The present invention relates to a device for measuring flame intensity having the characteristic features set out in the preamble of the main claim. Such a device is disclosed in GB 2 367 172 A.

The invention is applied in particular, but not exclusively, in the sector of systems for the control of the gas supply to burners of appliances for heating in general, whose flame is adapted to heat the environment or an intermediate fluid circulating in a boiler plant.

A typical application is in systems controlling the gas supply to burners of boilers for domestic heating and/or heating of domestic hot water.

In the technical sector of the invention it is known to equip the above-mentioned appliances, for safety reasons, with devices which detect the presence of a flame by measuring a current flowing in a flame sensor, currently known in the sector as "ionisation current", as disclosed for instance in Patent Specifications US 5 599 180, US 6 060 719 and JP 4 244 922.

It has nevertheless proved necessary in this field to provide devices not only for detection but also for the measurement of flame intensity in order to optimise the combustion parameters and therefore to reduce pollution and consumption.

In the reference technical sector it is preferably required for these devices to be substantially invariable with respect to temperature variations, at least in the temperature range in which the device is used, to the dispersion of the properties of components and to variations in the supply voltage, without production costs and times and design complexity being greatly affected.

A main object of the present invention is to provide a device for measuring flame intensity which is structurally and functionally adapted to satisfy the above-mentioned requirements and at the same time to remedy the drawbacks described with reference to the cited prior art.

These and other objects described in detail below are achieved by a device for measuring flame intensity in accordance with the accompanying claims.

Further characteristic features and advantages of the present invention are set out in the following detailed description of a preferred embodiment thereof, given solely by way of non-limiting example, with reference to the accompanying drawings, in which:

• Fig. 1 is a circuit diagram of a preferred embodiment of a device for measuring flame intensity of the present invention;
• Fig. 2 shows the duty cycle variation (in %) of the voltage V as a function of the ionisation current (in µA) of five different examples of a flame sensor at predetermined temperature, in accordance with the diagram of Fig. 1;
• Fig. 3 shows the temperature dispersion (in °C) of the magnitudes shown in Fig. 2;
• Fig. 4 shows a preferred embodiment of the input/output signal to various components of the device of Fig. 1;
• Fig. 5 diagrammatically shows a preferred embodiment of a control system of the invention comprising the device of Fig. 1.

With reference first to Fig. 1, a device for measuring flame intensity, adapted for instance to measure the flame intensity within burners, in accordance with the present invention, is shown overall by 1.

The device for measuring flame intensity 1 comprises a flame sensor U1 disposed within the burner so as to come into contact with the flame when the latter is present, and to generate an ionisation current proportional to the intensity of this flame.

The sensor U1 is preferably produced using conventional techniques in the sector and comprises, for instance, two electrodes. In a preferred embodiment of the invention, the operation of the flame sensor may be compared with that of a current generator connected to a one-way member, for instance a diode in series with a resistor (see diode D2 and resistor R10 in Fig 1): the current generated by the sensor as a result of the presence of the flame in practice flows in only one direction. This is in particular due to the fact that the burner is often connected to earth in boilers embodied according to the prior art, with the result that when current is flowing in the sensor U1 as a result of the presence of a flame, this current "drains" towards earth.

Generally, the currents generated by flame sensors coupled to known burners in the sector are of the order of microamperes. By way of example in the applications in question, the ionisation current generated in the sensor U1 preferably varies within a range of 0.8 µA to 7.5 µA. The device of the invention may nevertheless also be used in the case of ionisation currents having different orders of magnitude.

The term "flame intensity" has the following meaning in the preferred embodiment described below: a "low" flame is a flame which licks the sensor partially from a volumetric point of view and a "high" flame is when the sensor is completely surrounded by the flame. Various flame levels from low to high therefore correspond to a flame which licks an increasingly greater portion of the sensor until it is completely surrounded.

The device for measuring flame intensity 1 is supplied not just with the current signal from the flame sensor U1 but also with an alternating voltage signal V1 having a first predetermined duty cycle, for instance a voltage having a zero-centred duty cycle of 50%. The voltage V may for instance be the mains voltage from which the direct component is eliminated by means of a capacitor C1 (see Fig. 1, in which a resistor R1, the capacitor C1 and two further resistors R2 and R4 are disposed in series, this being one of the possible embodiments of the invention) in order to obtain a sinusoidal signal having a first duty cycle of 50% and substantially centred on 0 (i.e. having a mean value which is substantially zero). The voltage V1 may also be supplied by an alternating signal generator or by batteries (not shown).

According to a main characteristic feature of the invention, the device for measuring flame intensity 1 further comprises means 2 for varying the first predetermined duty cycle of the signal V1 whose input is supplied with both the ionisation current generated by the sensor U1 downstream of a resistor R6 and the alternating voltage signal V1. These means 2 generate as output an alternating voltage signal V having a second duty cycle which is a function of the first duty cycle (which is known and set when the device 1 is installed) and the ionisation current U1, as will be explained below. In particular, the second duty cycle of the output signal V increases as the ionisation current increases. By measuring the second duty cycle it is therefore possible to deduce the flame intensity as the intensity of the ionisation current is proportional to the intensity of the flame.

The variation means 2 comprise a switch member, preferably a transistor Q1, whose base is supplied by the signal V1 and by the ionisation current.

A positive supply voltage, shown in Fig. 1 as a direct voltage Vcc, is supplied to the collector of the transistor Q1. The collector resistor is also shown by R5 in Fig. 1.

The potential difference between the base and emitter of the transistor Q1 is determined by the base input signal (the emitter is preferably connected to earth). The values of the various components of the device 1, the values of the ionisation current and the value of the alternating current signal V1 having a first duty cycle are such that the input signal of the base of the transistor Q1 has a value which oscillates between saturation values (when the input signal of the base of the transistor is maximum) and cut-off values for the transistor Q1, causing the latter to operate substantially as a "switch".

It will be appreciated that, operating between saturation and cut-off and not in the linear zone, the transistor Q1 is substantially invariable with respect to temperature variations, which would not be the case if the transistor were operating in the linear zone. Moreover, as the transistor gain is not a relevant parameter, extremely economic transistors may be used in the device of the invention.

The measuring device 1 further comprises optional filter means 3 by means of which the alternating voltage signal V1 and the current signal from the flame sensor (if a flame is present) are advantageously filtered and then supplied as input to the base of the transistor Q1.

The optional filter means 3 preferably comprise a low-pass filter 3 which is more preferably formed by two low-pass filters in parallel, a first low-pass filter 4, for instance in series with a resistor R8' and a capacitor C3, and a second low-pass filter 5, for instance in series with a resistor R9 and a capacitor C2. One terminal of both the capacitor C3 and the capacitor C2 is preferably connected to earth.

A low-pass filter 3 comprising the two low-pass filters 4 and 5 in parallel is preferred in order to make it possible to lower the overall signal cut-off frequency by using commercially available, economic and reliable components. Disposing two low-pass filters in parallel in practice makes it possible to use resistors and capacitors of a lower value than in the case of a single low-pass filter. For the same reason, i.e. for cost savings and ease of location of components, the resistor R8' preferably comprises two resistors R7 and R8 in series.

It will be appreciated that the input signal to the low-pass filter 3 has a negative offset with respect to the signal V1, which offset depends on the value of the ionisation current through the flame sensor U1 to earth. Therefore, starting from the duty cycle of the signal V1, the signal supplied as input to the filter 3 and therefore to the base of the transistor Q1 has a duty cycle lower than that of the signal V1 as a result of the ionisation current which, when present, entails a negative offset of the signal V1, i.e. its "translation" by a particular value, determined by the intensity of the ionisation current, to negative voltage values.

The low-pass filter 3 is electrically connected as output to the base of the transistor Q1 by regulation means 6 of the base voltage of the transistor Q1. The regulation means 6 of the base voltage of the transistor Q1 for instance comprise a diode D4 in parallel with the base, i.e. joining the base and emitter of the transistor Q1, in order to limit the base voltage below a maximum limit authorised for the transistor Q1, so as to prevent damage to this transistor. It is preferable for the diode D4 to be disposed in parallel with the base (although this diode D4 may also be disposed in series with the base of the transistor) as this arrangement enables better temperature behaviour of the device 1. The diode D4 is further connected in parallel with a resistor R11 which is adapted to connect the base to earth.

The transistor Q1 further preferably comprises a collector resistor R5 and has a collector voltage V which represents the output signal from the measuring device 1: the output signal V substantially comprises a two-level voltage (for simplicity, known in short as "high" and "low") with a second duty cycle depending on the alternation of saturation (to which a "low" voltage V corresponds) and cut-off (to which a "high" voltage V corresponds) in the transistor Q1. The value of the duty cycle of the signal V depends, as the value of the first duty cycle of V1 is set and remains set, on the negative offset proportional to the ionisation current generated by the flame sensor U1 of the signal supplying the base of the transistor Q1.

Advantageously, the resistors R1-R11 are of the type which do not vary with variations in temperature, for instance resistors of the SMD (Surface Mounting Device) type, in order to ensure, together with the use of the transistor Q1 outside the linear zone, that the performance of the device 1 is reproducible in a predetermined temperature range equivalent to the range of use of the burner, for instance in the range between -40°C and 80°C.

Solely by way of example, the resistors used have the following value: R1=100 kΩ, R2=R4=470 kΩ, R3=2.2 MΩ, R5=R6=R7=1 MΩ, R8=22 MΩ, R9=15 MΩ, R10= 47 MΩ. The values of the capacitors used are as follows: C1=2.2 nF, C3=C2=1 nF, Lastly, Vcc = 5 V.

The alternating signal voltage V1 depends, however, on the type of mains supply to which the device is connected, i.e. a signal of 50 Hz in Italy. If the voltage varies, i.e. a mains voltage of 60 Hz, all that is needed is a software modification as the device may be used with any mains voltage.

A control system 7 of the invention, shown in Fig. 5, comprises the device for measuring flame intensity 1 and means 8 for comparing the second duty cycle of the output signal from the device 1 with a plurality of predetermined levels so as to assign a flame intensity to a specific value of the second duty cycle.

According to a preferred embodiment, there are four flame levels. A control function therefore assigns a predetermined level to specific values of the second duty cycle.

According to a preferred embodiment, the ionisation current generated in the sensor U1 may vary in a range of 0.8 µA to 7.5 µA which leads to a substantially linear variation of the second duty cycle (minimal when there is no flame and then increasing as the ionisation current increases).

The comparison means 8 preferably comprise a microprocessor µP having as input the collector voltage V sampled at high frequency, typically every 64 µs, which value may nevertheless be varied and depends, inter alia, on the frequency of the alternating signal V1 and on the ionisation current.

Preferably, the control system 7 further comprises, if the device 1 is supplied directly from the electrical mains, an isolation transformer 9 adapted to make the alternating signal V1 independent from the mains supply. The isolation transformer 9 has a supplementary winding on the secondary (not shown), isolating the "high voltage" part of the variation circuit 2 making it unnecessary to use photocouplers for the interface with the microprocessor.

The device 1 of the present invention operates as follows.

In the absence of a flame, the flame sensor U1 has no ionisation current flowing through it and therefore the input and output signals of the filter means 3, which attenuate solely the amplitude of the signal but do not change its duty cycle, are the alternating voltage signals V1 having a predetermined duty cycle, for instance that of the mains. In a preferred embodiment, the first duty cycle is 50%.

The potential difference between the base and emitter of the transistor Q1 therefore alternates between cut-off and saturation values during time intervals depending on the signal V1 and the second duty cycle of the collector voltage V is proportional and "inverted" with respect to the first reference duty cycle. In particular, when there is a positive input signal at the base of the transistor Q1, the signal V is negative, and when there is a negative signal at the base, the signal V is positive. In the absence of a flame, however, the signal V does not have a duty cycle coinciding with the duty cycle V1 because of the presence of the resistor R3 (of 2.2 MΩ in the preferred embodiment) in order to ensure more accurate detection of the signal V.

In the preferred embodiment of a device 1 having an output illustrated in Fig. 2, the duty cycle of the signal V at zero flame is set to be equal to 20%, although the duty cycle of the signal V at zero flame can be set as desired.

If there is a flame, the ionisation current in the flame sensor U1 is responsible for a negative offset with respect to the alternating signal V1 at the input of the low-pass filter 3 proportional to the intensity of this current: the greater the current, the greater will be the negative offset, i.e. the "translation" of the signal V1 to negative voltage values, and therefore the duty cycle of the signal supplied to the base of the transistor Q1 will be proportionally reduced. Reference should be made for instance to Fig. 4 which shows an example of the signal present as input/output at various points (in particular points A, B, C, D in the presence and absence of a flame) of the device 1. Although the embodiment illustrated shows a sinusoidal signal V1 with a duty cycle of 50%, it will be appreciated that the signal V1 may be any alternating signal having a predetermined set duty cycle.

In a first preferred embodiment, the signal V1 is sinusoidal with a duty cycle of 50%.

In a further preferred embodiment, the signal V1 is a triangular signal with a duty cycle of 50%. A triangular wave generator is in particular preferred in cases in which it is necessary to use an alternating current generator (for instance in cases in which connection to the electrical mains is impossible and it is therefore necessary to use, for instance, an appropriate battery). As a result of the negative offset, the potential difference between the base and emitter of the transmitter Q1 is such as to cut off the transistor Q1 for a time interval greater than the saturation interval as the signal supplying the base of the transistor is in negative voltage for a period longer than the period in which the base is supplied with positive voltage. The output signal V of the device 1 is therefore "high" for a longer period than when it is "low" with the result that its duty cycle increases (see the signals of Fig. 4). Consequently, the more the duty cycle of the input signal of the base of the transistor Q1 decreases, the more the duty cycle of the collector signal (i.e. the voltage V) increases.

The second duty cycle of the collector voltage V, determined by the passage of saturation current of the transistor Q1 into the resistor R5, is therefore proportional to the flame intensity detected, as shown in Fig. 2: the abscissa shows the ionisation current (proportional to the flame intensity) and the ordinate shows the resulting duty cycle of the signal V. The greater the current, the greater is the duty cycle until "saturation" is reached (the signal V is always "high"). The various curves shown in Fig. 2 refer to a plurality of different devices 1, in particular including different sensors U1, in order to verify that their behaviour is analogous.

The device 1 is appropriately dimensioned so that the saturation value is reached for the maximum flame levels generally encountered in the burners in which it is applied.

As a result of the use of fully SMD components and the fact that the transistor Q1 does not operate in the linear zone, the Applicants have been able to ascertain that there are maximum variations of 2% of the duty cycle of the signal V for temperature variations from -40 to +80, making the device 1 substantially invariable with respect to temperature in the normal temperature range of use. Fig. 3 shows a graph similar to that of Fig. 2 obtained for three different operating temperatures of the device 1 equal to -20°C, 25°C and 70°C (in this case as well, simulations of different devices 1 including different sensors are given for each temperature).

In the control system 7, the collector voltage V is sampled at high frequency and the sample values are compared by the microprocessor with the predetermined levels of the control function.

The invention thus achieves the proposed objects and provides the above-described advantages with respect to known solutions.

It will be appreciated in particular that the device for measuring flame intensity is invariable with respect to temperature variations, dispersions of the characteristics of components and variations of the supply voltage.