GB2468398A

GB2468398A - Pyroelectric detector array for the angle-resolved detection of flames or fires

Info

Publication number: GB2468398A
Application number: GB1003378A
Authority: GB
Inventors: Martin Ebermann; Silke Hoppe; Norbert Nuemann
Original assignee: INFRATEC GmbH
Current assignee: INFRATEC GmbH
Priority date: 2009-03-02
Filing date: 2010-03-01
Publication date: 2010-09-08
Anticipated expiration: 2030-03-01
Also published as: GB2468398B; DE102009012097A1; GB201003378D0; DE202009018974U1

Abstract

The invention relates to a sensor for the angle resolved detection of flames or fires. Pyroelectric detectors A1 to C3 are present in a one-dimensional or two-dimensional row and column arrangement or array. Electromagnetic radiation is directed via at least one focusing optical element such as a hemispherical lens 1 onto the pyroelectric detectors which are each connected individually to an electronic evaluation unit. The pyroelectric detectors are arranged in a plane at a spacing in front of or behind the focal plane of the focusing optical element 1. An optical diaphragm 2 with a circular opening may be arranged in the beam path of the radiation. The spatial angular position of a flame or fire may be determined by comparing measured signals detected at the same time by pyroelectric detectors arranged directly next to one another. A simple sensor of small construction is provided which is easy to produce and cost-effective and which also allows an angle resolution of a few degrees.

Description

Sensor and method for the angle resolved detection of flames or fires The invention relates to a sensor arid to a method for the angle resolved detection of flames or fires.

Pyroelectric detectors are present at the sensor in accordance with the invention in a field arrangement having a plurality of rows and columns onto which the electromagnetic radiation is incident which is emitted within an observation region, for example within a space in a building.

Pyroelectric infrared detectors are already used in fire and flame sensor systems to be able to discover * and combat fires at an early time. The spectral radiation of the carbonisation gases and combustion gases as well as the flickering of the flame are usually used as signals to be evaluated. Work is frequently carried out in different spectral channels, that is, while simultaneously evaluating electromagnetic radiation with different wavelengths to avoid false alarms, e.g. due to sunlight. Such a system is mostly designed and installed so that an observation region or space to be monitored, which typically extends over an angular range of � 45° in the horizontal and vertical direction and over a distance range of 20 -75 m, can be taken into account.

The flame is relatively small on the outbreak of a fire (geometric extents less than 30 cm * 30 cm are customary); a flame can thereby be considered approximately as spot-shaped radiation source at an infinite distance for the sensor. Simply designed sensors or those in which every single pyroelectric detector covers the whole observation region or space do not deliver any information on the location of the fire in the event of a fire so that automatically triggered extinguishing actions such as the activation of a sprinkler system will likewise cover the whole space.

If the outbreak of a fire can be localised fast, it is also possible to be able to carry out the *I* extinguishing actions in a direct, locally restricted and substantially more efficient manner.

A trivial, but very expensive solution for this :. 30 problem is the additional use of an IR camera with which the site and size of the fire can be determined very accurately.

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* An approach for a more cost-effective solution is e.g. described in EP 0 853 237 Bi. It is proposed therein to use a detector field having 10 to 10,000 elements and a focusing optical system integrated into the detector housing. This system basically represents a low-resolution camera. The resolution of the detector directly depends on the number of detectors and on the solid angle. A detector array of 9 * 9 detectors with an opening angle of +1-450 thus produces an angle resolution of 100. High costs thereby result because a high angle resolution requires many detectors and high demands on the quality of the optical elements.

It is the object of the invention to provide a simple sensor which is easy to produce and cost-effective and which also allows an angle resolution of a few degrees.

In accordance with the invention, this object is solved by a sensor having the features of claim 1. A method in accordance with claim 10 can be worked with for the spatially resolved determination of fires or flames.

Advantageous embodiments and further developments of the invention can be achieved using features designated in subordinate claims. * *

It is possible with the present invention to determine the direction in which a fire or a flame occurs within the monitored observation region or * 30 space.

With a sensor in accordance with the invention, pyroelectric detectors are present in a one-dimensional or two-dimensional row and column

arrangement (field arrangement) . Electromagnetic

radiation which is emitted from the respective observation region (for example an open depot) or space to be monitored is directed to pyroelectric detectors via at least one focusing optical element.

The pyroelectric detectors are each individually connected to an electronic evaluation unit.

The pyroelectric detectors are arranged in a plane with a spacing before (intrafocal) or behind (extrafocal) the focal plane of the focusing optical element, whereby the image of a spot-shaped radiation source at an infinite distance arising in the detector plane is not a sharp spot, but rather an extended spot with an intensity distribution. The intrafocal arrangement is to be preferred even further for the following reasons: It allows a smaller construction since then the pyroelectric detectors are arranged between a focusing optical element and its focal plane and not therebehind. The focal length of the optical element does not have to be so small as in the case of a focusing arrangement or of an extrafocal arrangement with the same opening angle, whereby the radiation throughput (speed) of the optical element can be higher. In addition, with an intrafocal arrangement, a more rectangular intensity profile can be achieved in the detector a plane, from which a better angle resolution results in connection with the evaluation process. * *

A plano-convex lens whose convexly curved surface *:. 30 faces in the direction of the pyroelectric detectors can be used as the focusing optical element.

Hemispherical lenses are particularly preferred since they have the focal lengths required for a large angular range and can nevertheless be manufactured cost-effectively.

An optical diaphragm can also advantageously be arranged in front of the pyroelectric detectors in the beam path of the electromagnetic radiation. An optical diaphragm can in this respect be arranged directly on or at the side of the focusing optical element facing in the direction of the observing region or space or can be attached thereto or formed thereon. The optical throughput and the intensity profile in the detector plane can be optimised using such an optical diaphragm.

The optical diaphragm can have a circular opening for the passage of electromagnetic radiation when a symmetrical two-dimensional arrangement of the pyroelectric detectors is formed at the sensor. The pyroelectric detectors are then present with the same number in a row and column arrangement. In this respect, the sensitive surfaces of the pyroelectric detectors and their central spacings should also be of the same size and the geometric design of the detectors should be selected to be the same.

Independently of this, pyroelectric detectors can have a sensitive surface of equal size, central spacings or centre of area spacings of equal size with one another and/or be arranged at equal spacings from the focal plane of the focusing optical element.

The number of pyroelectric detectors in rows and

columns and/or the extent or size of a field

30 arrangement (array arrangement) in the two axial directions can, however, also be different. This is : .. advantageous when respective different angular ranges should be covered in the two axial directions in which a monitoring should take place or when the required angle resolution in the two axial directions should be different. A larger angular range can thus be monitored as the case may be in the horizontal or vertical axial direction when the extent of the field arrangement in the respective axial direction is larger than in the other direction.

A larger angle resolution can be achieved as the case may be in the horizontal or vertical axial direction when the number of detectors in the respective axial direction is larger than in the other direction.

In this case, a focusing optical element adapted to such an arrangement of the pyroelectric detectors and a corresponding optical diaphragm can also be used.

If the determination of the angular position should be restricted to one axial direction, an optical element can preferably be used which only acts in a focusing manner in this axial direction. Such an optical element can e.g. be a cylindrical lens.

A detector array having at least nine pyroelectric detectors can thus be present in a favourable manner at a sensor in accordance with the invention, said detector array being received in integrated fashion together with a focusing optical element, an aperture ,. 25 diaphragm and nine pre-amplifiers in a housing.

S * S

Such a 3 * 3 detector array makes possible a * resolution of 30° in the horizontal and vertical directions in connection with a sharp-focusing optical element with an opening angle of 90°.

With the sensor in accordance with the invention, however, a blurred image is utiJ.ised in that the detector array is arranged, preferably intrafocally, at a spacing from the focal plane. The image of a flame (ray cross-section) is thereby blurred and approximately as large as the central spacing of the pyroelectric detectors. With non-square or non-circular sensitive surfaces of pyroelectric detectors, this can be the spacing of the centres of area. Square sensitive surfaces are, however, preferred since here the measured signals at the detectors in the horizontal direction and in the vertical direction take the same influence into account. In addition, the non-usable dividing surfaces between adjacently arranged pyroelectric detectors can be kept small.

Due to the small extent of a flame or of a starting fire and to the large distance, collimated electromagnetic radiation can be assumed with respect to the optical image.

As already expressed in the introduction to the

description, a detection with directional

determination should be carried out in the recognition of flames or of a fire. In this respect, it should be able to detect this state at least almost free of doubt. As customary in the prior art, the electromagnetic radiation emitted by the hot

S

carbon dioxide which lies at the wavelength of 4.3 pm * can also be selected in the invention. Taking this * ** into account, at least one optical filter can be present in the beam path of the electromagnetic *.S * 30 radiation which is optically transparent at least for electromagnetic radiation in front of the pyroelectric detectors with wavelengths of 4.3 � 0.1 ***S * * pm or such a focusing optical element can be present.

In this respect, a bandpass filter with a small passage interval or an edge filter whose lower boundary takes this wavelength of the carbon dioxide into account can also be used.

With a sensor in accordance with the invention, pyroelectric detectors should be arranged at a spacing from the focal plane of the focusing optical element in which the size of the image of a flame or of a fire corresponds to the size of the central spacings or to the centre of area spacings of the pyroelectric detectors arranged next to one another.

When the invention is used, measured signals detected at the same time by at least two pyroelectric detectors arranged directly next to one another, which were detected with a blurred image by the arrangement of the pyroelectric detectors at a spacing from the focal plane of the focusing optical element, are compared with one another and/or are put into relation with one another. The spatial angular position of a flame or of a fire within an observation region is determined therefrom.

The quotient of measured signals of the pyroelectric detectors arranged directly next to one another is preferably determined in this process and is taken *... 25 into account in the evaluation. S. *. S* eS

* S On the evaluation of the measured signals and on the * ** association with angular positions, the positions of the individual detectors in the field arrangement or the position of their centres or centres of area correspond to discrete angular positions in the observation region. By the formation of quotients of S.....

* S the measured signals of adjacent detectors, interpolation is carried out between their discrete angular positions, whereby the angle resolution can be considerably higher than the minimal resolution which is present due to the number of detectors present in the corresponding axial direction.

In this respect, the measured signals of at least two of the detectors disposed next to one another in the corresponding direction of the field arrangement must be compared for each axial direction in which the angular position of the flame should be determined.

If e.g. the horizontal angular position of a flame

should be determined, the field arrangement of the

detectors must have at least one horizontal row arrangement and measured signals of at least two detectors disposed next to one another in a horizontal row have to be compared with one another.

If the angular position of the flame should additionally be determined in the vertical axial

direction, the field arrangement of the detectors

must furthermore have a vertical column arrangement and measured signals from at least two detectors disposed next to one another in a vertical direction additionally have to be compared with one another.

More measured signals from pairs of pyroelectric detectors in the field arrangement can additionally also be compared with one another than are necessary for a clear determination of the angular position.

The accuracy and/or resolution of the angular determination can thereby be increased again under certain circumstances.

A trigger signal for a fire alarm system and/or a . 30 sprinkler system should be generated when a measured * *S* * signal has been detected with at least one *****e * S pyroelectric detector which is larger than a presettable threshold value.

The measured signals detected by pyroelectric detectors arranged directly next to one another should always be compared with one another or be put into relation with one another when a measured signal exceeding a presettable threshold value has been detected by at least one pyroelectric detector.

The measured signals of the pyroelectric detectors can additionally also be subjected to a frequency analysis in the frequency range 2 Hz to 20 Hz. Since changes in the intensity of flames or in fires occur in this frequency range, this can additionally be used for the secure detection of whether the electromagnetic radiation detected by the pyroelectric detectors is radiation which is emitted by a flame or a fire or not.

The invention should be explained in more detail by way of example in the following.

There are shown: Figure 1 in schematic form, an example of a sensor in accordance with the invention with incident electromagnetic radiation in a .... 25 sectional representation on the left hand side and a plan view of the detector plane on the right hand side; * * * * *.. * * Figure 2 a diagram of measured signal curves detectable in idealised form with three a... pyroelectric detectors which are present at a sensor in accordance with the invention in a horizontal row arrangement; Figure 3 a diagram with quotients associated with angles of measured signals in accordance with Figure 2 detected with three pyroelectric detectors arranged next to one another; and Figure 4 a diagram of the actually detected measured signal curves of the three pyroelectric detectors in the horizontal axial direction.

In the example shown in Figure 1, the pyroelectric detectors Al to C3 of the row and column arrangement or of the array have a size of 300 pm * 300 pm and a central spacing from one another of 500 pm. Their sensitive surfaces are square.

A hemispherical lens having the radius R = 1.0 mm and a focal length of 1.45 mm can be used as the focusing optical element 1 whose planar side faces in the direction of the observation region or space to be monitored. An aperture diaphragm 2 having a diameter of d = 1.0 mm is attached to this surface of the focusing optical element 1.

The array formed with the nine pyroelectric detectors Al to C3 is arranged intrafocally at a vertex *S S.* * distance of 0.385 mm from the focussing optical * ** element 1 so that the images in the plane in which * S * S.. * the pyroelectric detectors Al to C3 are arranged have *.

* 30 a diameter of 500 pm. This corresponds to the central spacing of the pyroelectric detectors Al to C3 which * S..

are arranged next to one another and which are * ..*S* * S arranged equidistant to one another.

If the measured signals of the individual pyroelectric detectors Al to C3 are evaluated simultaneously and if in particular the measured signals of the adjacent pyroelectric detectors Al to 03 are put into relation, interpolation can be carried out between the discrete detector positions.

In Figures 2 and 3, the principle is shown in simplified form for an axial direction (only horizontal angles) . In Figure 2, ideal typical normed measured signals of the detectors A2, 82 and C2, which arise by a flame occurring at horizontal angles between +45° and -45° and of the electromagnetic radiation emitted thereby are entered over the horizontal angle (normed angle sensitivity) In Figure 3, the quotients of the measured signals in accordance with Figure 1 of respective adjacent detectors 02/82 or B2/C2 and B2/A2 or A2/B2 are entered over the horizontal angle. These quotients serve as an association function for the angle position of the then currently detected event.

If a measured signal which is above a preset threshold value is registered at the sensor by the occurrence of a flame, the detector with the largest measured signal can first be determined.

* 25 Subsequently, the quotient from this measured signal and from that of the detector adjacent thereto **S.*S * delivering the second highest measured signal is * *** determined. The angular position can be determined *** . * from this quotient with the help of the association * 30 function. The association function can e.g. be stored in the form of a table. * * * *** *

S.....

* If e.g. electromagnetic radiation occurs in the case of the ideal typical situation in Figures 2 and 3 which was emitted by a flame at a horizontal angle of -33° (vertical angle O°)on the sensor, the measured signal of detector C2 is the largest. Detector B2 delivers a measured signal which is precisely one third of the size of the measured signal which was detected by detector 02. The measured signal of detector A2 (and the other measured signals) are zero. The quotient of the measured signals which were detected with detectors C2 and B2 is exactly 3. The angle -33° can be determined from the association function (Figure 3) . The principle can be extended in the same manner to the second axial direction (vertical angles) In the shown ideal typical case, the achievable angle resolution is only limited by the measured signal-to-noise ratio of the measurement.

In Figure 4, the measured signal curves are shown over the horizontal angle of a realised flame sensor.

The resolution reached is approximately at 5°. The association function of a real flame sensor can be determined experimentally and can be stored in a

table (calibration)

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A different number and arrangement of pyroelectric * .. 25 detectors can also be selected. Pyroelectric * * * *** * * detectors in X columns and N rows can then be present S' at a sensor in accordance with the invention. S. * . * S.. S.* a