Classifications

• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
  • G05D23/00—Control of temperature
  • G05D23/19—Control of temperature characterised by the use of electric means
  • G05D23/20—Control of temperature characterised by the use of electric means with sensing elements having variation of electric or magnetic properties with change of temperature
  • G05D23/24—Control of temperature characterised by the use of electric means with sensing elements having variation of electric or magnetic properties with change of temperature the sensing element having a resistance varying with temperature, e.g. a thermistor
• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
  • G05D23/00—Control of temperature
  • G05D23/19—Control of temperature characterised by the use of electric means
  • G05D23/1902—Control of temperature characterised by the use of electric means characterised by the use of a variable reference value

Definitions

• the present invention relates to a method for controlling a domestic electric liquid heating appliance, such as a kettle. Such a method is particularly intended to control the power supply of the electric heating element component such a household appliance.
• the present invention also relates to a device for controlling a domestic electrical liquid heating apparatus, for implementing the method that is the subject of the present invention.
• Domestic electrical appliances for liquid heating comprise a reservoir for containing the liquid to be heated, and a cup generally positioned on the bottom of the tank and for transmitting the calories to the liquid to be heated.
• this cup is heated by a heating element which, traversed by an electric current, dissipates electric energy by Joule effect.
• the electrical power dissipated by the device obviously depends on its dimensions, but it rarely exceeds 3000 W.
• such a household appliance necessarily comprises a control device for controlling the power supply of the heating element, depending in particular on the set temperature at which the user of the appliance wishes to heat the liquid.
• a control device comprises a thermometric element able to measure, directly or indirectly, the temperature of the liquid being heated.
• this thermometric element carries out measurements representative of the temperature of the heated liquid, which makes it possible to control the electrical supply of the heating element.
• thermometric element may indeed be positioned within the volume of liquid to be heated, thus making direct measurements, or under the cup at a distance from the heating element so that indirect measurements can be made.
• the conventional kettles of the prior art comprise an aluminum plate for distributing the heat, associated with the heating cup, generally made of stainless steel and a large electric heating resistance.
• a switch whose lever is triggered by a bimetallic spring formed of two materials of different expansion coefficients.
• the bimetal spring is placed at the outlet of a channel communicating with the upper part of the tank containing the heated liquid.
• a control device of the liquid heating apparatus does not allow to interrupt the heating cycle when reaching a predetermined temperature and below the boiling temperature.
• the bimetallic spring is a thermometric component of poor sensitivity. It reacts only to a large amount of steam transmitted through the channel. However, in practice, this quantity is produced only when the liquid has reached its boiling point, ie 100 ° C. for water. Therefore, a control device including a bimetallic spring does not control the heating cycle of the apparatus at different temperatures.
• the usual control methods of such household appliances most often provide for heating the liquid to a set temperature. This setpoint temperature can be adjusted by the user by means of a selection member. In most applications, the set temperature can be set between 50 and 98 ° C.
• the temperature of the liquid at the end of a heating cycle does not always correspond exactly to the set temperature.
• the temperature of the liquid is not necessarily uniform throughout the volume defined by the reservoir, and secondly it sometimes exceeds the set temperature or conversely it remains slightly lower than it. This approximation is a function of the performance of the device and the control method implemented.
• a first solution consists in continuously measuring the temperature of the liquid being heated and then interrupting the power supply of the heating element as soon as the set temperature is reached or exceeded.
• the prolonged exceeding of the set temperature may cause a loss of liquid by evaporation, or overflow if it is a liquid other than water, as for example in the case of a milk frother.
• US-4,948,950 proposes an alternative method of reducing the electric power supplied to the heating element so as to slow down the increase in temperature. liquid when approaching the set temperature.
• the decrease in the dissipated electrical power involves slowing down the heating of the liquid and thus lengthens the duration of the heating cycle. This elongation is all the more important as the volume to be heated is important. However, the duration of the heating cycle is a paramount parameter for the user of the device. Therefore, such a method is not really satisfactory either.
• a household appliance thus equipped and operating according to such a method therefore has drawbacks which may prove prohibitive for sale, given the high degree of competition existing in the field of household appliances.
• the object of the present invention is therefore to provide a method and a device for controlling a domestic electric liquid-heating appliance which does not have the drawbacks of the prior art, ie it can be heated without exceeding the temperature and does not require not the implementation of a bulky control device and expensive.
• the present invention therefore relates to a method for controlling a household electric liquid heating appliance, such as a kettle, to achieve a set temperature relatively accurately, quickly and economically.
• the household electrical appliance comprises a reservoir intended to contain the volume of liquid to be heated, a thermometric element able to carry out thermometric measurements representative of the temperature of the liquid, and a cup associated with an electric heating element whose electrical supply is controlled according to these thermometric measurements, so as to heat the liquid to a set temperature, in particular a temperature programmed by the user.
• this method comprises the steps of:
• the iterative step ending when the current temperature level is higher than the target level.
• the actual heating time is corrected in real time as a function of the quantity of water to be heated.
• this volume of water contained in the reservoir is “evaluated” by measuring the temperature variation over a predetermined period of time, and then the target temperature level is adjusted according to the evaluated volume, so as to interrupt the heating as soon as possible. from the moment the set temperature is reached.
• the total heating time of the liquid is adapted to the volume of liquid to be heated, so as to take into account thermal and / or metrological inertia may occur during the control process of the device.
• the duration of the heating cycle is the optimal time to reach the set temperature. In other words, there is no unnecessary increase in the duration of the heating cycle.
• the purpose of this method is to interrupt the heating cycle precisely, that is to say when the liquid is at a temperature as close as possible to the set temperature.
• the method may include an initial step of verifying that the current temperature level is below the initial target level.
• the initial value of the target level can be determined by a calculation of multiplying the set temperature by a first determined coefficient and removing the product obtained from a number depending on the characteristics of the thermometric element.
• the value of the target level can be determined by a calculation consisting of forming the difference between the first and second registers, to multiply said difference by a second determined coefficient. Then, according to this embodiment, it is necessary to subtract from the product obtained a product of the set temperature by a third determined coefficient and add to the result obtained a number. For this calculation, the coefficients depending on the characteristics and the position of the thermometric element, the number depending on the temperature of the
• the temperature variation here consists of the difference between two of the levels indicated by the thermometric element.
• the set temperature may be between 50 ° C. and 98 ° C.
• the heating cup can be thin and the thermometric measurements can be performed by a negative temperature coefficient resistor sized and positioned at this cup so as to produce potential differences between its terminals representative of the temperature of the liquid.
• thermometric component is indeed able to provide measurements in a relatively fast and accurate manner, which leads to a good speed in the control of the domestic liquid heating apparatus.
• thin means a cup having a thickness typically between 0.8 and 1.2 mm, this cup being advantageously made of a metal, so a good thermal conductor.
• the heat capacity of the cup is thus determined by the material which constitutes it and by this thickness.
• this negative temperature coefficient resistance can be positioned on this thin cup within a region free of the electric heating element, which therefore remains relatively cold during heating of the liquid.
• thermometric measurement is quickly and directly representative of the temperature of the water contained in the kettle, because it limits the number and thermal inertia components involved in the thermometric measurement.
• the power supply of the electric heating element can be controlled by an electronic device positioned within this so-called "cold" region.
• this electronic device can be positioned near the negative temperature coefficient resistor.
• the invention also relates to a device for controlling a domestic electrical liquid heating apparatus according to the method of the invention.
• this device comprises a resistor with negative temperature coefficient sized and positioned in said apparatus so as to produce potential differences between its terminals representative of the temperature of said liquid, a switching element, such as a relay or a TRIAC, for switching on or off the power supply of the heating element, and a storage unit having at least two registers for storing temperature levels.
• a switching element such as a relay or a TRIAC
• the duration of the heating cycle and its accuracy are optimized.
• the duration of the heating cycle is interrupted when the liquid is at a temperature as close as possible to the set temperature.
• the device may comprise a thermal safety component controlling the stopping of the heating method that is the subject of the invention when the rate of increase of the temperature of the thermometric element is greater than a predetermined limit speed, so as to interrupt the heating in case of absence of liquid.
• Figure 1 is a flowchart showing a sequence of steps to achieve the method object of the present invention.
• FIG. 1 thus represents the succession of steps making it possible to complete a complete heating cycle, that is to say until the liquid contained in the tank of a household electric heating appliance reaches the set temperature.
• the liquid to be heated is usually a liquid food, such as water, milk etc. In the example developed below, it is water.
• the liquid heating apparatus comprises a reservoir for containing the volume of liquid to be heated, at the bottom of which is a stainless steel cup capable of transmitting the heat dissipated by Joule effect in the heating element located under the cup.
• the apparatus is equipped with a thermometric element able to carry out measurements representative of the temperature of the liquid being heated.
• the thermometric element is a variable resistance with a negative temperature coefficient (abbreviated as "CTN resistance").
• a CTN resistor has between its terminals, at constant current, a potential difference U which decreases as its temperature increases.
• the resistance CTN is positioned in the dry, under the cup in the area where the heating element is located. It is thus able to play the role of a thermometric element performing measurements representative of the temperature of the liquid.
• this resistance CTN is positioned under the cup if we consider the kettle with the cup at the bottom.
• the NTC resistance is positioned "on" the cup in the sense that it is attached to its surface.
• the cup has a relatively small thickness, for example of the order of 0.8 to 1.2 mm.
• the measurements made by the NTC resistance are little affected by the thermal inertia of the cup and they are more representative of the temperature of the liquid being heated.
• the NTC resistance is reported on the cup within a "cold" zone, from which the heating element is absent, which may be in the form of an electrically resistive track screen printed on the cup.
• the "cold" zone further houses an electronic circuit for controlling the power supply of the heating element.
• the CTN resistor is positioned near this electronic circuit.
• the first step shown in the flowchart of Figure 1 is to turn on the kettle.
• the user switches a switch which in turn authorizes the power supply of the element. heated by the device control device.
• the set temperature that the user can select is between 50 ° C. and 98 ° C.
• control device of the apparatus comprises a thermal safety component capable of detecting the dry operation of the appliance, in the event that the user has not introduced liquid into the tank.
• a safety component generally comprises a fuse element and its role is to interrupt the heating in the absence of liquid. Such an interruption, not shown in the flowchart of Figure 1, occurs if necessary very quickly after switching on the device in step 1 in case of vacuum heating.
• step 2 also comprises the detection of the set temperature T con set by the user by means of a selection member, for example a potentiometer, provided in a conventional manner on the heater.
• a selection member for example a potentiometer
• Uc Uc
• Uc (O) initial value
• NTC the characteristics of the resistance
• Uc the resistance of the tank of the apparatus
• T con the set temperature selected by the user.
• Uco is a reference value depending on the supply voltage or current of the CTN resistor
• Co is a correction coefficient also dependent on the supply voltage or current of the CTN resistor.
• Uco is 310 and C 0 is 2.
• step 3 is a test step of verifying that the current temperature level, represented by Ui, is lower than the initial target level, represented by Uc.
• the NTC resistance is by definition negative temperature coefficient, this condition equals inequality [Ui ⁇ Uc] -
• a guard number Uci in this case 10, which is subtracted in the first member of the inequation, is added to the inequality.
• the Uc - Uci value corresponds to a temperature located at 5 0 C below the desired temperature. This limits the effects of thermal inertia, which can be significant. In addition, this makes it possible to avoid starting a heating cycle when the temperature of the liquid is close to the set temperature, ie when the difference is less than 5 ° C., which does not justify starting a cycle. of heating.
• the next step 4 is to stop the power supply of the heating element, thus to stop the heating of the liquid.
• the current temperature of the water is lower than the set temperature T con , so that it is necessary to continue heating the liquid.
• the temperature of the liquid that is being heated is measured in known manner by means of the voltage between the terminals of the NTC resistor, which is itself proportional to the temperature of the NTC resistor.
• step 5 is a standby step while the liquid continues to heat up.
• This waiting step ts has a duration, here 8 s, which is predetermined according to the characteristics of the device.
• an iterative step 6 is then carried out consisting of: "feeding the heating element;” modifying the target level as a function of the set temperature and the variation of the current temperature level over the reference period slippery having a predetermined duration.
• the iterative step ends when the current temperature level, measured at regular intervals, is higher than the target level.
• the variation of this current level of temperature is calculated by difference, but it could also be a ratio.
• step 6 comprises measuring the current temperature level and assigning it to a second register U 2 .
• Step 6 it can also be verified that the user has not changed the setting of the set temperature, as this may require premature shutdown of the liquid heating. Indeed, if the user sets a new setpoint temperature lower than the previous and especially the current temperature of the liquid, there is no need to continue to heat the latter.
• Step 6 also consists, in accordance with a characteristic of the method that is the subject of the invention, of modifying the target temperature level as a function of the target temperature and of the variation of the current temperature level over the sliding reference period exhibiting a predetermined duration.
• the value of the target level Uc is determined by a calculation consisting of forming the difference between the first and second registers, to multiply this difference by a determined second coefficient. , subtracting from the product obtained a product of the set temperature T con by a determined third coefficient, then adding to the result obtained a given digital voltage value.
• the coefficients and the numerical value making it possible to carry out this calculation depend on the characteristics of the apparatus, and in particular those of the thermometric element, namely the NTC resistance, the capacity of the tank of the apparatus and the electric power. dissipated in the heating element, at most 3000 W.
• the numerical value of the voltage Uco is 295
• the second coefficient Ci is 2.5
• the third coefficient is 2 and it corresponds to the correction coefficient Co defined in relation to step 2.
• the correction term is then : [-15 + 2.5 (Ui - U 2 )].
• the iterative step 6 ends when the current temperature level is higher than the target level, that is to say when the value of the second register U 2 is lower than the value of the calculated target level Uc
• This iteration stop therefore implies the implementation of a test step 7, characterized by the inequality [U 2 ⁇ Uc] -
• this inequation is not verified, that is to say when the temperature is still lower than the set temperature (target level Uc not reached), it is necessary to continue heating the liquid. For this, this iterative step is repeated after a time delay t "of a given duration, here of one second.
• the reference period has a fixed duration, in this case 8 s, but it "slides" at each iteration so as to continuously evaluate the temperature variation throughout the heating cycle.
• This fixed duration is predetermined and corresponds to the duration of step 5.ElIe is determined according to the characteristics of the apparatus. This predetermined duration is in this case 8 s.
• step 6 makes it possible to measure the variation of the current temperature level over the sliding reference period having a predetermined duration of 8 s. Therefore, the storage unit equipping the control device object of the invention must have enough registers to store the current temperature levels at regular intervals during the reference period. In practice, the number of registers required therefore depends on the duration of the reference period and the storage frequency chosen. This number is therefore [1 + ts / 1 "] registers.
• the beginning of the reference period corresponding to the register Ui slides one second at each iteration, so as to remain constantly eight seconds behind the end of this reference period corresponding to the value recorded in the second register U 2 .
• a storage unit is implanted in the control device of the apparatus, so as to memorize eight values each corresponding to the temperature of the liquid being heated, respectively at each second marking a new iteration of the device. loop materialized by steps 6 to 8.
• the voltages at the terminals of the resistance CTN which represent the temperature levels of the liquid being heated, are thus stored according to the principle of "first in last out” (or FILO of the English “First In hast Out “).
• the storage unit therefore has eight sliding values renewed at each iteration by recording the current temperature level.
• the method which is the subject of the invention is therefore designed so as to modify the target level of temperature as a function of the volume of heated liquid.
• the voltages across the NTC resistance measured over a reference period having a predetermined duration, typically of the order of 8 s, are measured in a sliding manner.
• the invention offers the possibility of modulating the target temperature level, so the heating time, depending on the volume.
• the device and method that is the subject of the invention make it possible to adjust the duration of a heating cycle as short as possible. necessary to reach the set temperature and this, without resorting to a device for controlling and varying the electrical power delivered to the heating element.
• the present invention thus makes it possible to achieve the target temperature with improved accuracy, while achieving savings in terms of cost and space for a home appliance operating according to its principle.
• the speed of the heating cycle is not obtained to the detriment of the accuracy of the heating by the device to achieve the target temperature with a minimum overshoot.
• the liquid heating appliance described in the example could equally be a milk frother, a sauce warmer or a device for the preparation of any hot beverage, such as tea, chocolate or coffee.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Automation & Control Theory (AREA)
• Cookers (AREA)
• Heat-Pump Type And Storage Water Heaters (AREA)

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• 2006
  • 2006-08-21 FR FR0653418A patent/FR2905016B1/fr not_active Expired - Fee Related
• 2007
  • 2007-08-17 WO PCT/FR2007/051828 patent/WO2008023131A2/fr active Application Filing
  • 2007-08-17 EP EP07823728A patent/EP2054784A2/de not_active Withdrawn

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