FR2772463A1 - DEVICE FOR ASSISTING THE MANAGEMENT OF THE OPERATION OF A HOME BURNER BY FLAME DETECTION - Google Patents

Abstract

The device according to the invention relates to any type of domestic burner for gaseous fuel present for example on gas stoves, gas ovens or hobs. It helps to manage the operation of the burner (B) by detecting the flame (F) resulting from the combustion of gas at the burner (B) .To this end, the device includes a thermocouple (TC) positioned in the vicinity of the flame (F) and delivering an electrical signal stc depending on the local temperature. It further comprises actuation means (MA) of the burner and a control unit (COM), the unit (COM) processing the electrical signal stc in order to control the means (MA) for actuating the burner at predetermined times independently of each other with respect to the moments of appearance or disappearance of the flame (F). The means (MA) comprise, according to an example, an actuator (AEM) of a gas inlet valve (V) and an ignition device (ALL) of the burner. The device according to the invention makes it possible to significantly reduce the inertia inherent in the devices of the prior art, while using a standard thermocouple.

Description

The invention relates to a device for assisting operation management

  a domestic burner by flame detection for any type of burner for gaseous fuel present on domestic appliances operating on gas such as for example

  gas, gas ovens, hobs.

  The burners are the elements of the device where the mixture of combustible gas and air is burned. Some burners are equipped with a device to detect the presence of the flame and to activate the opening and closing of a gas inlet valve. This is in particular a safety measure in the event of accidental extinction of the flame which can lead to a leak of toxic gas, following for example a violent draft or the spillage of a liquid on the flame. The most common detection device on household burners includes a thermocouple positioned at the level of the flame and an electromagnet which actuates a core enabling the gas supply to be shut off. When the flame is present, the thermocouple heats up, delivering a current which feeds the electromagnet. Beyond a certain threshold, called the catching current, the core is attracted to a position which allows the circulation of the gas. On the contrary, if the flame goes out, the current delivered by the thermocouple decreases and when it becomes lower than the latching current, the core moves away, for example thanks to the action of a spring, and comes to close off the arrival of gas. These devices are simple, compact, easy to implement and extremely widespread. Their main disadvantage is however their inertia both at the time of lighting the flame and at the time of its extinction. It takes several seconds for the current to reach the latching current in the presence of the flame; this therefore requires keeping a gas inlet valve open, for example by manual control, for this period. After the flame has gone out, it usually takes a few tens of seconds for the gas supply to be shut off, during which time the gas escapes freely. To overcome this problem, other detection systems can be used, whose response times are much faster. For example, optical sensors sensitive to ultraviolet light make it possible to detect the presence of a flame almost instantaneously. However, these sensors are bulky, fragile, difficult to install and not very adaptable to hob burners. Other sensors, such as ionization detectors which exploit the electrical properties of the flame, are very sensitive; but they are also very reactive to variations in the flame and therefore require very high positioning constraints relative to the burner. The invention proposes using a standard thermocouple, connected by a control unit to actuation means of the burner, for example to an electromechanical actuator of a gas inlet valve, but in an operating mode which allows to overcome inertia

  devices of the prior art.

  More specifically, the invention consists of a device for assisting in the management of the operation of a domestic gas burner by flame detection, comprising a thermocouple positioned in the vicinity of the flame resulting from the combustion of the gas and delivering an electrical signal sc as a function of the local temperature, characterized in that it further comprises means for actuating the burner and a control unit, the unit processing the electrical signal stc in order to control said means for actuating the burner at predetermined times independently each other with respect to the moments of onset or

disappearance of the flame.

  The actuating means may for example comprise an electro-mechanical actuator of a gas inlet valve or a

burner ignition device.

  The invention makes it possible to control the inertia of the detection device while using standard thermocouples; the device according to the invention is therefore easily adaptable to existing burners. In addition, since it is no longer the signal delivered by the thermocouple which itself triggers an actuator, the device has the advantage of being less sensitive to the absolute values of this signal and therefore to the

  positioning of the thermocouple in the flame.

  Other advantages and features of the invention will become apparent

  more clearly on reading the description which follows and the figures

  attached which represent: - Figure la, the diagram of a device according to the prior art and Figure lb, a timing diagram illustrating an operating mode of such a device; - Figure 2, the diagram of a device according to the invention; - Figure 3a, a block diagram of the control unit of an exemplary device according to the invention and Figure 3b, a timing diagram illustrating an operating mode of such a device; - Figures 4a and 4b, a block diagram and a timing diagram illustrating another mode of operation; In these figures, the homologous elements are referenced by the

same benchmarks.

  FIG. 1a schematically represents an example of a flame detection device according to the prior art for controlling a valve V for the arrival of gas in a burner B. The device comprises a thermocouple TC positioned in the vicinity of the resulting flame F of the combustion of the gas in burner B. The thermocouple is composed of two different metals and placed so that the junction points between the metals are brought to different temperatures in the presence of flame F. This temperature difference is at the origin of the electrical signal delivered by the thermocouple. In known devices, it is the current itc delivered by the thermocouple which is used for

  supply an electromagnet EA symbolized in figure l a by a coil.

  When the current in the electromagnet is greater than a threshold value called latching current, the core N is attracted, this core blocking the valve V of the gas inlet when the current is less than the threshold value. The device described in FIG. 1 a further comprises an ALL ignition device for the burner B, triggered manually, which can operate for example by the emission of a train of electrical pulses

  or by heating near the burner of an incandescent wire.

  FIG. 1 b illustrates an operating mode of such a device. The curve 11 represents as a function of the time noted t the presence (noted YES in the figure) or the absence (noted NO) of the flame F. The instant to represents the moment when the ignition device is actuated. This action is maintained until time t1 of the appearance of the flame. Between instants to and t1, the valve V is kept open by means different from those used in the device based on the detection of the flame. These means are here called external means. It may for example be a manual control. Curve 11 also shows between instants t2 and t3, a momentary and partial extinction of the flame, for example due to the spillage of a liquid on a zone of the flame close to the thermocouple and sufficiently small so that the flame can reignite completely. and quickly. Finally the instant t4 corresponds to the disappearance of the flame. Curve 12 gives, as a function of time t, the current itc delivered by the thermocouple during the same period as that represented by curve 1 1. Depending on the value iacc of the latching current, a certain period of time Atm, called here time of rise or time of presence detection, is necessary for the device based on the flame detection to take over from the external means to allow the opening of the valve V. The detection of the presence of the flame which results here in l opening of the valve using the device is represented in FIG. 1b by a quantity Y taking the value 1. The latching current can be reduced to reduce the time Atm. But in this case, at the extinction of the flame, the time Atd, here called the descent time or extinction detection time, necessary for the current itc to fall below the threshold iacc and therefore for the core N to close off the valve V will only be larger; in FIG. 1 b, the detection of the extinction of the flame, resulting in the closing of the valve by means of the device is represented by the quantity Y taking the value 0. Thus for a rise time Atm of a few seconds, the fall time Atd is typically a few tens of seconds. It can be seen from this example that the times for detecting the presence or extinction of the flame are linked. Between instants t2 and t3, corresponding to a momentary extinction of the flame, if this time is short enough, the current itc has not fallen below the threshold iacc and the valve therefore remains open, which is an advantage of

Inertia of such a device.

  Let us now consider a device according to the invention, a diagram of which is shown in FIG. 2. It also includes a thermocouple TC, which can be the same as those used in the devices of the prior art. The TC thermocouple when subjected to a temperature variation delivers an electrical signal sc. The device further comprises means MA for actuating the burner B and a control unit COM. The COM box processes the stc signal in order to control the means MA which allow the burner to start when they

are activated.

  The actuating means MA of the burner may include, as illustrated in FIG. 2, an electromechanical actuator AEM allowing the opening or closing of a valve V for the arrival of gas in the burner. The COM box processes the stc signal in order to control the AEM actuator to trigger the opening or closing of the V valve. The AEM electromechanical actuator may include an electromagnet of the same type as those used in the devices of the prior art, but controlled in this case by the COM control unit. Other types of actuators can be used in the device according to

  the invention. For example, the actuator may include a piezo device

  electric which allows, under the action of an electric signal, the displacement of a membrane; this membrane coming for example to shut off the gas supply at the valve V. Note that a piezoelectric device requires high supply voltages to obtain small displacements, also such an actuator would have been difficult

  conceivable in a device of the prior art.

  The means MA can also include an ignition device ALL of the burner, the control unit COM making it possible to control the starting or stopping of the ignition device. The ignition device can for example, as in certain devices of the prior art, operate by the emission of a train of electric pulses or by the heating near the burner of an incandescent wire. The processing functions performed by the COM unit are such that the activation or deactivation of the means MA can be determined independently with respect to the moments of appearance or disappearance of the flame F. For example, these functions can be applied to the signal stc and to the variations of this signal, which also amounts to processing the derivative s'tc of the signal stc. By treating two quantities in this way, it is possible, for example by comparing these quantities with two independent threshold values, to independently determine the instants of opening or closing of the valve V or the instants d activation or deactivation of the ignition device with respect to

  moments of appearance and disappearance of the flame.

  An example of such an operating mode of the COM control unit is shown in FIG. 3a. The COM box comprises means 31 for calculating the derivative s'c of the electrical signal sTC delivered by the thermocouple. It further comprises means 32 for comparing the signal stc with a first threshold value s1 and means for comparing the derivative s'tc with a second threshold value S2. The results of these comparisons are called A1 and A2 respectively. For example, A is 1 if stc is greater than or equal to s1 and A2 is 1 if s't is greater than or equal to s2. A, and A2 are worth 0 otherwise. The housing also includes means 34 making it possible to operate a logic function between the quantities A1 and A2. In the example chosen in FIG. 3a, this function is a logical OR. The result of this operation is the quantity Y which materializes the flame detection. In the example chosen, Y equals 1 when the presence of the flame is detected, Y equals 0 otherwise. The value of Y determines the activation of the actuation means MA of the burner. To better understand the time response of such a device according to the invention, we can use the example of the timing diagram 1 1 described in FIG. 1 b and repeated in FIG. 3 b. At the instant t1 of the appearance of the flame, it is for example the value of s'tc which determines the detection of the flame; a strictly positive threshold value s2 is fixed, so that upon ignition of the flame, the variation of heat at the thermocouple almost instantaneously causes the relation s'tc greater than or equal to s2, the quantity A2 taking the value 1; it follows that the result Y of the operation between A1 and A2 also changes to 1, which results in the detection of the flame and therefore the control of the actuation means MA of the burner B with a rise time Atm close to 0. In the example chosen, this results in the opening of the valve V thanks to the electro-mechanical actuator AEM and the automatic shutdown of the ignition device ALL also controlled by the COM box. The derivative s'tc decreases rapidly but the signal stc increases, passing beyond the threshold sl; s1 can be chosen so that, when s'tc is less than s2, stc is greater than sl, leading to the change to 1 of the quantity A1. Thus, Y is always equal to 1. When the flame is extinguished (instant t4), it is the passage of stc below the threshold s1 which determines the flame extinction detection instant (Y goes to 0) . This is reflected in the example chosen either by closing the valve V by means of the AEM actuator (the gas which supplies the burner is therefore cut off), or by restarting the ALL ignition device in order to relaunch the flame. The choice of s1 thus determines the descent time Atd; it is then necessary to find a compromise on the value of s, to reduce the descent time Atd while fixing a duration (corresponding on the curve 1 1 to the period between the instants t2 and t3) during which the flame can locally s' extinguish without closing valve V or restarting the ignition device; typically, Atd is worth a few seconds. The functions performed by the COM unit can be performed by means of analog electronic functions or digitally,

  by means of software programmed for example on a microcontroller.

  The electrical signal stc delivered by the thermocouple TC and processed by the control unit COM is for example the voltage measured at the terminals of the thermocouple. This makes the operation of the device more reliable because the signal is then independent of the values of the

  electrical connection resistors.

  Another example of operation of the COM control unit is illustrated in the operation diagram in FIG. 4a and the timing diagram in FIG. 4b. The housing comprises means making it possible to construct, from the electrical signal stc delivered by the thermocouple TC, a signal rtc (in dotted lines in FIG. 4b) and means of comparison between the signal stc and the signal rtc. The result Y of the comparison is transmitted to the actuation means MA of the burner to control their activation. In the example chosen, the detection of the presence of the flame, corresponding to Y = 1 in FIG. 4b, is obtained when stc is greater than rtc, and the detection of extinction of the flame then corresponding to Y = 0 is obtained otherwise. The quantity Y is sent to the actuation means which can be, to use the previous example, the electromechanical actuator AEM of the valve V and the ignition device ALL. The signal rtc can for example be constructed as follows. The first step 40 (FIG. 4a) consists in controlling the operation of the burner by

  the user. It corresponds to time to on the timing diagram 4b.

  During the period 41 (FIG. 4a) waiting for the appearance of the flame (Y = 0), the signal rtc takes a minimum value rs constant. At the instant t1 of appearance of the flame, the signal stc begins to grow; when sc becomes greater than rtó (42), Y goes to 1 and the detection of the presence of flame is obtained with a rise time Atm which therefore depends on the minimum value rs. This rise time can be of the order of a second. During the period 43 during which the signal stc is increasing or constant, and greater than the sum of the minimum value rs and a threshold value vs, the signal rtoc is equal to the difference of the signal stc and a value of shift sg. The threshold value can be equal to the offset value, as is substantially the case in Figure 4b. During the period 44 which follows the disappearance of the flame (instant t4), period during which the signal stc is decreasing, the signal rtc decreases with a predetermined time constant, this constant being greater than that of decrease of the signal stc. If stc increases again before sic has become less than rtc, rtc takes the value equal to the difference of the signal stc and the offset value sg. When the signal stc becomes lower than the signal rtc, this results in the detection of the extinction of the flame (Y = 0) after a period of time corresponding to the descent time Atd which depends on the time constant of the signal rtc ; typically Atd is worth a few seconds. According to a first option, the value Y can be maintained at 0 for a duration Atb (blocking period 46) corresponding to the time it takes for the signal rtc to decrease to the minimum value rs. The blocking period allows, when a new appearance of the flame occurs, to obtain the detection of the presence of the flame and therefore the control of the actuation means MA of the burner with always the same time constant Atm. According to another option (shown in dotted lines in FIG. 4a), there is no blocking period and if again, the operation of the burner is controlled by the user (step 40), the detection of the extinction flame (Y = 0) is only obtained during the period 45 corresponding to stc less than rtc An operating mode such as that which has just been described can very easily be programmed by means of a microcontroller for example. We can also use an electronic circuit which performs

  analogically the different functions.

  The device according to the invention thus makes it possible to independently set the rise and fall times and thus significantly reduce the inertia inherent in the devices of the prior art, while using a standard thermocouple. The position of the thermocouple relative to the flame is less critical than in known devices because it is no longer the signal delivered by the thermocouple which directly feeds an actuator; the operation is therefore less sensitive to the flame temperature. The device according to the invention makes it possible to improve the safety measures relating to gas leaks due to the accidental extinction of the flame. It also makes it possible to facilitate the implementation of certain operating modes of the gas burner, such as for example a simmer mode obtained thanks to an automatic sequential operation of the burner. Indeed, this mode, the alternation period between the ignition phase and the flame extinction phase can be of the order of a few tens of seconds, requires short rise and fall times. On ignition in particular, the device according to the invention makes it possible to limit the duration of operation of the ignition device which persists as long as the flame has not been detected, and which, if it is produced for example by means of

  the emission of a train of electric sparks is noisy.

  Other optional functions can be easily implemented using the device according to the invention. For example, the device may include means for blocking the general operation of the domestic appliance on which the burner is located. These means are activated by the COM control unit when an abnormal presence of the flame on the burner is detected; for example when the flame is detected while the gas inlet valve is closed. This blocking can consist, for example, of a temporary impossibility for the user to have access to the commands of the device. The device according to the invention may also include display means, 5 controlled by the control unit (COM), and making it possible to inform the user about the detection of the presence or of the extinction

of flame.

Claims (9)

  1 - Device for assisting in the management of the operation of a domestic gas burner (B) by flame detection, comprising a thermocouple (TC) positioned in the vicinity of the flame (F) resulting from the combustion of the gas and delivering an electrical signal sc depending on the local temperature, characterized in that it further comprises actuation means (MA) of the burner (B) and a control box (COM), the box (COM) processing the electrical signal stc in order to control said means (MA) for actuating the burner (B) at predetermined times independently of one another with respect to the moments of appearance or disappearance of the flame (F).   2 - Device according to claim 1, characterized in that the box (COM) comprises means of calculation (31) of the derivative s'tc of the signal Bag, comparison means (32) of the signal Sc with a first value of predetermined threshold (sj), means of comparison   (33) of the signal s'tc with a second predetermined threshold value (s2).   3 - Device according to claim 2, characterized in that the housing (COM) further comprises means (34) for performing a logical OR function between the result A of the comparison between the signal sc and the first threshold (sj) and the result B of the comparison between the derivative s', c and the second threshold (s2), the actuation of the burner (B) by the   means (MA) being determined by the result Y of said function.   4 - Device according to claim 1, characterized in that the housing (COM) comprises means making it possible to build from the signal sc an rtc signal and means for comparing the signal sc with the signal rtc, the actuation of the burner ( B) being determined by the result Y of said comparison, the signal rc being constructed so that - it is worth a constant minimum value (rs) during the period (41) of waiting for the appearance of the flame, - it is equal to the difference of the signal sc and an offset value (sg) during the period (43) during which the signal stc is increasing or constant, and greater than the sum of the minimum value (rs) and a value threshold (vs), - it decreases with a predetermined time constant during the period (44) during which the signal stc is decreasing, said   constant being greater than that of decrease of the signal sc.   - Device according to claim 5, characterized in that the housing (COM) further comprises means allowing the blocking of the actuating means (MA) during the period (46) which follows the disappearance of the flame and during which the signal rtc is still   greater than said minimum value (rs).   6 - Device according to any one of claims   10 previous, characterized in that the box (COM) comprises a microcontroller allowing the programming of at least some of the functions necessary for processing the stc signal and for controlling the actuation means (MA).   7 - Device according to any one of claims   i 5 above, characterized in that the actuating means (MA) comprise an electromechanical actuator (AEM) of a valve (V) for supplying gas to the burner (B), the housing (COM) thus making it possible to   control the opening or closing of said valve (V).   8 - Device according to claim 7, characterized in that the actuator comprises an electromagnet which allows the movement of a   core, thus opening or closing the valve (V).   9 - Device according to claim 7, characterized in that the actuator comprises a piezoelectric device which allows the deformation of a membrane, thus causing the opening or closing of the valve (V).   - Device according to any one of the claims   above, characterized in that the actuating means (MA) comprise an ignition device (ALL) of the burner (B), the box (COM) thus making it possible to control the starting or stopping of said ignition device.   11 - Device according to any one of claims   above, characterized in that it further comprises means for blocking the general operation of the domestic appliance on which the burner is located (B), said means being activated by the control unit (COM) when an abnormal presence flame on the   burner (B) is detected. 12 - Device according to any one of the preceding claims, characterized in that it further comprises means   display controlled by the control box (COM) and allowing to inform the user on the detection of the presence of the flame.