GB2463937A

GB2463937A - A self-calibrating temperature sensor for a kettle

Info

Publication number: GB2463937A
Application number: GB0818098A
Authority: GB
Inventors: Colin Lawrence Amess
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-10-03
Filing date: 2008-10-03
Publication date: 2010-04-07
Anticipated expiration: 2028-10-03
Also published as: GB2463937B; GB0818098D0

Abstract

A kettle comprises an adaptive self-calibrating temperature sensor 5. A heating plate 3 and temperature sensor 5 are arranged in thermal contact with water, or with a vessel containing water. Electronic control means (not shown) and computing means (not shown) calibrate the temperature sensor 5, and derive the heat transfer functions between the heating plate, the temperature sensor, and the quantity of water to be heated (i.e., determine how much power is required and for how long). A non-volatile memory means (not shown) retains the calibration and heat-transfer functions even when power is removed from the kettle.

Description

SELF CALLIBRAT1NG ADAPTIVE PREDICTIVE TEMPERATURE CONTROL FOR AN ELECTRIC WATER HEATING VESSEL OR KETTLE.

This inventions relates to the control of electric kettles, and in particular, to allow the kettle to predict and apply the minimum amount of power to heat whatever quantity of water should be present in the kettle to the desired temperature.

In the case of electric kettles, existing kettles generally use a generic topology comprising an electrical heating source to indirectly heat a thermally conductive distribution plate whereby the surface of the distribution plate is in contact with or comprises the the body of the vessel containing the water to be heated. Alternatively, the temperature sensor is mounted inside a waterproof tube or conduit that is rooted to the inside base of the kettle, the heating base forming a composite of the bottom of the kettle.

According to the teachings of W02007131271 if the temperature sensor is mounted to the heat distribution plate, the mounting location greatly reduces the accuracy of the temperature sensor. The temperature sensor senses the temperature of the heat distribution plate and does not directly sense the temperature of the vessel's contents. Because of this, discrepancies may arise between the measured temperature and the actual temperature of the contents. W02007131271 asserts that for a kettle, this may result in the kettle switching off before the water is actually boiling.

Furthermore, an inaccurate temperature sensor limits the potential functionality of the heating vessel. Since the temperature of the vessel's contents is not accurately sensed, only a limited range of functions controlled with reference to an approximate temperature reading are possible. For example, in the case of a kettle, it is only possible to stop the kettle boiling based on an approximate boiling point.

This invention describes a means wherein the limitations due to the geometry applied in mounting the temperature sensor as described by the teachings of W02007131271 are substantially overcome by introducing an electronic self calibrating and adaptive control system that compensates for the temperature effects due to the the migration of heat from the heat distribution plate into the temperature sensor and thereby allowing discrimination between the temperature rises of the sensor due to the effects of heating by the water and heat distribution plate.

The invention also overcomes the need to provide accurate a priori calibration of the temperature sensor during the manufacturing process and to provide compensation for factors dependent upon the geometry of the distribution plate and degree of thermal insulation between distribution plate and temperature sensor implicit within the teachings and inventions of W02007131271. 0 The invention further overcomes the need to factory fix the setting of a priori time delays within the sensor controls to compensate for differing mounting sensor and conduction plates geometry and the resulting thermal lag/lead relationships between the heat conducting plate, temperature sensor and water to be heated.

The invention also provides for the automatic calibration the temperature sensor and further provides for use in differing geographic locations where the local atmospheric pressure causes water to boil at temperatures above or below 100 degrees Celsius.

Summary of the invention.

According to the invention as described in claimi There is provided a heating and control means to heat water wherein the control means is capable of automatic and adaptive self calibration. The control means is able to adapt its control of the heating source to allow the precise amount of power required to raise the temperature of the water to the required temperature level. The controller is also able to adapt a calibration profile to compensate for the different temperatures that water boils at under differing atmospheric pressures as experienced in differing geographic locations.

The controller monitors the electrical output generated by a temperature transducer that is in thermal contact with a vessel holding water that requires heating. The temperature transducer may be in varying degrees of uncalibrated thermal contact with the heat distribution plate. Electrical signals from the temperature transducer are input to a controller. The controller is programmed to enable power to the heat distribution plate and measure incremental changes in the output of the temperature transducer over time. Under the control of program algorithms, the micro controller creates a predictive model of the kettles heating performance or transfer functions by forming a multi dimensional parametric relationship between the amount of water being heated, the electrical characteristics of the temperature sensor, the temperature lead or lag relationships between heat transfer and temperature between the heated vessel,the water in the vessel, the heat distribution plate and the temperature transducer. The controller may also have signal or data inputloutput means to provide a data path between the controller and external computer or memory means wherein the external computing or memory means is able to effect the modification of the control algorithms and program means within the controller. Such input/output data means may also be used for the transfer of kettle performance and usage data from the kettle into external means for usage or diagnostic purposes. The controller may also have further signal means communicating input output signals to one or more of the controllers input/output ports for ancillary switches, controls and/or indicators to allow the user to input commands to the controller and receive visual or audible confirmation that the command has been accepted or completed, for example, pressing a switch a certain number of times in quick succession, could be used to initiate a new calibration profile. A light or sounder output by the controller could signal that a new calibration profile has been initiated and completed. Once automatic calibration of the kettle has been achieved, it should be obvious that the ancillary controls and indicators may facilitate the means to select numerous heating functions that may be made available by controller, for example keep warm', or low water level' or boil in one hour, sound an alarm and switch off' functions

Brief description of the drawings

Embodiments of the invention will now be described with reference to the drawings, in which: Fig 1 shows a schematic cross sectional representation of a generic kettle base.

Fig 2 shows a schematic cross sectional representation of a generic kettle base wherein the temperature sensor is mounted in a tube or conduit inside the kettle.

Fig 3 is a graph showing the thermal response of components within the kettle base and water temperature within the kettle.

Fig 4 is a graph showing the temperature of the temperature transducer A computing means such as a microprocessor or micro controller with internal or external flash memory is used to compute and store calibration constants into operational memory. The microprocessor or micro controller contains the following means, analogue and digital input ports, output ports, ALU, volatile and non-volatile memory means, the computing means is able to compute or operate on input data or information according to a program or programs held in memory. The computing means is operationally connected to the temperature sensor and a power switch or switches to a heating element or elements. Henceforth these means will be collectively referred to as to as the controller.

Referring to figure 1, a heating element I is used to heat a heat conduction plate 3, the heating element us electrically insulated from the conduction plate 3 by a thin layer of electrical insulation 2. An electrical temperature transducer 5 is mounted in a substantively thermally decoupled portion of the heat conduction plate. The thermal decoupling is effected by the use of a heat insulating jacket 6 acting as a barrier between heat conduction plate 3 and temperature transducer 5. When a kettle is placed on the kettle base, the under surface of the kettle is substantially in thermal contact with the heat distribution plate surface 4 and the top surface of the electrical temperature transducer 7.

Referring to figure 2, a heating element 1 is used to heat a heat conduction plate 3, in this case, the conduction plate also forms base of the kettle in direct contact wth water inside the kettle. the heating element 1 is electrically insulated from the conduction plate 3 by a thin layer of electrical insulation 2. The temperature sensor 5 is mounted inside a tube or conduit 13 rooted to the kettle base and extending up into the inside of the kettle.

Referring to fig3, the graphical representation shows four traces monitoring temperature against time for components of a kettle base while heating a kettle of water. Trace 8, represents the temperature of the heat conduction plate. Trace 9 represents the temperature of a temperature transducer only lightly thermally decoupled from the heat distribution plate.

Trace 11 represents the temperature of a temperature transducer heavily thennally decoupled from the heat conduction plate. Trace 10 represents the temperature of* water within the kettle.

Referring to fig 4, trace 12 shows the output from the temperature sensor when the heating of the kettle is carried out in controlled steps. the controller is employing an adaptive predictive switching algorithm that adapts duration of successive power switching to the heater element 1, wherein a successive approximation method is based upon the incremental quantity of energy supplied, the responses from the temperature transducer and the heating lag, cooling tag, rate of rise of temperature and the absolute temperatures achieved.

According to the first aspect of the invention hereafter referred to as Casel, The controller effects an absolute calibration of the kettles temperature sensor 5 and extracts the transfer functions of the kettle heater in parametric terms relating to the rate of conduction of energy from heating element to into the water and the lag/lead relationships between the temperature of the water being heated, the energy being conducted by the heat distribution plate 3 and the temperature of the temperature sensor 5.

As can be seen from figure 2, The degree of thermal decoupling of the temperature transducer 5 from the heat conduction plate 3 will directly affect the lag or lead the temperature of the water in the kettle and the temperature of the temperature sensor as shown by traces 9 and 10. if the temperature transducer is heavily thermally de-coupled from the heat conduction plate, the temperature of the temperature transducer can lag the temperature of the water in the kettle as shown by traces 10 and 11.

In case 1, the kettle is filled with a measured quantity of water. At switch on, the controller enables power to the heating element 1 and the controller times and records such period ti until the electrical output of the temperature sensor sensor 5 as measured by the analogue input of the micro controller, reaches a substantially constant rate of rise of voltage. The controller maintains power to the heating element for a further period t until the sensor reading is increased to a convenient multiple, for example but not exclusively two times the value returned at room temperature.

The controller then records the change of sensor output voltage as a function of the linear temperature change, DT, of the sensor temperature in the period t.

The controller then removes power from the heating element and times a cooling period t2 until the temperature transducer 5 output becomes substantially constant steady state.

The controller computes and records the temperature lag/lead heating and cooling time constants in periods ti and t2.

From the known energy (DQ) applied in period t and known mass of water (lvi) in the kettle, the controller computes the change in temperature (DT) and since DT K*DQIM the linear sensor transfer constant in volts/degree (K) is determined from DQ the quantity of heat applied in period (t) Watt seconds.

The controller again allows power to the heating element until kettle boils and sensor output is detected as being steady state by the controller. The controller then equates the temperature sensors output to denote boiling point at the locations altitude.

the controller computes a linear temperature fit on T = (K*Q/M) +C where C is a sensor bias or offset, the value of C is also computed by the controller.

the controller then stores the temperature sensors absolute calibration constants in non volatile memory along with the heating and cooling thermal lag/lead time constants.

According to the second aspect of the invention hereafter referred to as Case2, The controller effects a calibration relative to the kettle filled to an inexact level, for example, very approximately half full.

Water is added to the kettle, to say approximately half full, the controller applies power to the heating element and conducts a linear calibration fit as in case 1 excepting the derived values now form an estimated K from estimated M. The * controller also computes and records the temperature lag/lead time constants in periods ti and t2. as in case 1 and records them into non volatile memory.

The next and subsequent times the kettle is used, controller measures the ratio metric rate of rise of the temperature sensors output after a period ti compared to the amount of water used in the original approximate half filling. The controller then computes * the ratio of mass of water in the kettle relative to the amount of water used in the approximated first half filling and computes the total time that power will have to be maintained to just bring the water to boiling point.

Adaptive relative calibration.

According to the third aspect of the invention hereafter referred to as case3, The controller effects an adaptive relative calibration.

As a first iteration, the controller uses an estimated K as in case 2. The controller then monitors the relative rate of rise of temperature over a number of uses of the kettle to establish a statistical profile of the relative quantities of water used over these usages.

The controller computes a rolling estimate of most frequently used quantity of water and compute a value K freq or K modal along with their associated values of ti and t2 The controller then stores the value of K freq of K modal as as the new relative sensor calibration constants for the N+ 1 use of the kettle.

Predictive switching According to the fourth aspect of the invention hereafter referred to as case 4, The kettle is switched on and the controller computes the rate of rise of temperature of the temperature transducer 5 and calculates the time it will take for the sensor change from its output reading as reported at the moment heating power is applied to reach the output reading required to indicate that there has been sufficient power applied to the kettle to bring the water to the desired temperature. It should be noted that the temperature velocity constants and ti and t2 temperature lead/lag time constants used in the computations can be derived from case 1, case2, or case3.

Adaptive Predictive switching.

Case 5 Parametric Adaptive Predictive switching The controller computes the estimated required heating time as described in case 1 and applies power for a reduced period, for example, but not exclusively, 80% of the Case 4 estimate. The controller then waits until the temperature sensor returns a slow rate of change of temperature, the rate of change being either negative or positive depending on the level of thermal decoupling between the heat conduction plate and the temperature sensor. The rate of change of sensor temperature is used to identify that the sensor is very close to the temperature of the heated water. It should be obvious that for heat exchange systems where the heat exchange plate has any significant thermal coupling to the temperature sensor, the sensor may read above boiling and once power is removed the sensor will experience a rapid temperature drop until it reaches a temperature close to the water temperature. The controller again switches on the heater and computes the new estimated heating time in the same manner as previously described. The iterations are repeated until a minimal estimated heating time is returned. The controller then creates an entry on a look up array linking rate of rise of temperature, the duration of each heating period and the percentage level of temperature sensor signal away from a boiling point signal at the beginning and end of each successive heating cycle. The controller then performs a piecewise linear fit or integration and stores the corrected estimated heating time required in a second array also recording the associated estimated mass of water and temperature differential to be heated through. The next time the user switches on the kettle with substantially the same volume of water at a very similar start temperature, the deviation from boiling from the temperature sensor together with the value returned from the rate of temperature rise may be used by the controller as a two dimensional address pointing to the total time required to switch on the heating source to enable the water to just reach boiling point If sufficient memory is provided to create large arrays, the controller can interpolate between values returned in caseS and infill the timing array thereby improving the resolution of the estimated heating time required.

Fuzzy logic switching.

The Controller may compute a fuzzy logic' approximation of case or cases, 1,2,3, 4,or 5 above.

Standby temperature mode.

Once the Kettle has been through a self setting temperature calibration cycle, the controller may be instructed with a command to either maintain or cycle the water temperature at any percentage point between room temperature and boiling point. It should be obvious to one skilled in the art, that any number of combinations of electrical switches and indicators may be interfaced with the controllers input/output ports to allow the communication of the command selection of the chosen percentage point and mode.

The controller may also be have its low voltage power supplies and one ore more data ports connected to a plug or socket arrangement to facilitate the updating of any control algorithms to be held in flash ram. The data port/ports may also be used to transfer the kettles performance data to a second communicating device such as a personal computer or other dedicated communications device.

In all cases previously described, The controller may be programmed to remove power if boiling is detected to exceed a period or a too great a rate of change of temperature is detected. Power may then only be re applied if a reset mechanism is invoked under user intervention.

Claims

CLAIMS.1 / A self calibrating adaptive predictive temperature control for a kettle or electric heating vessel comprising: a heating plate and temperature sensor arranged to be in mutual thermal contact with water or a vessel containing water that is required to be heated to a specific temperature, an electronic control and computing means to auto calibrate the temperature sensor and to adaptively compute the parametric heating transfer functions between the heating plate, the temperature sensor and the quantity of water to be heated with respect to time and the power applied, a non volatile memoty means to retain the calibration and parametric transfer functions when power is removed from the kettle.

2/ A self calibrating adaptive predictive temperature control for a kettle or electric heating vessel as claimed in claim 1, further comprising: a computing means to construct a virtual predictive model to determine the time the controller needs to apply power to the heating element in order to bring the temperature of the quantity of water in the vessel to the required temperature.

3/ A self calibrating adaptive predictive temperature control for a kettle or electric heating vessel as claimed in claim 1 or claim 2, further comprising: a display means to indicate the status of the calibration cycle, the operating mode and the temperature of the water in the vessel, a switch or switches to allow the user to select the operational modes of the controller and the desired temperatures of the water to be heated.

4/ A self calibrating adaptive predictive temperature control for a kettle or electric heating vessel as claimed in claim 4, further comprising: an embedded programming algorithm or algorithms to allow the controller to respond to the selection from a range of heating modes and temperature cycling profiles.

5/ A self calibrating adaptive predictive temperature control for a kettle or electric heating vessel as claimed in any predicating claim, further comprising: an embedded fuzzy logic algorithm or algorithms to allow the controller to emulate the parametric modelling the heating system.

6/ A self calibrating adaptive predictive temperature control for a kettle or electric heating vessel as claimed in any predicating claim, further comprising: an audible sounder to signal the status of the calibration or heating cycle.

7/ A self calibrating adaptive predictive temperature control for a kettle or electric heating vessel as claimed in any predicating claim, further comprising: an overheating detection circuit and excessive rate of rise of temperature detector wherein the controller removes power from the heating element until a reset button is activated via user intervention.

8/ A self calibrating adaptive predictive temperature control for a kettle or electric heating vessel as claimed in any predicating claim, further comprising: an electronic communication port allowing the kettles control algorithm/s to be updated or otherwise modified via an external serial device.

9/ A self calibrating adaptive predictive temperature control for a kettle or electric heating vessel as claimed in any predicating claim, further comprising: an electronic communication port allowing the kettles performance data and usage history to be extracted via an external serial device.