolate power to the resistive heating element responsive to determining, in dependence on the light signal, that at least a portion of the resistive heating element is overheating, irrespective of whether the temperature signal is indicative of the resistive heating element overheating.

5. The device of claim 4, wherein: when the controller reduces or isolates power to the resistive heating element responsive to determining, in dependence on the light signal, that at least a portion of the resistive heating element was overheating, the controller causes storage of an indication that the resistive heating element was reduced or isolated responsive to the light signal.

6. The device of claim 4 or 5, wherein: when the controller reduces or isolates power to the resistive heating element responsive to determining, in dependence on the light signal, that at least a portion of the resistive heating element was overheating, the controller: causes the device to enter a resistive heating element cleaning mode in which the airflow generator controls the airflow through the air path so as to attempt to dislodge any blockage that might have caused a local hotspot on the resistive heating element leading to the light signal being indicative of the resistive heating element overheating; outputs, to the user, an indication that the user should place the device into a resistive heating element cleaning mode; output, to the user, an indication that power to the resistive heating element was reduced or isolated responsive to the light signal; output, to the user, an indication of a fault condition; and / or output, to the user, an indication that the device requires repair or servicing.

7. The device of any preceding claim, wherein the range of wavelengths has a lower cut-off wavelength of 550 nm.

8. The device of any preceding claim, wherein the range of wavelengths has an upper cut-off wavelength of 780 nm.

9. The device of any preceding claim, wherein a detection peak of the range of wavelengths is centred at between 550 nm and 650 nm.

10. The device of any preceding claim, comprising a light guide comprising at least one reflective surface, the light guide being configured and positioned to communicate light from the resistive heating element to the light sensor.

11. The device of claim 10, wherein the light guide comprises at least two mirrors.

12. The device of claim 10 or 11, wherein the light guide is elongate in a direction generally between the resistive heating element and the light sensor.

13. The device of any preceding claim, wherein the device is a hair-styling appliance.

14. The device of any one of claim 1 to 12, wherein the device is a heater.

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