EP2282613A1

EP2282613A1 - Cooking apparatus comprising a lighting device and method for controlling the activation thereof

Info

Publication number: EP2282613A1
Application number: EP10180461A
Authority: EP
Inventors: Valerio Aisa; Paolo Santonicola; Brunetta Vinerba; Valeria Animobono
Original assignee: Indesit Co SpA
Current assignee: Whirlpool EMEA SpA
Priority date: 2005-03-09
Filing date: 2005-11-03
Publication date: 2011-02-09
Also published as: ITTO20050148A1; CN101124854B; EA010991B1; CN101124854A; WO2006095217A1; EA200701513A1; EP1856951A1; EP1856951B1

Abstract

The present invention relates to a cooking apparatus (1) comprising at least one inner compartment (2A), at least one lighting device (3) to illuminate the inner compartment (2A), and a control device (5) to operate the lighting device (3); according to the invention, the control device (5) is adapted to apply a supply voltage being variable in time to the lighting device (3), so as to extend the life of the lighting device (3).

Description

The present invention relates to a cooking apparatus according to the preamble of claim 1 and to a method for controlling the activation of a lighting device of a cooking apparatus according to the preamble of claim 9.
The structure of many household appliances includes at least one inner compartment in which one can place what one wants the household appliance to perform its function on. Such an inner compartment may for example be the cell of a refrigerator or of a freezer or of a refrigerator-freezer, wherein a user places foodstuffs to be cooled or frozen, or the muffle of an oven, wherein a user places foods to be cooked. The term refrigerator-freezer designates a household appliance fitted with at least two cells having different temperatures, thereby allowing for at least two different preservation states for the foodstuffs kept inside the household appliance. Said inner compartments of household appliances are often provided with a lighting system which helps the user to lay objects in or take objects from said inner compartment and/or to check the condition of the objects contained in said inner compartment. As regards a refrigerator or a freezer, the lighting system helps the user to position the foodstuffs correctly when loading the household appliance, as well as to easily find those foodstuffs that one wants to take out from the appliance. As far as an oven is concerned, the lighting system helps the user to monitor the cooking process, thereby preventing the food from burning during the cooking or from being extracted from the appliance when still partially uncooked. Said lighting system usually comprises, for each compartment to be illuminated, an incandescent lamp comprising a metal filament (typically a filament made of tungsten or a metal alloy including tungsten among its components). The current flowing through said metal filament causes the filament to become incandescent, thereby producing the diffusion of visible light in the inner compartment of the household appliance.
According to a known method for operating an incandescent lamp installed inside a household appliance, a constant and predefined current (typically the 50Hz or 60Hz mains alternating voltage) is applied across the metal filament for the whole lamp activation time. This method has the drawback of reducing considerably the lamp's life, which turns out to be only a small fraction of the average life obtained under uninterrupted operation, this being due to the fact that, when the lamp is switched on, it passes instantly from the off condition to an on condition in which it provides the entire luminous flux it can generate.
The shock caused to the lamp increases even further in refrigerators, because of the low temperature of the lamp when the refrigerator door is closed.
Incandescent lamps currently available on the market have different power ratings and are characterized by an average life under uninterrupted operation of approximately 1,000÷1,200 hours. This average life falls sharply as the ratio between the number of lamp activations and the lamp on time increases, i.e. it decreases considerably if the lamp, in the course of its life, is subjected to an operating condition involving frequent on/off operations.
Such an operating condition typically applies to lamps installed inside household appliances. For instance, the lamp of a refrigerator, which is usually turned on automatically when the door is opened, is switched on and off several times every day when a user places foodstuffs to be cooled in the refrigerator and/or removes cooled foodstuffs. On the other hand, in order to prevent the temperature inside the refrigerator compartment from rising significantly, it is advantageous to keep the refrigerator door open for a time as short as possible. Consequently, the operating time of the refrigerator lamp, once switched on, is usually very short.
Such a sharp reduction of the average life of incandescent lamps implies serious inconveniences, especially for the user of household appliances, whose inner compartments are generally illuminated by an incandescent lamp. In fact, such a user frequently finds him/herself in a situation wherein an inner compartment of a household appliance remains in the dark due to a burnt-out lamp. This condition is not compatible with a comfortable and safe use of the household appliance by the user. Moreover, the operational comfort and safety of the household appliance are further reduced by the necessity of replacing the burnt-out lamps several times during the life of the household appliance.
The inconvenience caused to the user by the failure of one of these lamps is increased also by the fact that these lamps are not easily available on the market. The object of the present invention is to solve the above problems by extending the life of incandescent lamps being subject to frequent on/off operations, said life preferably tending to match the average life under uninterrupted operation. This allows, in fact, to limit or avoid any failures of the lamp during the life of the household appliance wherein the lamp is installed. As a consequence, remarkable advantages related to the comfort and safety of the household appliance are obtained.
Said object is achieved through the method and household appliance incorporating the features described in the annexed claims, which represent an integral part of the present description.
The general idea at the basis of the present invention is to regulate the supply voltage supplied to the lighting device typically during its on/off operation.
The method and household appliance according to the present invention will become apparent, together with their further advantages, from the following detailed description and from the annexed drawings, which are supplied by way of non-limiting example, wherein:

Fig. 1 schematically shows a freezer-refrigerator whereto the present invention may be applied;
Fig. 2 schematically shows a lighting device and an associated control device according to the present invention; and
Fig. 3 schematically shows a possible application of the method being the subject of the present invention to a voltage signal supplying a lighting device, in particular an incandescent lamp supplied by mains voltage.

It should be stated beforehand that the term "voltage value" will often be used in the following description; when referring to alternating voltages, this expression will not mean the instantaneous voltage value, but the effective voltage value referred to one or more periods of the alternating signal.
Fig. 1 shows a refrigerator-freezer 1 having two cells 2; the upper cell 2A is used for fresh food, whereas the lower cell 2B is used for frozen food. In the cell 2A there are two differently sized shelves and one drawer. The refrigerator-freezer 1 is fitted with a lighting device for illuminating the cell 2A; Fig. 1 shows the bulb of an incandescent lamp 3 used as a lighting device. The two cells 2 are closed by means of two doors 20; in particular, the door 20A closes the cell 2A, whereas the door 20B closes the cell 2B.
Fig. 1 is clearly very schematic and merely exemplificative; many elements usually included in a refrigerator-freezer are not shown in Fig. 1 for simplicity's sake; the illustration however shows a triggering device 6 used for switching on the lamp 3 mechanically. In many refrigerators (as in the one shown in Fig. 1), such a device consists of a push-button which is kept pressed by the refrigerator door, i.e. the door 20A in Fig. 1, and which is released when the door is opened. For switching off the lamp 3, the same triggering device, i.e. a push-button, may be used; solutions using a lamp being switched off after a preset delay time are also known.
In general, the household appliance according to the present invention comprises at least one inner compartment, at least one lighting device for illuminating said inner compartment, and a control device for operating said lighting device.
Said household appliance is typically a refrigeration apparatus such as a refrigerator, a freezer or a refrigerator-freezer, or a cooking apparatus such as a cooking oven.
According to the present invention, the control device is adapted to apply a supply voltage being variable in time to the lighting device, so as to extend the life of the lighting device. Such a control is most important during the period in which the lamp is switched from the off condition to the on condition.
Typically, the lighting device is an incandescent lamp preferably comprising a filament, in particular being made of tungsten or of a tungsten alloy; it is exactly in this case that the problem of extending the life of the lighting device is particularly felt.
A control device according to the present invention is shown in Fig. 2; in this illustration, the control device is connected on one side to an incandescent lamp 3 fitted with a filament 4 and on the other side to an alternating voltage source VAC, typically the public supply mains.
The control device of Fig. 2 may be connected to a triggering device for switching on the lighting device, e.g. the device 6 shown in Fig. 1. The triggering device for switching on the lighting device may be provided in many different ways and may be either manual or automatic. If the household appliance is a refrigerator or a freezer or a refrigerator-freezer, the triggering device could be the push-button normally used for activating the lamp when the door is opened, whereas if the household appliance is a cooking oven, the triggering device could be the knob or push-button located in the appliance control panel, through which the user can activate or deactivate the lamp.
The control device of Fig. 2 comprises a power supply circuit 7, a power actuator, in particular a triac 11, being able to operate the lighting device 3, a mains voltage coupling impedance 12 and a microcontroller 8 supplied by the power supply circuit 7; the microcontroller 8 comprising in turn a CPU 9, a memory 10 and an interface peripheral 13; the memory 10 in the example of Fig. 2 includes at least a non-volatile portion (EPROM, EEPROM or Flash) and at least a ROM or EPROM portion for programmes and a RAM portion for data. The mains voltage coupling impedance 12 has the purpose of limiting the current applied to the microcontroller 8 and allows to generate the zero crossing signal of the mains voltage. This signal is used as a synchronism signal for operating the triac 11, which regulates the voltage level applied to the lighting device by using the phase partialization technique (according to the known process called "phase control").
A control device of the electronic type, being preferably programmable, allows to obtain better results in terms of extension of the life of the lamp, thanks to its control accuracy.
The emission of a luminous flux by the incandescent lamp 3 is achieved by heating up, through the application of electric current, the metal filament 4, which is thus brought to a high temperature at which incandescence occurs. As a non-limiting example, it will be assumed herein that the metal filament 4 comprises tungsten. The resistivity of tungsten depends on temperature in a directly proportional way: at room temperature (the temperature of the filament when no current flows through it), the resistivity of tungsten is approximately 5·10^-8 Ω·m, whereas at a temperature of 2000^°C (the approximate temperature of the filament when current flows through it in steady state), the resistivity of tungsten increases to approximately 65·10^-8 Ω·m. From this variation in resistivity which is greater than an order of magnitude, it derives that, for the same value of the voltage applied across the tungsten filament, the current flowing through the filament will be more than an order of magnitude greater in the case of cold filament (room temperature) than in the case of hot filament (temperature of approximately 2000^°C). Hence, by applying a constant voltage across the filament for the whole duration of the operating period of the lamp (i.e. by performing the operating method according to the prior art), when the lamp is switched on there will be a peak in the current value (more than ten times the steady state current) flowing through the metal filament, with a consequent peak in the power drawn by the lamp. In the transient represented by the filament warm-up period, the power absorption always remains above the rated value, i.e. above the value of the power drawn by the lamp once it has reached the steady state (at approximately 2000^°C). Experiments have shown that the duration of the transient is approximately 80÷100 ms.
It has been found that it is exactly the above-mentioned transient of power absorption (which remains higher than the rated value from the peak value to the achievement of the steady state) which subjects the metal filament of incandescent lamps to thermal stress, thereby causing a reduction of the lamp life, if the lamp is operated according to the method known in the art. The fact that said reduction of the life of the lamp becomes more relevant as the on/off operation frequency of the lamp increases is perfectly in accordance with such a discovery. Indeed, the higher the on/off operation frequency of the lamp, the higher the number of transients generated and the higher the number of power absorption peaks suffered by the lamp.
The method being the subject of the present invention, aimed at extending the life of an incandescent lamp, is based on lessening of the above-mentioned power absorption peak occurring during the initial transient following the activation of the lamp. Even better results are obtained if the absorption peak is completely eliminated, rather than only attenuated: in this case, in fact, the life of an incandescent lamp will tend to match the average life of the same lamp under uninterrupted operation.
The method according to the present invention finds a typical application in lighting devices for household appliances, and is particularly advantageous in refrigerators and refrigerator-freezers, wherein the lamp is usually switched on and off (number of times the refrigerator door is opened and closed) several dozens of times a day and the lamp on time is typically less than one minute.
In general, the method according to the present invention is useful for controlling the activation of a lighting device, in particular an incandescent lamp comprising a filament; according to this method, a supply voltage being variable in time, preferably gradually, is applied to the lighting device so as to extend the life of said lighting device; the objective is to prevent or at least reduce current peaks in the lamp filament.
A very effective way to implement the method according to the present invention includes the following phases:

i) application across the filament of a supply voltage having a first preset value, so as to prevent or lessen the formation of an electric power absorption peak by the incandescent lamp as it is switched on, followed by
ii) application across the filament of a supply voltage having a value gradually increasing to a second preset value, said second value corresponding to the rated voltage of the incandescent lamp.

This implementation is particularly suited to electronic control devices of the digital type; besides, it allows to obtain any desired degree of accuracy.
For this implementation, three parameters must be set: the duration of phase ii), the first preset voltage value, and the second preset voltage value; these three parameters are not strictly correlated to one another.
According to a good choice, phase ii) lasts from a minimum of 50 mS to a maximum of 1,000 mS, the first value is preferably comprised between 0 Volt and 50 Volt, and the second value is preferably comprised between 180 Volt and 230 Volt.
The method according to the present invention will be described below by referring to an embodiment example.
According to this example, an incandescent lamp comprising a metal filament is be activated in soft mode, i.e. by applying across the filament a sequence of increasing supply voltage values. Said sequence starts from a minimum value representing a pedestal voltage value. Thus the metal filament is pre-heated in soft mode, the current flowing through the filament being similar to the current flowing through the same in steady state (at approximately 2000^°C), i.e. much lower than that flowing through a cold filament (which has a very low electric resistance) if the entire supply voltage is applied immediately. Besides extending the life of the incandescent lamp comprising the metal filament, such a soft activation mode also offers the additional advantage of a remarkable aesthetic improvement compared to the traditional activation method. As a matter of fact, by slowing down considerably the speed at which the lamp passes from the off condition to the condition in which it provides the entire luminous flux it can generate, the human eye is prevented from meeting the maximum luminous flux at once and can therefore adapt itself comfortably to the transition from dark to light.
More in detail, with reference to Fig. 3, the method according to this example provides for applying to the incandescent lamp a variable supply voltage V instead of a constant supply voltage V^*. Said variable voltage V is such that the absorbed electric power W=V²/R is never greater than the rated power value W^* of the incandescent lamp. This completely avoids the formation of any power absorption peak when the lamp is switched on. The initial resistance of the metal filament has a value R₀=R^*/K², wherein R^* is the resistance of the filament at steady state and K is a numerical coefficient taking into account the difference in resistivity of the filament material between the temperatures T₀ and T^*. Said numerical coefficient K can advantageously have a value of 4 for a tungsten filament. For the power W₀ absorbed by the lamp during its activation not to be greater than W^*, it must be V₀ ²≤V^*2·(R₀/R^*), i.e. V₀≤V^*/K. As the filament temperature T increases by Joule effect due to the current flowing through it, the filament resistance increases as well and a sequence of voltages V gradually increasing up to V^* is then applied across the filament. The application mode of said sequence of voltages V is such that the electric power W absorbed by the lamp is never greater than the rated power W^* of the incandescent lamp for the whole duration of the transient represented by the warm-up period of the metal filament.
If the voltage V applied across the metal filament is an alternating voltage, the above voltage values are to be understood as effective values, i.e. RMS values, which are regulated by means of a power actuator; with reference to Fig. 2, said regulation takes place through the triac 11, which is controlled by the microcontroller 8. During the transient represented by the warm-up period of the metal filament 4, the triac 11 is activated by the microcontroller 8 with a delay time t relative to the zero crossing of the mains voltage, said time t having to be set as a function of the voltage value V to be supplied to the incandescent lamp 3. The voltage value V depends on the temperature T of the filament 4, and therefore the time t must be gradually decreased in order to create a sequence of gradually increasing wave fractions until the steady state is achieved. The time to after which the triac 11 must intervene to cut the first half-wave supplied to the lighting device must not be shorter than the value t₀=t_halfperiod(1-(arcsin(1/K)/π)).
Considering as a non-limiting example an incandescent lamp fitted with a tungsten filament having a numerical coefficient K=4 and a mains alternating voltage with V^*=230 V and frequency f=50 Hz, one can assume a voltage value V₀=55 V as a voltage pedestal. Being t_halfperiod=10 ms when f=50 Hz, the first activation of the triac will then occur with a delay time to relative to the zero crossing of the mains voltage not being shorter than 9.4 ms, if the formation of the initial absorption peak is to be completely avoided. Subsequent activations of the triac will occur after gradually decreasing delay times t relative to the zero crossing of the mains voltage, until the steady state is achieved and the triac remains constantly on.
In the event that a complete elimination of the power absorption peak is not judged to be indispensable and that said peak must be simply reduced, the same method described above can still be used, with the only difference that the first activation of the triac occurs after a delay time t relative to the zero crossing of the mains voltage being shorter than to. Under the same conditions of the above example (K=4, V^*=230V and f=50Hz), the values t₀=9 ms or t₀=7 ms may be used to merely reduce the initial peak in the power absorption of the incandescent lamp. The shorter said time t, the smaller the reduction of said power absorption peak.
The gradualness with which the lighting device, in particular the incandescent lamp, starting from the activation at voltage V₀, reaches the steady state (corresponding to the voltage V^*) depends on the number N of transitions P₁-P₂-...-P_i-...-P_N illustrated in Fig. 3. The higher said number N, the more gradual and progressive the activation of the lighting device, in particular an incandescent lamp. It is also possible to use, in accordance with the present invention, different sequences of transitions P₁-P₂-...-P_i-...-P_N: for instance, once the first transition P₁ has taken place at a time t₀=t_halfperiod-τ, each subsequent transition P_i can be performed at a time t_i=t_halfperiod-i·τ, or as an alternative one can conceive a sequence with more gradual transitions in the initial phase of activation of the lamp and with less gradual transitions in the subsequent phase (more specifically, each transition P_i may occur at a time t_i=t_halfperiod-i²·τ).
From a practical viewpoint, a table corresponding to at least one profile for the gradual activation of the lighting device can be stored in the non-volatile portion of the memory 10 of the microcontroller 8. Said table comprises at least one numerical sequence consisting of the delay times t relative to the zero crossing of the mains voltage at which the progressive activation of the power actuator, in particular the triac 11, takes place during the gradual activation process of the lighting device.
In order to implement the method according to the present invention, it is necessary to prepare an appropriate control device adapted to apply a supply voltage being variable in time to the lighting device.
A first peculiarity of the control device could be its suitability for applying to the lighting device a supply voltage having a first preset value when the lighting device is switched on, typically caused by the triggering device.
A second peculiarity of the control device could be its suitability for applying to the lighting device a supply voltage gradually increasing to a second preset value after said device has been switched on.
It is apparent that these two peculiarities combined together offer the best results and provide a gradual activation profile for the lighting device.
Such an activation profile can be very easily and effectively stored in non-volatile memory means of the control device; if said control device comprises a microcontroller, said activation profile can advantageously be stored in table format in a portion of the internal non-volatile memory of the microcontroller.
As said, in the quite typical case wherein the lighting device is supplied by alternating voltage, the control device must be adapted to apply to the lighting device a supply voltage having a variable active value; if the control device is an electronic and digital one, the supply voltage variation can advantageously be accomplished through switching techniques.
The present invention focuses on the behaviour of the lighting device, in particular a filament-type incandescent lamp, when it is switched on and/or off. In the simplest and yet effective case, the control device is adapted to apply a variable supply voltage to the lighting device only when the lighting device is switched on and/or off.
The household appliance according to the present invention has the advantage of ensuring, by means of the method described above and substantially defined in the annexed claims, an average life of its lighting device, in particular an incandescent lamp comprising a metal filament, being comparable to the average life of the other components of the household appliance. Furthermore, it also offers the advantage of being much more suited to usages requiring frequent on/off switchings of its lighting device, compared to the household appliances according to the prior art. A few explanatory and non-limiting examples of such usages are listed below:

i) the lighting device, through an appropriate modulation of the luminous flux emitted, communicates to the user information concerning the operating state of the household appliance wherein the device is installed. Such a communication can take place through different flashing frequencies or different sequences consisting of flashes and pauses repeated regularly. An example of such a usage can be found in EP0686818B1 , wherein the household appliance is a refrigerator and the lamp is capable of warning the user when the refrigerator door has remained open for a certain time, that it would be appropriate to close the door again so as to avoid an excessive temperature rise in the refrigerating compartment;
ii) the lighting device, through an appropriate modulation of the luminous flux emitted, communicates spontaneously or when requested by a suitable mechanism information concerning the proper operation of the household appliance wherein the device is installed. An example of such a usage of the lighting device for diagnostic purposes can be found in EP0686818B1 , wherein the household appliance is a refrigerator and the lamp is capable of signaling the results of an internal self-diagnosis process through appropriate flash sequences to be interpreted by the service technician;
iii) the lighting device is used as a resistive load in order to allow the household appliance to communicate with an external auxiliary device through its power cable, according to the communication technique described in WO02/21664A1 . According to said technique, the household appliance is capable of sending functional, statistic and diagnostic information to the external auxiliary device by means of predefined power absorptions which can be coded as a sequence of binary logic values (bits);
iv) the lighting device is used during the final testing of the household appliance. For example, the lighting device can be used for carrying out the automatic testing of household appliances according to the teachings of EP1465320A2 , wherein said device is used as a resistive load for the transmission of data by means of predefined power absorptions, according to the above-mentioned communication technique described in WO02/21664A1 .

The present invention has been described with particular reference to a specific embodiment example, but it is clear that many changes are possible for those skilled in the art without departing from the scope defined by the annexed claims.

Claims

Cooking apparatus comprising at least one inner compartment (2A), at least one lighting device (3) to illuminate said inner compartment (2A), and a control device (5) to operate said lighting device (3), characterized in that said control device (5) is adapted to apply to said lighting device (3) a supply voltage being variable in time, so as to extend the life of said lighting device (3).
Cooking apparatus according to claim 1, wherein said control device (5) is of the electronic type and comprises a microcontroller (8) and a power actuator, preferably a triac (11).
Cooking apparatus according to claim 1 or 2, wherein said lighting device is an incandescent lamp (3), preferably of the type comprising a filament (4), in particular made of tungsten or a tungsten alloy.
Cooking apparatus according to any one of claims 1 to 3, wherein said control device (5) is adapted to apply to said lighting device (3) a supply voltage having a first preset value (V₀) when said lighting device (3) is switched on.
Cooking apparatus according to claim 4, wherein said control device (5) is adapted to apply to said lighting device (3) a supply voltage gradually increasing to a second preset value (V^*) after the lighting device has been switched on.
Cooking apparatus according to claims 2 and 5, wherein said microcontroller (8) comprises memory means (10) of the non-volatile type storing at least one table corresponding to a gradual activation profile of said lighting device (3).
Cooking apparatus according to any one of claims 1 to 6, wherein said supply voltage is an alternating voltage, wherein said control device (5) is adapted to apply to said lighting device (3) a supply voltage having an effective value being variable in time, and wherein the variation of said supply voltage is preferably obtained through switching techniques.
Cooking apparatus according to any one of claims 1 to 7, wherein said control device (5) is adapted to applying to said lighting device (3) a supply voltage being variable in time when the lighting device (3) is switched on and/or off.
Method for controlling the activation of a lighting device of a cooking apparatus, in particular an incandescent lamp (3) comprising a filament (4), characterized in that a supply voltage being variable in time is applied, preferably gradually, to said lighting device so as to extend the life of said lighting device.
Method according to claim 9, comprising the following phases:
i) application across said filament (4) of a supply voltage having a first preset value (V₀), so as to prevent or lessen the formation of an electric power absorption peak by said incandescent lamp (3) as it is switched on, followed by

ii) application across said filament (4) of a supply voltage having a value (V) gradually increasing to a second preset value (V^*), said second value preferably corresponding to the rated voltage of said incandescent lamp (3).
Method according to claim 10, wherein phase ii) lasts from a minimum of 50 mS to a maximum of 1,000 mS, wherein said first value is preferably comprised between 0 Volt and 50 Volt, and wherein said second value is preferably comprised between 180 Volt and 230 Volt.
Method according to claim 10 or 11, wherein phase ii) is accomplished by using values contained in a table stored in non-volatile memory means.
Method according to any one of claims 9 to 12, characterized by the application of a supply voltage having an effective value being variable in time, the variation of said supply voltage being preferably obtained through switching techniques.