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GB2564657B - Cooking hob monitoring method - Google Patents

Cooking hob monitoring method Download PDF

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Publication number
GB2564657B
GB2564657B GB1711491.9A GB201711491A GB2564657B GB 2564657 B GB2564657 B GB 2564657B GB 201711491 A GB201711491 A GB 201711491A GB 2564657 B GB2564657 B GB 2564657B
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temperature
hob
pan
cooking
fire
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GB2564657A (en
GB201711491D0 (en
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Gerard Bailey Samuel
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    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24HEATING; RANGES; VENTILATING
    • F24CDOMESTIC STOVES OR RANGES ; DETAILS OF DOMESTIC STOVES OR RANGES, OF GENERAL APPLICATION
    • F24C7/00Stoves or ranges heated by electric energy
    • F24C7/08Arrangement or mounting of control or safety devices
    • F24C7/082Arrangement or mounting of control or safety devices on ranges, e.g. control panels, illumination
    • F24C7/083Arrangement or mounting of control or safety devices on ranges, e.g. control panels, illumination on tops, hot plates
    • GPHYSICS
    • G08SIGNALLING
    • G08BSIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B17/00Fire alarms; Alarms responsive to explosion
    • G08B17/12Actuation by presence of radiation or particles, e.g. of infrared radiation or of ions
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24HEATING; RANGES; VENTILATING
    • F24CDOMESTIC STOVES OR RANGES ; DETAILS OF DOMESTIC STOVES OR RANGES, OF GENERAL APPLICATION
    • F24C15/00Details
    • F24C15/20Removing cooking fumes
    • F24C15/2021Arrangement or mounting of control or safety systems

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  • Engineering & Computer Science (AREA)
  • Business, Economics & Management (AREA)
  • Emergency Management (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Electric Stoves And Ranges (AREA)
  • Frying-Pans Or Fryers (AREA)

Description

Cooking hob monitoring method
Hob fires are by far the single largest cause of dwelling fires, accounting for over 50% of fires in the UK and 40% in the USA.
Hob fires are typically caused by unattended pans, which overheat if left on the hob. This can be a pan which boils dry and overheats, or a frying pan (particularly a deep fat fryer) which if left on a high heat can reach the auto-ignition point of the oil in the pan, which can then ignite. They can also be caused by tea towels or other combustible items being left on or close to the hob.
Most homes will have a smoke alarm ora heat alarm fitted in the kitchen. However by the time these trigger, the fire is likely to have already spread to some other part of the kitchen, and may already be too dangerous for a householder to tackle. They will then evacuate the building and the fire will be left to the fire brigade to tackle, substantial damage will have been done to the building and there is a risk of death or injury to occupants of the building.
Prior to the dangerous hob usage becoming a fire, there is time period of around 1 to 3 minutes between the pan exceeding the temperature it would experience during normal cooking, and the pan reaching a temperature at which there is a risk of fire. We refer to this as a pre-ignition situation. If this condition is detected, and alarm triggered alerting the user, they can simply turn off the hob or remove the pan from the heat, or the heat to the hob can be turned off automatically.
Being able to accurately discriminate between a pre-ignition and ignitions situations and normal cooking, and providing an alert, thus has substantial benefit for reducing the risk of fires on the hob.
Three typical conditions leading up to a hob fire would be:
Oil fire - User places saucepan on hob with oil in. Pan is heated up. Temperature rises until the oil reaches a temperature suitable for frying. If the pan is not attended by the user, either by reducing the heat or adding food to cool the oil, the temperature will continue to rise. The temperature will initially reach the smoke point of the oil, and if still unattended will reach the auto-ignition temperature of the oil and a fire will start.
Dry pan fire - User places saucepan on hob with water or water based food in it e.g. soup or stew. Pan is heated up. Temperature rises until the water reaches boiling point. Temperature stabilises at around the boiling point of water (100C). Pan is not attended to by the user and water from the pan continues to evaporate. When no more water is present to maintain the 100C temperature, the temperature starts to rise. When the contents of the now dry pan reaches the combustion temperature of the remaining food, then a potential hob fire arises.
Combustible materials fire. User places a combustible material (e.g. tea towel) in the vicinity of the hob. The hob heats up and ignites the material and a fire is started. US20150196161A1 describes a method for detecting dangerous cooking by monitoring the temperature as measured by a single heat sensor above the hob and triggering above a certain temperature or rate of change or temperature. However as this only uses a single sensor, the sensor sees an average temperature of the region above the hob. This is means that it effectively measures the total heat output of the hob. The heat output of the hob can be high if a user is cooking something that requires the hob to be on high, e.g. if using a wok, even if the temperature of the wok itself is below a safe level. It can also alarm when the rate of heat measured rises rapidly, even if the temperature is low. This can occur if a warm pan is placed on the hob, as the temperature will rise more rapidly than if the pan had been on the hob and had been heated up, however the temperature of the pan itself can be quite low and consequently safe with regard to fire risk. The system thus false alarms very frequently, and is likely to be ignored or removed by a user to avoid annoyance. This reduces its effectiveness in preventing fires. A study by the Consumer Product Safety Commission in the United States (https://www.cpsc.gov/s3fs-public/pdfs/3512ba2b.pdf) into detecting pre-ignition conditions of cooking related fires concluded that measuring the temperature of the pan bottom or pan contents was the best indicator that a pre-ignition situation existed. They concluded that a pan bottom temperature of 386°C to 494°C was required to cause ignition (depending on the pan contents) and that holding the pan temperature below 315°C to 330°C would prevent 99% of ignitions (the lower number is to allow for the thermal inertia - a pan may continue to increase in temperature after the heat is switched off as energy from the hot plate continues to be transferred to the pan).
However measuring the temperature of the base of a pan is difficult. Either the pan must have a thermocouple in the base, which would require the use of specifically modified pans, or the hob must have a sensor built in. Pans exist with sensors in but they do not tend to be a commercial success. Some electric hobs have sensors built in which can automatically limit or cut out over certain temperatures. However these are not ubiquitous and users without these hobs either have to upgrade their hob or be exposed to the increased fire risk.
This invention describes a fire pre-ignition sensor that can be easily fitted to a cooker or cooker hood and detect ignition and pre-ignition conditions on an existing hob without use of any other specialist pans or equipment.
Typically, pre-ignition situationswill follow a recognisable pattern if viewed in the thermal infrared part of the electromagnetic spectrum. Thermal imaging cameras detect the surface temperature of objects within their field of view. However, to mount a thermal imaging camera where it can view the base of a ban is hard. It would have to be mounted on the hob and look upwards, where it is likely to be damaged, splashed or exposed to high temperatures. A thermal imaging camera can be mounted above the hob, for example on a wall or cooker hood. It will then have a field of view of the hob and any pans on the hob. It will be less likely to be damaged, and will not be exposed to high direct temperatures. However, it will not be able to directly detect the temperature of the pan bottom if the pan has food in it, so must infer the presence of a pre-ignition condition based on the temperature image it sees a view from above.
This patent describes a method for detecting the existence of pre-ignition conditions on a hob from a thermal imaging sensor mounted above the hob.
This can be done by a method comprising one or more of the following steps.
Fig 1. Shows a possible layout for an infrared thermal imaging sensor 1. mounted above a hob 2.. The field of view 3. is such that for a typical mounting height, the hot plate(s) are within the field of view.
The sensor will have a series of pixels, each of which will measure the average radiant surface temperature of whatever is within the field of view of that pixel. The result is a 2D image of the hob, with a pixel measuring temperature rather than colour. The detection is performed using an algorithm that processes this image.
Testing with a thermal imaging sensor mounted in such a way show that it is possible to detect pre-ignition conditions of the type described above.
The characteristics of the pre-ignition conditions vary depending upon the cooking type.
These would be frying pan overheats, pan with water or other water based food in boils dry, hob left on and other combustion e.g. tea towel.
While a simple threshold on the maximum temperature detected would detect many pre-ignition conditions, because the bottom of the pan temperature is the hottest part, and the part where ignition will likely take place, it is necessary to adjust the alarm threshold depending on the cooking type and the contents ofthe pan.
Fig 11 shows the profile of the temperature of a pan of baked beans as they are left on the hob. Of note is that the beans ignited at around the point marked 3. on the figure, where the temperature as viewed from above is around 120C in the middle ofthe pan, but because the beans are poor conductors of heat, as will many thicker stews or other sauces be, and the pan was not stirred as is likely to be the case in a dangerous scenario, the temperature at the pan bottom is far higher and hit the temperature necessary to start the beans burning. However 120C as viewed from above is not a pre-ignition condition for a frying pan containing oil. The method must therefore identify the appropriate threshold to use based on classifying what the cooking type is.
Also because the user may put a small pan on a large hob, a higher temperature may be visible around the edge ofthe pan than in the centre ofthe hob. Fig 10. Shows an example of this, where the centre ofthe pan is 140C, but the pan walls and flames passing up the side of them are >200C. Whilst this is inefficient, it is not necessarily a pre-ignition condition and would be a nuisance alarm to a user if it was alerted based on merely the presence of high temperatures in the field of view.
The first step in identifying the pre-ignition conditions is to identify the cooking type or hob with no pan on it. A deviation from the normal heat signature for that cooking type would then be identifiable as a pre-ignition condition.
When the hob is lit, if the hob is a gas hob and there is no pan on the hob, there follows a sudden rise in temperature in the region of the lit burner. Fig 2. Shows a thermal contour map taken using a low resolution thermal imaging sensor of a gas burner. The change in temperature with time for the pixel viewing the centre of the burner is shown in Fig 3. The temperature reaches a temperature of around 350C almost immediately and fluctuates +/- 15C. If this condition persists without a cooling when a pan is placed on the hob, this characteristic can therefore be used to identify that a gas burner has been left on. It can also be used to locate the position of the burners within the field of view. An electric hob will have a similar profile, but with a longer warm up time and a likely larger hot region.
Figure 4 shows the thermal contour image of a pan of boiling water on the hob. The centre of the pan region is fluctuating at around 100°C.
An algorithm can be used which comprises the some or all ofthe following steps:
Step 1 is to identify the location ofthe hot plates or pans within the field of view. As the imaging sensor can be fitted at a variety of heights and angles, the algorithm may first identify the hot regions within the field of view.
From a starting point of all regions being at approximately ambient temperature (<40C), the hob can be assumed to be in an off state.
If a region of the image rises to a temperature of >250C within less than a defined period, say 5 seconds, then a gas burner can be assumed to have been ignited. A pan may be placed on the burner, in which case the viewed temperature will drop almost instantly to a lower temperature, that will depend on the temperature of the pan {the pan may already be hot).
If the temperature stays above a threshold indicative of a hob being on, but below the pan temperature encountered during normal cooking, say 300C, but may be lower e.g. 250C or higher e.g. 350-400C, for a timeout period, then an alert is sounded to indicate to the user that a hob has been left unattended. This may be combined with information from a presence detector of a type known in the field e.g. a Passive Infrared Motion sensor directed in front of the stove, which indicates whether there is a person attending the stove. If a person is present, the alarm may be suppressed or delayed. If no person is present then the alarm may be sounded immediately or after a shorter timeout. This sensing logic will also aid in detecting other fires on the hob e.g. burning tea towels.
Alternatively, the pan may be placed on the hob either hot or cool, and the burner or hob turned on underneath it. In this case a slower rise in temperature in part of the image will be detectable by the sensor.
The presence and location of a pan can be detected as by looking for square or circular regions of higher temperature than the surrounding pixels. One way of doing this is as follows.
By taking the first spatial differential of the temperature in the x and y direction, where the x and y direction are the pixel coordinates in the thermal imaging sensor and T is the temperature as below, the gradient of the temperature can be measured. dT dT dx ’ dy
By combining these, for example by linear summing or by taking the sum of the squares or the root of the sum of the squares, a measurement of the overall temperature gradient, T' can be found e.g. , Ι/3Λ2 /3 A2 T ~ JGx) + Gy)
Applying a threshold to all the values of T' above a certain value (the threshold will depend upon the resolution of the sensor, the hob and pan temperature, so could be chosen automatically e.g. the top x pixels with the highest gradient value Τ', where x is a number that approximately gives the circumference of a pan in pixels as viewed from the sensor) gives a set of pixels that detect the edges of the pan, as these will be the regions with the highest temperature gradient. Fig 5. shows a cross section of the temperature of a pan containing boiling water. As can be seen, pixels 3-4 and 12-13 have the largest gradient difference, and hence show the edge of the pan.
The thresholded pixels on the edge of the pan could then be put through a circle finding algorithm such as the circular Hough transform which is reproduced here in pseudocode, or an alternative known in the field e.g. the RANSAC algorithm.
Circular Hough Transform
Where x and y are the pixel coordinates.
For each pixel(x,y)
For each radius r - 5 to r - 10 // the possible radius in pixels
For each theta t = 0 to 360 // the possible theta 0 to 360 a = x - r * cos(t * PI /180); //polar coordinate for center b - y - r * sin(t * PI / 180); //polar coordinate for center A[a,b,r] +-1; //voting end end end
The value in the accumulator matrix, A, with the largest value would then yield the coordinates a,b ofthe centre ofthe pan, and it's radius, r.
Note that this would only work for a circular pan, a square Hough transform for a square pan could also be applied, and the accumulator with the highest value from both taken to be the one that indicates the shape ofthe pan. Fora circular pan:
Having found the location ofthe pan in the image, the pixels located inside the pan i.e. (x - a)2 + (y - b)2 < r2 can then be measured and taken as an indication ofthe temperature ofthe contents ofthe pan on the hob.
If the temperatures at the edge of the pan e.g. where (r - d)2 < (x - a)2 + (y - b)2 < r2 and d is an estimate of a typical pan wall thickness as viewed in pixels, are higher on average than those in the centre of the pan by a significant margin, for example 10C higher, then this is an indication that the edge of the pan itself is warmer than the contents or that the pan is not fully covering the hotplate or gas flame. Fig 9 shows the input from a sensor where this is the case. The pixels around the edge of the pan are >100C, where as those in the centre are around 80C. This indicates the sensor is detecting the heat being transmitted up the metal pan sides by the burner flames passing around the sides of the pan. These hotter edge pixels should then be discarded, and the pixels in the centre region, R as those where (x - a)2 + (y - b)2 < (r - d)2 taken as being measures ofthe top surface temperature ofthe pan contents.
An alternative would be to apply a simple threshold to the temperatures in the image, and then use another circle finding algorithm such as counting all the pixels above the threshold which are less then a radius r from each possible pan centre. The threshold can then be lowered step by step, and when a sudden change in the number of pixels is detected, then those counted pixels above the threshold are assumed to be part of the pan.
Having found the region, R containing the pixels in the pan, these can then be used to find a representative value of the temperature of the top surface of the pan contents, Tp. This could be done by taking a mean of the pixel temperatures within R, or a median to exclude outliers, or a weighted average to reduce the effect of pixels close to the edge of the pan, or a single pixel in the centre of the pan, or the maximum or minimum of any pixel or pixels within the region R.
Having detected and located the presence of the pan and the region R over which to calculate Tp, Tp can then be monitored over time to determine the type of cooking being done, and consequently detect any pre-ignition conditions that may occur. The determination of R may be repeated, to allow for the pan being moved by a user as they stir or check the cooking.
Figure 5 shows the temperature as measured by the pixel viewing the centre of the pan as a function of time. After an initial warm up period, the duration of which will depend upon the hob heat setting and the amount of water in the pan, the temperature, Tp, stabilises at approximately 100°C, labelled as 1., and fluctuating +/-5°C or so due to convection currents and bubbles rising through the water, and also sensor noise.
At approximately 17:30 in the test time, the temperature of the centre of the pan starts to rise above 100C, due to all the water having evaporated from the pan (the pan has boiled dry), and continues to rise (labelled 2.). At 17:33 the temperature in the pan has reached 250C, at which point it has started to smoke, and the hob is switched off.
The region from 17:30 to 17:33 are pre-ignition conditions. The pan has overheated and is in danger of igniting, but it hasn't yet reached ignition temperature. This condition can now be detected using a classifier such that an alert can be sent to the user.
This can be achieved by an algorithm comprising some or all of the following steps. A flow diagram of the steps is shown in Fig 12.
Smooth the measured Tp by using a smoothing filter such as a low pass filter or moving average, or a fit a line or curve to Tp using least squares or a similar method.
Monitor the rise rate of Tp by measuring the gradient with respect to time of either the smoothed or unsmoothed Tp using an algorithm known in the field such a difference equation e.g. , dTp _ Tpt-T t-i
Rise rate - ----=---------- dt h
Where h is the step size between samples
If the temperature is below 90°C +/-f, where f is a tolerance value that could be 15°C, but may be a value up to 25°C, or as low as 0°C or any value in between of 1-25C and the rise rate shows it is rising, then the pan is in a heating up state.
Continue to measure the rise rate, and if it drops to a value below g, where g can vary between different sensors, but is around 0.2°C per second or as low as 0.002C per second or as high as 2C per second or any value in between, and the average temperature is 90°C +/- f°C, then the pan contains boiling water or a water based food such as stew, soup etc.
Maintain the state that the pan contains a water based food and continue to measure Tp,
If Tp suddenly jumps by a threshold amount e.g. 20C, but possibly as low as 5Cor as high as 100C or any value in between, then the pan has been removed from the hob. The algorithm will start a timer which if the temperature doesn't start to drop or go below a safe temperature e.g. 40C within a set time interval an alert will be set indicating that a hob is left on.
If the rise rate increases above g plus a hysteresis value e.g. an additional 0.2°C per second, then the pan has boiled dry. If the temperature continues to rise above a second threshold, say 120C, but could be as low as 105Cor as high as 300C or any value in between e.g. 105,11,120,130,140,150, 160,170,180,190, 200,210, 220, 230, 240, 250, 260, 270 280, 290C, then an alert set indicating that a pan has boiled dry. The threshold may be changed based on whether a user is present based on the presence sensor, or on a sensitivity that the user can set, or based on whether a similar temperature has been experienced previously by the device but the alarm was cancelled.
If the temperature failed to stabilise below 90°C +/-f, then the pan is either dry or contains cooking oil. Continue to monitor the temperature, but this time monitor for the maximum value of Tp. If the value of Tp exceeds the smoke threshold for the likely range of oils encountered in cooking, then the alert would be triggered. The threshold for this value could be similar to the threshold of 315C recommended by CPSC, or it could be lower e.g. 180C to allow for temperature gradients in the oil, where the bottom is hotter than the top, or to allow for oils with lower smoke temperatures such as butter and to be alerted before the smoke appears, or it may be set higher to minimise the risk of false alarms. Fig 7 shows the temperature image of a frying pan just after the oil started to smoke. The centre of the pan is over 240C. The temperature of the centre of the pan during heating up is shown in Fig 8. The gradient of heating, reaching 150C after 60 seconds is indicative of a shallow frying pan on a reasonably high heat. A deep fat frying pan would take longer to reach the desired frying temperature, typically 5 minutes or more. Monitoring the warm up time and detecting a longer warm up time, above a threshold of say 3 minutes, but as much as 10 minutes, indicates that the user is deep fat frying, which can then be used to measure likely riskiness of cooking.
Note that the accuracy of the infrared sensor used for the test may not be very high, so thresholds in and temperatures in this patent and in actual devices may need to be adjusted to allow for variations in the sensor accuracy.
In an alternative embodiment of the algorithm, the step of locating the pan in the image using the circle or square detection may be skipped. All pixels in the image may be monitored for their temperature profile over time.
If a group of pixels which are close together exhibit the conditions outlined previously where they heat up, then stabilise at a temperature < 90C +/- f, then they are labelled as an image of a pan containing a water based food. They are then labelled as the region R is in the previous analysis and the monitoring overtime of a water based food operates as previously described.
If a group of pixels which are close together exhibit a rise rate which is below that previously measured for a hob with no pan on, and then continue to rise above a smoke or pre-ignition temperature for an oil filled pan as above, then an alert is triggered.
The device may keep a record of which locations it has previously detected heat, and is the location of the hot plates. Once it has located all hot plates (e.g. because it has counted 4 or 5 hot plates, or it has been set with the number of hot plates by means of user input, or a sufficient amount of usage has been detected that it is confident it has seen all hob locations), it can then detect fires caused by combustible materials. If a region outside the previous known hob location shows a temperature above a likely cooking temperature e.g. 250C, then it can trigger a potential fire alert. Alternatively, if a larger region of the image than is feasible for the most likely large pan that consumer might use is above a likely cooking temperature threshold of say 250C, then it can trigger a potential fire alert.
If large area ofthe image is above a threshold temperature e.g. 250C, but possibly as high as 350C or a value in between, and the temperature flickers above a threshold amount, e.g. +/-10C per second or as low as +/- 1C or as high as +/- 50C or any value in between, then an alert indicating a fire can be triggered.
The alerts for the different alarm conditions may be audible (e.g. a beep or siren noise, or a spoken alert e.g. 'warning, switch off hob' or another sound), and/or visible e.g. flashing light particularly for elderly or deaf users and/or remote e.g. a message to a smartphone or other device that would alert a user that had left the house or was in another room, or a carer who wasn't present but may be responsible for an elderly or vulnerable person.
There could also be different alert levels, e.g. 'warning, pan overheating' if it is a pre-ignition condition that is likely to still be easy to tackle by a user turning off a hob or removing a pan if safe to do so, or a 'warning, fire' if the temperature has hit a likely combustion point, or heat can be detected coming from regions of the image away from the pan or hob region R, indicating a fire has spread to other combustible materials.
The alert could be sent out either directly by radio link, or by other communication means known in the field e.g. Local Area Network connection or LAN such as WiFi®, Zigbee®, Z Wave or Bluetooth®, or a Wide Area Network, such as GSM®, SigFox®, LoRa® or other communication technology.
The alert could also go out to a central monitoring station, as could a message saying whether the pre-ignition condition was controlled or whether it led to a fire situation. An automated alert to the fire services could be sent based on this.
In an alternative embodiment, additionally or alternatively to an alert, the device could automatically switch of the gas or electric power to the hob.
In another alternative embodiment, the device could automatically trigger a fire suppression system such as a sprinkler or other device.
The device is likely to be of interest to insurance companies, and also to careers of vulnerable people, who in addition to wanting to reduce the likelihood of fires, would like to be able to measure and characterise the cooking behaviour of households to determine likely fire risk.
The data gathered could therefore comprise any of:
Frequency of cooking
Frequency and duration of leaving cooking unattended.
Frequency and duration of frying food.
Time for oil pan to heat up (longer heat up period indicating deep fat frying) and frequency of occurrence of deep fat frying.
Frequency of pre-ignition conditions.
Response time to pre-ignition alerts.
Time of cooking (late night cooking may be more likely to be performed whilst tired or under the influence of alcohol, increasing fire risk)
This data could then be sent back to a monitoring system which could determine likely fire risk based on the above metrics.
Because the device may not have access to mains power, it is preferable that it draws low power to extend battery life, or be able to run on energy harvesting technology. The device may therefore stay in a sleep state to reduce power consumption, and then wake up intermittently e.g. once per minute, but maybe as low as once every 10 seconds or as high as every 5-10 minutes or any period in between. When it wakes up it may take a single heat measurement, either using the infra-red sensor, or a secondary heat sensor, and if the measured temperature does not indicate the hob is in use, go back to sleep. The wake interval could be varied by time of day, for example being more frequent at meal times and less frequent in between.
Alternatively, or additionally, it may monitor a presence sensor e.g. a PIR and only go into full power mode when motion near the hob is detected.

Claims (25)

1. A method for monitoring the use of a cooking hob using an infrared imaging sensor to take images of the cooking hob and infer from them fire, fire risk or pre-ignition conditions based on a) locating hot plates, burners or pans within the images based on their temperature; b) using the images to measure the temperature of the hot plates, burners or pans during usage; c) classifying each hot plate, burner or pan into likely cooking types, or as a hob with no pan on, based on temperature rise rate and level; d) applying different fire, fire risk or pre-ignition conditions for each of the different cooking types or hob with no pan on; and e) alerting and/or recording when the fire, fire risk or pre-ignition conditions are met.
2. A method as claimed in Claim 1 wherein alerting comprises sending alerts to a remote monitoring system.
3. A method as claimed in Claim 1 or Claim 2 wherein alerting comprises an audible alert.
4. A method as claimed in any preceding claim wherein alerting comprises a visual alert.
5. A method as claimed in any preceding claim wherein a presence sensor is used to detect if a user is attending to the hob.
6. A method as claimed in Claim 5, which varies the alerting method and fire, fire risk or pre-ignition conditions depending on whether a user is attending to the hob.
7. A method as claimed in Claim 1 or Claim 5 wherein other usage data such as type of cooking, frequency of alerts, time of cooking, whether cooking was left unattended, is collected or transmitted to a monitoring system.
8. A method as claimed in Claim 1 where the location of the pans or hot plates is based on detecting circles or squares in the images.
9. A method as claimed in Claim 1 where the location of the pans is based on measuring a temperature profile over time of pixels in the images and comparing the temperature profile to known profiles for different cooking types.
10. A method as claimed in Claim 1, implemented with a device in which the device switches off the cooking hob if fire, fire risk or pre-ignition conditions are met.
11. A method as claimed in Claim 1 implemented with a device wherein a user can reduce the sensitivity of the device to reduce false alarms.
12. A method as claimed in any preceding claim, in which having located the hot plates, alerting occurs when a temperature above a threshold is detected in a region that does not correspond to the locations of the hot plates.
13. A method as claimed in any preceding claim, in which if an area of the image is above a threshold temperature, and the temperature flickers above a threshold amount, this is taken as indicating the presence of flames and an alert is provided.
14. A method as claimed in any preceding claim wherein the alerts differ depending upon the conditions that trigger them.
15. A method as claimed in any of claims 1-9 or 12-14, implemented with a device wherein the device is switched on and off intermittently between a wake state and a sleep state.
16. A method as claimed in Claim 10 or Claim 11, wherein the device is switched on and off intermittently between a wake state and a sleep state.
17. A method as in Claim 15, or Claim 16, wherein the wake state is activated by a presence sensor detecting a user attending to the hob.
18. A device comprising an infrared sensor for taking an image of a cooking hob, and configured to implement the method of any any of Claims 1-9 or 12-14.
19. A device as claimed in Claim 18, which reduces its power consumption by operating in a sleep mode when no hob use and/or user presence is detected.
20. A device as claimed in Claim 18 which having located the hot plates, alerts when a temperature above a threshold is detected in a region that does not correspond to the known locations ofthe hot plates.
21. A device as claimed in Claim 18 which alerts when a large area of the image is above a threshold temperature
22. A device as claimed in Claim 18 which alerts if an area of the image is above a threshold temperature, and the temperature flickers above a threshold amount, indicating the presence of flames.
23. A device as claimed in Claim 18, adapted to switch off the cooking hob if fire, fire risk or preignition conditions are met.
24. A device as claimed in Claim 18, wherein a user can reduce the sensitivity of the device to reduce false alarms
25. A device as claimed in Claim 18, wherein the device is adapted to be switched on and off intermittently between a wake state and a sleep state.
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