Patent Publication Number: US-10775318-B2

Title: System and method for detecting a level of dirtiness of a filter mat of an airflow cooling system for telecommunications equipment

Description:
RELATED APPLICATIONS 
     This application is a 35 U.S.C. 371 national stage filing of International Application No. PCT/EP2017/063370, filed on Jun. 1, 2017, which claims priority to European Patent Application No. 16174335.6, filed on Jun. 14, 2016. The entire contents of these applications are incorporated herein by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention is in the field of telecommunications equipment. In particular, the present invention relates to the detection of the amount of dirt accumulated in a filter mat of an airflow cooling system for telecommunications equipment. 
     BACKGROUND OF THE INVENTION 
     Telecommunications equipment produces during operation a significant amount of heat. Therefore, cooling systems are employed to avoid overheating and to keep the involved electrical components within their recommended temperature range of operation such that correct functioning thereof can be guaranteed. 
     Telecommunications equipment components are typically held in housing structures designed for such purposes, such as racks containing several subracks stapled on top of each other, in which components of equipment, such as cards, can be inserted and secured. An airflow cooling system may be employed to generate an airflow through the rack or subracks that dissipates away the excess heat produced by the telecommunications equipment installed therein. In that case, the air used for cooling has to be previously filtered in order to avoid that dirt suspended in the air is deposited on sensible components of the telecommunications equipment, such as printed circuit boards or sensors. Air filters are used for such purposes. 
     An air filter is a device typically composed of fibrous materials able to capture solid particles from the air flowing through it such that the airflow is substantially free from such solid particles once it has crossed the filter. In the course of operation of an airflow cooling system for telecommunications equipment, airborne solid particles such as dust, pollen, mould, and bacteria are captured by the filter and accumulate therein. Such solid particles are generally referred to herein as “dirt” or “dirtiness”. 
     Eventually, the amount of dirt accumulated in the filter is such that the pore structure is occluded and airflow through it is reduced to an extent that proper dissipation of the heat generated by the telecommunications equipment can no longer be guaranteed. Therefore, air filters need to be replaced when such a situation occurs. It is hence useful to monitor the amount of dirt accumulated in the filter, such that the situation can be identified and the replacement of the filter can be initiated in due time. 
     Commonly known systems for detecting the level of dirtiness of an air filter generally make use of airflow sensors that measure the airflow through the filter or the subrack and signal a decrease thereof below a predefined threshold. However such airflow sensors are technically involved, highly sensible and very costly components. Thus there is room for technical improvements in the detection of the amount of dirt accumulated in a filter mat of an airflow cooling system for telecommunications equipment. 
     SUMMARY OF THE INVENTION 
     The problem underlying the invention is a need for a better solution for determing and addressing a level of dirtiness of a filter of an airflow cooling system for telecommunications equipment. This problem is solved by an apparatus, a system, and a method, according to the present invention. 
     The system of the invention comprises a detector for detecting fluorescent light or reflected light backscattered at at least one part of a filter mat, wherein the at least one part of the filter mat comprises or is treated with a fluorescent or a reflective material. The detector comprises a light source for illuminating with sampling light said at least one part of the filter mat and a photosensor for detecting fluorescent light or reflected light backscattered at said at least one part of the filter mat caused by the illumination thereof with sampling light. The system is further configured for inferring the level of dirtiness of the filter mat from the amount of detected fluorescent or reflected light. 
     Herein, a “fluorescent or reflective material” is understood to refer to any material able of backscattering electromagnetic radiation incident upon it, be it elastically (reflection) or inelastically (fluorescence) or of emitting electromagnetic radiation stimulated by said incident electromagnetic radiation. Fluorescence is a physical phenomenon wherein a material absorbs electromagnetic radiation incident upon it and emits thereupon fluorescent electromagnetic radiation. In most cases, the emitted radiation has a longer wavelength and thus lower photon energy than the absorbed radiation. This phenomenon is most widely known from materials absorbing light in the ultraviolet (UV) region of the spectrum and emitting light in the visible region of the spectrum. Thus, the incident radiation may be invisible for the human eye, while the emitted radiation may be visible. Herein, the term “light” shall refer to any kind of electromagnetic radiation without limitation to any wavelength range. 
     Backscattering is a physical effect wherein light incident on a surface is at least in part reflected back from the surface. In the narrower sense, the incident light is reflected back to the direction from which it came. Typically, it also refers to a diffuse reflection. Herein, terms like “backscattering” and “backscattered” are understood in a broad sense to refer to any kind of radiation emitted from a surface due to the incidence of radiation on this surface, irrespectively of whether the backscattered light has the same wavelength as the incident light (classical reflection) or the backscattered light has a wavelength different from that of the incident light (fluorescence). In particular, the term “backscattered light” refers herein to any kind of light propagating away from a surface or material regardless, for example, of whether incident light has been reflected or the backscattered light has been generated by fluorescence. 
     According to the invention, at least one part of the filter mat comprises or is treated with a fluorescent or reflective material, wherein a filter mat treated with a fluorescent or reflective material herein may in particular refer to a filter mat impregnated or coated with said fluorescent or reflective material. In the course of operation of an airflow cooling system for telecommunications equipment, the air used for cooling purposes is filtered by the filter mat such that dirtiness is captured thereby. Dirtiness accumulates in the parts of the filter mat exposed to airflow forming an external layer that covers the outer surfaces thereof and increasingly impedes the penetration of sampling light into the filter mat. Consequently, the amount of photons of the sampling light which are absorbed by dirt before reaching the surface of the filter mat increases with the amount of dirtiness accumulated in the filter mat and hence leads to a gradual decrease in the fluorescence and/or the reflectivity of the at least one part of the filter mat comprising or treated with fluorescent or reflective material. 
     Therefore, a measure of the amount of fluorescent light or reflected light backscattered by a filter mat comprising or treated with a fluorescent or reflective material can be interpreted as a measure of the level of dirtiness of the filter mat. For this purpose, the detector of the invention comprises a light source, typically an LED emitting in the UV part of the spectrum, and a photosensor, typically a photodiode, that measures the illuminance of the at least one part of the filter mat comprising or treated with fluorescent or reflective material. The photosensor accomplishes this by absorbing photons of the resulting fluorescent light or reflected light and generating a corresponding photocurrent. Since the photocurrent is proportional to the illuminance absorbed by the photodiode, a measure of the photocurrent provides a measure for the amount of dirt accumulated in the filter mat. The detectors of the invention are preferably placed on the side of the filter mat facing the incoming airflow. 
     The invention provides a system able of detecting a level of dirtiness of a filter mat of an airflow cooling system for telecommunications equipment such that the necessity of replacing the filter mat can be noticed at due time and the necessary replacement of the filter mat can be initiated. Since the system of the invention, other than previously known systems, does not rely on the use of expensive airflow sensors, a novel cost efficient manner of noticing the need of replacing a filter mat in an airflow cooling system for to the communications equipment is provided by the present invention. 
     In a preferred embodiment of the invention, the system further comprises a fluorescent or reflective material for treating said at least one part of the filter mat. This way, an existing filter mat may be treated, typically impregnated or coated, with the reflective or fluorescent material such that it becomes suitable for a system according to the invention to operate. For example, an existing filter mat may be impregnated with a fluorescent material such that upon installation of a system according to the invention, the level of dirtiness of the filter mat can be inferred. 
     According to preferred embodiments of the invention, the system further comprises a first additional detector for detecting fluorescent or reflected light backscattered at a part of the filter mat comprising or treated with a fluorescent or reflective material, wherein said part is not exposed to airflow during operation of said airflow cooling system. Thereby, a decrease in the amount of detected fluorescent or reflected light due to a natural timely decrease in the fluorescence or reflectivity of the fluorescent or reflective material can be properly accounted for. The intensity of the light backscattered by the fluorescent or reflective material might naturally decrease over time even without accumulation of dirt due to the fluorescent or reflective material not being sufficiently stable. A detector not exposed to airflow during operation of the airflow cooling system is not subject to a decrease in the amount of detected fluorescent or reflected light due to the accumulation of dirt in the filter mat. Hence by comparing the amount of fluorescent or reflected light detected by the detector to that detected by the first additional detector, a contribution to the decrease in the fluorescence or reflectivity of the fluorescent or reflective material not due to the accumulation of dust can be detected and properly taken into account so as to avoid a premature diagnose of the need to replace the filter mat. 
     In a preferred embodiment of the invention, the system further comprises a second additional detector for detecting the fluorescent or reflected light backscattered at a part of the filter mat not comprising or treated with the fluorescent or reflective material. Since the second additional detector is placed such that it detects the fluorescent or reflected light backscattered at a part of the filter mat not comprising or treated with the fluorescent or reflective material, any backscattered light detected by the photosensor of the second additional detector must be ambient light, that is, light other than fluorescent or reflective light caused by the illumination of said at least one part of the filter mat with sampling light. Hence the signal measured by the second additional detector can provide a measure of the impact of ambient light upon the amount of light detected by any detector, such that the contribution of such ambient light can be subtracted from the measurements of the detector and/or the first additional detector. Note that the term “second additional detector” is only chosen to distinguish it from the “first additional detector”, but shall not rule out that such second additional detector is employed in embodiments without a “first additional detector”. 
     Additionally or alternatively, a photosensor of at least one of said detector and additional detectors can be covered with a filter structure for filtering out said ambient light. Such filter structures might be used to suppress unwanted light in wavelengths ranges that do not correspond to the sampling light or that are excluded from the fluorescence mechanism. Further, at least one of said detector and additional detectors may be placed in an enclosure keeping away ambient light. This way, ambient light can be more efficiently suppressed so as to increase the accuracy of the detection of the level of dirtiness of the filter mat. 
     According to preferred embodiments of the invention, the sampling light is modulated with at least one predetermined frequency, wherein at least one of said detector and/or additional detectors is configured for filtering out or selectively amplifying a frequency component of the fluorescent or reflected light corresponding to said at least one predetermined frequency. Since the lifetime of fluorescence is relatively short, a timely modulation of the sampling light translates into a time dependence of the photocurrent measured by the photosensor that allows easily distinguishing the contribution to the photocurrent of the fluorescent or reflected light from that of ambient light. For these purposes, a lock-in amplifier technique may be used. It is also possible to use on-off keying digital modulation and to determine the extinction ratio, i.e. the ratio of the maximum photocurrent and the minimum photocurrent. This way, the contribution of ambient light to the signal measured by the detector can be accounted for so as to avoid a premature diagnosis of the need to replace the filter mat even without use of parts of the filter mat not comprising or not treated with the fluorescent or reflective material as a reference in the manner described above. Hence the filter mat may be completely treated with the fluorescent or reflective material. Further, no additional detectors are required for accounting for the contribution of ambient light. Other physical parameters may be modulated and used to distinguishing the contribution to the photocurrent of the fluorescent or reflected light from that of ambient light, like propagation direction and polarization. 
     In a preferred embodiment of the invention, the light source and the photosensor of at least one of said detector and/or additional detectors comprise a protected surface and are configured for being arranged with respect to a filter mat such that the protective surface is not directly exposed to airflow flowing through the filter mat during operation of the cooling system, wherein the sampling light is emitted from the protected surface of the light source and the fluorescent light or reflected light is detected at the protected surface of the photosensor. This way, the amount of dirt accumulating on the emitting or detecting surfaces of said detector and/or additional detectors and hence the influence thereof on the detection of the amount of fluorescent or reflected light is minimised. This is an advantageous effect inasmuch as the system of the invention aims at detecting the level of dirtiness of the filter mat and not of the components of detector. The extent to which said protected surfaces are protected from air flow may be influenced by the design of the detector. Preferably, the shape of the at least one of said detector and additional detectors might be such that the formation of turbulence in the airflow surrounding said detector and/or additional detector during operation of the cooling system is avoided. Thereby the deposition of dust on the light source and the photosensor due to turbulent air reflux can be prevented. 
     According to preferred embodiments of the invention, the system further comprises an impregnator for impregnating at least a part of the filter mat with the fluorescent or reflective material. Thereby, the filter mat or a portion thereof may be impregnated with the fluorescent or reflective material only immediately before said filter mat or portion thereof starts being employed for filtering purposes. This way, effects of the aforementioned limited durability of the fluorescent or reflective properties of the fluorescent of reflective material, possibly influenced by ambient conditions during manufacturing, transportation, or storage of the filter mat, can be minimised. Further, the system of the invention can directly be applied to an existing cooling system without the need to previously replace or treat the filter mat. 
     Additionally or alternatively, the fluorescent or reflective material can be in the form of a stripe or band configured for being attached to the filter mat. For instance, said stripe or band might be an adhesive band that can easily be adhered to the filter mat. 
     In a preferred embodiment of the invention, at least one part of the filter mat is provided with a characteristic pattern, wherein at least one of said detector and additional detectors or a dedicated detector is configured for detecting the characteristic pattern. By recognizing such a pattern, the presence of a proper dedicated filter mat comprising or treated with the fluorescent or reflective material may be confirmed, such that for example, the system can identify the insertion of an intended replacement filter mat after a filter mat has previously been removed. Further properties of the filter mat and/or of the fluorescent or reflective material may be encoded in the characteristic pattern, such that for instance, the properties of the sampling light and/or the operating conditions of any of the detectors can be adjusted accordingly. 
     A further aspect of the invention relates to a filtering device comprising a control unit, and a system according to any of the embodiments described above, wherein the control unit is operatively connected to at least one of said detector and additional detectors, and wherein the control unit is configured for inferring a level of dirtiness of the filter mat from the amount of detected fluorescent or reflected light. The filtering device preferably comprises a filter mat. 
     According to preferred embodiments of the invention, the filter mat comprises a filtering portion, which is exposed to airflow during operation of the cooling system, a usable portion, which can be used for filtering, and a used portion, which has previously been exposed to the airflow. The filter mat comprised in the filtering device is of a size, shape and extension such that, at a given time only a portion of it, referred to herein as the “filtering portion”, is exposed to airflow and hence being used for filtering purposes. Preferably, the filtering device comprises a usable mat storage region and a used mat storage region, wherein the usable portion of the filter mat is stored in the usable mat storage region and the used portion of the filter mat is stored in the used mat storage region. Then, parts of the filter mat that have previously been used for filtering purposes and whose level of dirtiness is possibly such that they may no longer be used for such purposes, may be stored in the used mat storage region, whereas usable parts of the filter mat, which may still be used for filtering purposes, may be stored in the usable mat storage region. 
     The operation of replacing the entire filter mat needs not be carried out each time a part of the filter mat exposed to airflow needs replacing. Instead, once this happens, a part of the filter mat stored in the usable mat storage region may be conveyed into the filtering portion while the part of the filter mat previously situated in the filtering portion, which has now accumulated a substantial amount of dirt, can be conveyed into the used mat storage region. 
     Hence, replacement of the entire filter mat is only necessary after several such cycles, such that the operation lifetime of a single filter mat is maximised and the maintenance effort is minimized, which is particularly advantageous for telecommunications equipment at remote locations. 
     Preferably, the filtering device further comprises two roller cylinders, wherein the used and usable portions of the filter mat are respectively wound up on a respective one of the roller cylinders. Thereby, a simple and compact configuration for controlling the different portions of the filter mat and storing the same is provided. The roller cylinders may allow conveying a portion of the filter mat from the usable mat storage region into the part of the filtering device exposed to airflow, such that said portion may now constitute the filtering portion, while the part of the filter mat previously constituting the filtering portion may be conveyed into the used mat storage region by simply rotating the roller cylinders accordingly, such that a usable portion of the filter mat may unwind from the roller cylinder in the usable mat storage region and be conveyed into the filtering portion, while the part of the filter mat previously constituting the filtering portion may be conveyed into the used mat storage region and wound up on the roller cylinder in the used mat storage region. 
     Once the filter mat has been used up, i.e. all portions of the filter mat have accumulated an amount of dirt rendering them unsuitable for further filtering, most of the filter mat may be wound up on the roller cylinder in the used mat storage region, such that it may easily be replaced by extracting the filter mat that has already been used up and inserting a new filter mat by appropriately winding it on the rolling cylinders. The replacement of the filter mat may optionally include extracting and/or replacing at least part of the roller cylinders, for example an extractable part. Then, the filter mat may be provided by the manufacturer already totally or partially wound up on the roller cylinders, such that the filter mat can easily and quickly be replaced by replacing the roller cylinders it is wound up on. 
     Preferably, the filtering device may further comprise at least one driving mechanism for conveying the filter mat in a forward direction such that the portion thereof exposed to airflow during operation of the cooling system is changed. For instance, a portion of the filter mat may be conveyed from the filtering portion into the used mat storage region a portion thereof is moved from the usable mat storage region into the filtering portion. For example, the at least one driving mechanism may be configured for driving a rotation movement of one or both of the rolling cylinders when necessary. The at least one driving mechanism may also be integrated within one or both of the roller cylinders. Hence, when a portion of the filter mat in the filtering portion needs to be replaced, this might be done by operating the at least one driving mechanism, be it manually or preferably automatically under control of the control unit. 
     Advantageously, the at least one driving mechanism may allow for the automation of the process of conveying the filter mat in a forward direction, such that once the filtering portion of the filter mat has accumulated an amount of dirt rendering it at least partially unsuitable for further filtering, the filtering device may autonomously replace the portion of the filter mat constituting the filtering portion, such that the intervention of a human operator for supervising the filtering device is not required. Alternatively, the at least one driving mechanism may be configured for continuously conveying the filter mat in the forward direction such that the portion thereof constituting the filtering portion is continuously being renewed. In that case, the velocity with which the at least one driving mechanism conveys the filter mat in the forward direction may be conveniently adjusted, for instance such that a given portion of the filter mat remains within the filtering portion as long as the level of dirtiness accumulated therein does not render it at least partially unsuitable for filtering. 
     For example, the at least one driving mechanism may slow down the conveyance movement of the filter mat when a level of dirtiness that is below a first predefined threshold level is detected. Furthermore, the at least one driving mechanism may speed up the conveyance movement of the filter mat when a level of dirtiness that is above a second predefined threshold level is detected. In addition, the velocity of the at least one driving mechanism might be increased when a card in the subrack sends an alarm indicating that its temperature is above a specified operating temperature range. Such an alarm might also trigger a decrease of the second threshold level. 
     According to preferred embodiments of the invention, the detector is configured for detecting fluorescent or reflected light backscattered at a used portion of the filter mat and/or at the filtering portion thereof. Further, the first additional detector may be configured for detecting fluorescent or reflected light backscattered at a usable portion of the filter mat. Preferably, the detector and the first additional detector are respectively disposed such that the distance along the conveyance path of the filter mat between the first additional detector and the end of the filtering portion closest to the first additional detector is longer than the distance along the conveyance path of the filter mat between the detector and the end of the filtering portion closest to the detector. Most of the time, the detector measures fluorescent or reflected light at portions of the filter mat that have previously been exposed to the airflow, whereas the first additional detector measures fluorescent or reflected light at portions of the filter mat that have not been previously exposed to the airflow yet. However, the filter mat may be moved by a length equivalent to the distance between the detector and the end of the filtering portion closest to the detector, such that the detector can measure fluorescent or reflected light backscattered at the portion of the filter mat that is currently being used for filtering purposes as filtering portion. Once the measurement has been carried out, the filter mat can be moved back by the same length. Since the distance between the first additional detector and the end of the filtering portion closest to it is longer than said length, the measurements of the first additional detector are not disturbed by fluorescent or reflected light backscattered at the portion of the filter mat currently constituting the filtering portion. 
     According to a preferred embodiment of the invention, the sampling light is modulated with at least one predetermined frequency, and the control unit is further configured for filtering out or selectively amplifying a frequency component of the fluorescent or reflected light corresponding to said at least one predetermined frequency. This allows easily distinguishing the contribution to the photocurrent of the fluorescent or reflected light from that of ambient light. This way, the contribution of ambient light to the signal measured by the detector can be accounted for so as to avoid a premature diagnosis of the need to replace the filter. Other physical parameters may be modulated and used to distinguishing the contribution to the photocurrent of the fluorescent or reflected light from that of ambient light, like propagation direction and polarization. 
     In a preferred embodiment of the invention, the control unit is further configured for detecting when the level of dirtiness exceeds a predefined dirtiness threshold. Any configurations of the filtering device related to the replacement of a used filter mat can then be referred to this dirtiness threshold. Preferably, the control unit is further configured for providing a warning signal for signalling the necessity of replacing the filter mat or the portion thereof exposed to airflow when the control unit detects that the level of dirtiness has exceeded the predefined dirtiness threshold. The warning signal may then be appropriately interpreted by a human supervisor or by the control unit itself such that the required action for replacing said filter mat or portion thereof exposed to airflow be initiated. For example, the control unit may be operatively connected to the at least one driving mechanism described above, wherein when the control unit detects that the level of dirtiness has exceeded the predefined dirtiness threshold, the control unit operates the at least one driving mechanism in order that a portion of the filter mat is conveyed in the forward direction, such that the portion of the filter mat exposed to airflow during operation of the cooling system is changed. This renders human supervision of the filtering device unnecessary until an entire filter mat needs to be replaced. 
     According to preferred embodiments of the invention, the filtering device further comprises an electrical contact for operatively connecting the filtering device to a housing for holding said telecommunications equipment when the filtering device is disposed into the housing. The housing in the sense of this invention is typically formed by a rack for holding telecommunications equipment. Preferably, the filtering device is a pluggable filtering device configured for being plugged into a housing for holding telecommunications equipment. Said housing is preferably a rack or a subrack for holding telecommunications equipment. Thereby, an operative connection between the housing and the filtering device can be established in spite of the filtering device being a replaceable device. For instance, the filtering device, or parts thereof, like the filter mat, may be replaced or extracted for carrying out maintenance tasks. Once a filtering device is disposed in the housing again, the operative connection between the housing and the filtering device is guaranteed by said electrical contact. This allows, for example, a recycling configuration according to which the filtering device might be extracted, the filter mat thereof replaced, and then the filtering device might be inserted back into the housing and operation is resumed. Alternatively, the filtering device as a whole may be replaced once the filter mat thereof needs replacing. 
     According to preferred embodiments of the invention, the control unit is further configured for storing an initial value of the amount of fluorescent or reflected light detected by at least one of said detector and additional detectors. A usable portion of the filter mat is a portion that has not been previously exposed to the airflow during operation of the cooling system to an extent rendering said portion of the filter mat at least partially unsuitable for filtering purposes. For example, said initial value may be stored when starting operation of the cooling system with a previously unused filter mat or previously unused portion thereof. The control unit may then be further configured for comparing said initial value with the current value of said amount of detected fluorescent or reflected light in order to detect a change in said amount of detected fluorescent or reflected light with respect to said initial value. Preferably, the control unit is further configured for storing a new value of said initial value when the filter mat or the portion thereof exposed to airflow during operation of the cooling system is changed and when a significant increase in the current value of said amount of detected fluorescent or reflected light is detected. Thereby, the control unit can recognise if the filter mat has been removed and/or replaced and/or whether the detector and/or additional detectors have been cleaned. For instance, irrespectively of whether the detector and/or additional detectors have been cleaned, a new initial value is stored when the filter mat is replaced such that a corresponding predefined dirtiness threshold can be updated with respect to the new initial value. This way, the possibility that dust accumulated on detectors triggers a premature diagnose of the necessity of replacing the filter mat can be avoided. 
     Further, in case the filtering device is for instance removed from the housing, for example to carry out control or maintenance tasks, and inserted again to continue operation without having replaced the filter mat, the control unit will detect that the filter mat has been replaced but since no significant increase in the amount of detected fluorescent or reflected light is detected, no new initial value is stored such that operation of the filtering device may be resumed unaffected by the extraction and subsequent reinsertion of the filtering device. 
     A further aspect of the invention refers to a subrack for holding telecommunications equipment configured for receiving the filtering device described above, characterised in that the subrack comprises an electrical contact for operatively connecting a filtering device to the subrack when the filtering device is disposed in the subrack. Preferably, the subrack further comprises cleaning means disposed such that when the filtering device is inserted to or extracted from the subrack, the detectors of the system of the filtering device are cleaned by the cleaning means, such that dirt deposited on the detectors is cleaned away. This way, the dirt deposited on the detectors, that could possibly disturb the measurement of the detected fluorescent or reflected light, is automatically cleaned away each time the filtering device is inserted to or extracted from the subrack, for example for replacing the filter mat. Advantageously this makes a correct functioning of the filtering device independent of whether a human operator manually removes dust accumulated on the detectors or not. For example, the cleaning means may comprise a brush disposed such that when the filtering device is inserted to or extracted from the subrack, the detectors of the system of the filtering device are brushed by the brush, such that dirt deposited on the detectors is brushed away 
     A further aspect of the invention refers to a method for inferring a level of dirtiness of a filter mat from the amount of detected fluorescent or reflected light backscattered at at least one part of the filter mat with any of the systems and/or filtering devices described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  Shows a system according to an embodiment of the invention. 
         FIG. 2  Illustrates how modulation of the sampling light may help eliminating the impact of ambient light:
         a. illustrates sinusoidal modulation of the sampling light;   b. illustrates on-off keying-based digital modulation of the sampling light.       

         FIG. 3  Shows an exemplary design of a detector according to embodiments of the invention. 
         FIG. 4  Shows an embodiment of a filtering device comprising two roller cylinders. 
         FIG. 5  Shows an embodiment of the invention using two roller cylinders and fluorescent stripes attached to the filter mat. 
         FIG. 6  Illustrates the comparison of the current amount of detected fluorescent or reflected light to an initial value. 
         FIG. 7  Shows a subrack and a filtering system disposed within the subrack according to an embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to a preferred embodiment illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated apparatus and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur now or in the future to one skilled in the art to which the invention relates. 
       FIG. 1  shows a system  10  for detecting a level of dirtiness of a filter mat  20  of an airflow cooling system for telecommunications equipment according to an embodiment of the invention. The system  10  comprises a detector  12  for detecting fluorescent light emitted due to incident UV light at at least one part  22  of the filter mat  20  impregnated with a fluorescent material. The system  10  further comprises a first additional detector  14  for detecting fluorescent light backscattered at a part  24  of the filter mat  20  impregnated with the fluorescent material, wherein said part  24  is not exposed to airflow during operation of the airflow cooling system. Further, the system  10  comprises a second additional detector  16  for detecting light backscattered at a part  26  of the filter mat  20  which is not impregnated with the fluorescent material. The detector  12 , the first additional detector  14 , and a second additional detector  16  comprise a light source (not shown) for illuminating the corresponding parts  22 ,  24 , or  26  of the filter mat  20  with sampling light and a photosensor for detecting fluorescent light backscattered at said parts  22 ,  24 ,  26  of the filter mat  20  caused by the illumination thereof with sampling light. The light sources of the detectors  12 ,  14 , and  16  are LEDs emitting in the UV part of the spectrum. The photosensors of the detectors  12 ,  14 , and  16  are photodiodes that absorb photons of the resulting fluorescent light and generate a corresponding photocurrent. 
     The system  10  is configured for inferring the level of dirtiness of the filter mat  20  from the amount of detected fluorescent light detected by the detectors  12 ,  14 , and  16 , a measure of which is provided by the photocurrent generated by the photodiodes thereof. For that purpose, the detector  12 , the first additional detector  14  and the second additional detector  16  are connected to a control unit  44 . The control unit  44  processes the photocurrents generated by the detectors  12 ,  14 , and  16  and infers the level of dirtiness of the filter mat  20 . Since the first additional detector  14  detects fluorescent light backscattered at a part  24  of the filter mat  20  impregnated with the fluorescent material but not exposed to airflow, the first additional detector  14  provides a measure of the timely evolution of the fluorescence of the fluorescent material over time that is not influenced by the accumulation of dirt. Further, the second additional detector  16  detects the fluorescent light backscattered at a part  26  of the filter mat  20  that is exposed to airflow but has not been impregnated with the fluorescent material. Hence, any backscattered light detected by the photodiode of the second additional detector  16  must be ambient light, that is, light other than fluorescent light caused by the illumination of said part  26  of the filter mat  20  with sampling light. Thus, the amount of backscattered light measured by the first additional detector  14  and by the second additional detector  16  may be taken into account when evaluating the amount of fluorescent light measured by the detector  12  such that the contributions of ambient light and of a possible natural timely decrease of the fluorescence properties of the fluorescent material can be subtracted from the signal measured by the detector  12 . This way, the system  10  is able to infer the level of dirtiness accumulated in the filter mat  20  in spite of the effects of ambient light and of an eventual timely deterioration of the fluorescence properties of the fluorescent material. 
     The presence of the second additional detector  16  can be rendered unnecessary in some embodiments of the invention by covering the photosensors of the detector  12  and of the first additional detector  14  with a filter structure for filtering out ambient light. Additionally or alternatively, the sampling light of the detectors  12  and  14  can be modulated as shown in  FIG. 2 . Modulation with at least one predetermined frequency is shown in  FIG. 2 a   , which illustrates sinusoidally modulated sampling light in the upper row and the corresponding detected photocurrent in the lower row. As opposed to sampling light, ambient light is not modulated, so the contribution to the detected backscattered light not corresponding to fluorescent light can be easily subtracted by configuring the detectors  12  and  14  to filter away the signal components not corresponding to the predetermined frequency of modulation. In an alternative embodiment, the modulated signal can be amplified as compared to any background signal due to ambient light by lock-in amplification in a manner per se known in the art. 
     Emitted optical power of the sampling light and the resulting photocurrent when using a digital modulation signal based on on-off-keying (OOK) are illustrated in  FIG. 2 b   . The optical power of the sampling light alternatingly takes two values, namely a maximum value P 1  and a minimum value P 0 . The modulation signal is characterized by the extinction ratio ε source =P 1 /P 0 , which typically amounts to around 13 dB. The photocurrent values corresponding to these power levels are denoted by I 1  and I 0 , respectively. Assuming a linear relationship between the photocurrent and the optical power incident on the surface of the photodiode and denoting the extinction ratio of the photocurrent by ε current =I 1 /I 0 , the contribution I mod  of the modulation signal to the average photocurrent Ī is given by the equation 
     
       
         
           
             
               
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                     ) 
                   
                 
               
               . 
             
           
         
       
     
     Thus, the contribution of ambient light to the overall detected photocurrent can be removed by measuring the extinction of the photocurrent, since the extinction of the modulation signal is typically known. 
       FIG. 3  shows a preferred embodiment of the detectors  12 ,  14  and  16  of  FIG. 1 . As illustrated therein, each detector comprises a light source  12   a  and a photosensor  12   b . The light source  12   a  and the photosensor  12   b  each comprise a protected surface  18   a  and  18   b . When arranged with respect to the filter mat  20  in the way shown in the figure, the protected surfaces  18   a  and  18   b  are substantially parallel or slightly tilted with respect to the airflow during operation of the cooling system and hence not directly exposed thereto, such that no dirt accumulates on the protected surfaces  18   a  and  18   b . The sampling light is emitted by the light source  12   a  from its protected surface  18   a  and the resulting fluorescent light is detected by the photosensor  12   b  at its protected surface  18   b . Further, the shape of the detector shown in the figure, i.e. of the light source  12   a  and of the photosensor  12   b , is such that the formation of turbulence in the airflow surrounding the light source  12   a  and the photosensor  12   b  is avoided, as indicated by the arrows and lines signalling the airflow. Thereby, the deposition of dust on the protected surface  18   a  of the light source  12   a  and on the protected surface  18   b  of the photosensor  12   b  due to turbulent air reflux is prevented. 
       FIG. 4  shows a filtering device  40  comprising a control unit  44 , two driving mechanisms  46 , a filter mat  20  and a system for detecting a level of dirtiness of the filter mat  20  of an airflow cooling system for telecommunications equipment equivalent to the system shown in  FIG. 1 . The filtering device  40  comprises a filtering portion  23  of the filter mat  20  exposed to the airflow during operation of the cooling system, a usable mat storage region  30  for storing usable portions  21  of the filter mat  20 , and a used mat storage region  32  for storing used portions  25  of the filter mat  20 , wherein said used portions  25  have previously been exposed to the airflow. The filtering device  40  further comprises two roller cylinders  36   a  and  36   b . In the embodiment shown, the driving mechanisms  46  are each operatively connected to one of the roller cylinders  36   a  and  36   b  and configured for driving a rotation movement thereof. However, it is also possible that a driving mechanism be operatively connected to one or both of the roller cylinders  36   a ,  36   b  and that it be integrated within one or both of the roller cylinders  36   a ,  36   b  or attached to them. 
     The used portion  25  of the filter mat  20  is wound up on the roller cylinder  36   b  and the usable portion  21  of the filter mat  20  is wound up on the roller cylinder  36   a . By rotating the roller cylinders  36   a  and  36   b , a part of the filter mat  20  can be conveyed from the usable mat storage region  30  into the region of the filtering device  40  exposed to airflow such that said part of the filter mat  20  now constitutes the filtering portion  23  thereof, exposed to airflow, whereas the part of the filter mat  20  previously constituting the filtering portion  23  thereof is conveyed into the used mat storage region  32 . 
     As illustrated in the figure, the rotation of the roller cylinders  36   a  and  36   b  as indicated by the curved arrows leads to a displacement of the filter mat  20  in the forward direction signalled by the black arrow that causes a usable portion  21  of the filter mat  22  to unwind from the roller cylinder  36   a  in the usable mat storage region  30  and to be conveyed into the part of the filtering device  40  exposed to airflow such that it now constitutes the filtering portion  23 . Meanwhile, the portion of the filter mat  20  previously constituting the filtering portion  23  is conveyed into the used mat storage region  32  and wound up on the roller cylinder  36   b . The filtering device  40  further comprises an impregnator  42  for impregnating the filter mat  20  while it unwinds from the roller cylinder  36   a  when the roller cylinders  36   a  and  36   b  rotate. The impregnator  42  is placed before the additional detector  14  along the conveyance path of the filter mat  20 , such that the parts of the filter mat  20  reaching the position of the additional detector  14  have already been impregnated with the fluorescent material by the impregnator  42 . The rotation of the roller cylinders  36   a  and  36   b  is driven by the driving mechanisms  46 , which are controlled by the control unit  44 . 
     By way of example,  FIG. 4  shows a configuration in which two driving mechanisms  46  respectively drive one of the roller cylinders  36   a  and  36   b . Of course, the driving mechanisms  46  can be integrated in a single device. Configurations in which a driving mechanism drives only one of the roller cylinders are also possible. The convenience of a given configuration of the driving mechanism  46  depends on the kind of conveyance movements of the filter mat  20  that have to be driven. For example, a driving mechanism acting on the roller  36   b  can be sufficient for conveying the filter mat forward, i.e. in the direction signalled by the black arrow in the figure. However, means to actively rotate both rollers are typically required for conveying the filter mat back and forth. Note however, that the filter mat  20  can be conveyed back and forth even if only one of the roller cylinders  36   a ,  36   b  is directly driven by the driving mechanisms  46 , for instance by means of additional means if necessary, such as a spring that executes force or torque on the roller cylinder not directly driven by the driving mechanisms  46 . For example, a spring exerting a torque on roller  36   a  in a direction is opposite to the curved arrow shown in this roller in the figure can be used to convey the filter mat  20  backwards without directly applying any torque to roller  36   b . A forwards conveyance movement in the direction indicated in the figure by the black arrow can be achieved by exerting a torque on the roller cylinder  36   b  by means of the driving mechanisms  46 . Applying the same torque on both roller cylinders  36   a ,  36   b  would let the filter mat  20  rest in its current position. 
     The filtering device  40  further comprises a detector  13  configured for detecting fluorescent light backscattered at a part of the filter mat  20  momentarily stored in the used mat storage region  32  and a first additional detector  14  configured for detecting fluorescent light backscattered at a part of the filter mat  20  momentarily stored in the usable mat storage region  30 . The detector  13  and the first additional detector  14  are disposed such that the length of the filter mat  20  along the conveyance path thereof between the detector  14  and the end  38   a  of the filtering portion  23  of the filter mat  20  closest to the first additional detector  14  is longer than the length of the filter mat  20  along the conveyance path thereof between the detector  13  and the end  38   b  of the filtering portion  23  of the filter mat  20  closest to the detector  13 . Furthermore, as can be seen in the figure, the chosen position of the detectors  13  and  14  minimizes the amount of ambient light incident upon them. 
     The detector  13  measures most of the time fluorescent light backscattered at a used portion  25  of the filter mat  20  whereas the detector  14  measures most of the time fluorescent light backscattered at a usable portion  21  of the filter mat  20 , which has not been exposed to airflow yet. However, it is possible for the driving mechanisms  46  to provisionally convey the filter mat  20  in the forward direction so that a part of the filtering portion  23  is moved forward into the used mat storage region in order that the detector  13  can detect fluorescent light backscattered at the portion of the filter mat  20  constituting the filtering portion  23 . The filter mat is then conveyed in the forward direction by a length equivalent to the distance between the detector  13  and the end of the filtering portion  38   b  closest to the detector  13 . Once the detection has been carried out, the driving mechanisms  46  conveys the filter mat  20  back to the initial position. Since the length of the filter mat  20  along the conveyance path thereof between the additional detector  14  and the end of the filtering portion  38   a  closest to it is longer than the length along the conveyance path of the filter mat  20  between the detector  13  and the end of the filtering portion  38   b , the detection by the detector  14  is prevented from being disturbed by a detection corresponding to a part of the filter mat  20  which has provisionally been exposed to the airflow during the aforementioned measurement. 
     The control unit  44  is operatively connected to the detector  13  and to the first additional detector  14  and is configured for inferring the level of dirtiness of the filter mat  20  from the amount of detected fluorescent light detected by the detectors  13  and  14 . Further, the control unit  44  is configured for detecting when the level of dirtiness in the filter mat  20  exceeds a predefined dirtiness threshold. When that happens, the control unit  44  of the filtering device  40  provides a warning signal that signals the necessity of replacing the filter mat  20  or the portion  23  thereof that is exposed to airflow. In that case, a human operator may replace the filter mat  20  or operate the driving mechanisms  46  such that the part of the filter mat constituting the filtering portion  23  is replaced by a part of the filter mat previously stored in the usable mat storage region  30 . In the embodiment shown in  FIG. 4 , the control unit  44  is operatively connected to the driving mechanisms  46  and configured for automatically operating the driving mechanisms  46  when the control unit  44  detects that the level of dirtiness of the filter mat  20  has exceeded the predefined dirtiness threshold, in order that a portion of the filter mat  20  is conveyed in the forward direction, such that the portion of the filter mat  23  exposed to airflow under operation of the cooling system is replaced. 
     Another embodiment of the invention comprising rolling cylinders  36   a  and  36   b  is shown in  FIG. 5 .  FIG. 5  shows a configuration in which the filter mat  20  is split in a usable portion, mostly corresponding to the part of the filter mat  20  wound up on the rolling cylinder  36   a , a used portion, mostly corresponding to the part of the filter mat  20  wound up on the rolling cylinder  36   b , and a filtering portion of the filter mat  20  disposed between the usable portion and the used portion of the filter mat  20 . The filter mat  20  comprises two stripes  27 ,  29  of fluorescent material, which are attached to the filter mat  20 . The stripe  27  of fluorescent material is exposed to airflow such that light backscattered by the stripe  27  can be detected by a detector, e.g. the detector  12  of  FIG. 1 , to provide a measure of the level of dirtiness of the filter mat  20 . The stripe  29  is not exposed to airflow so that light backscattered by the stripe  29  can be detected by a detector, e.g. the detector  14  of  FIG. 1 , to provide a measure of a natural deterioration of the fluorescence properties of the fluorescent material to be accounted for by properly interpreting the measurement corresponding to the stripe  27 . 
     The control unit  44  of of  FIG. 1  is configured for storing an initial value of the amount of fluorescent light detected by the detector  12 , that is of the amount of fluorescent light backscattered at the part  22  of the filter mat  20  impregnated with the fluorescent material. This initial value corresponds to the first maximum peak shown in the plots of  FIG. 6 . The control unit  44  is configured for comparing the current value of the amount of detected fluorescent light detected by the detector  12  of  FIG. 1 , in order to detect a change in the value of said amount of fluorescent light. As shown in the figure, the control unit  44  is further configured for storing a new value of said initial value when the filter mat  20  or the portion thereof  22  exposed to airflow is replaced and a significant increase in the current amount of detected fluorescent light is detected. The situations in which the necessity of replacing the filter mat  20  (or the portion thereof  22  exposed to airflow) is signalled by the control unit  44  are marked by vertical lines in the figure. The plot in the upper row shows the ideal scenario in which, upon replacement of the filter mat  20 , the corresponding detectors of the system  10  are free of dust or properly cleaned. Then, the current amount of detected fluorescent light reaches the initial value upon replacement of the filter mat  20 . The middle row of the figure displays the situation in which, when the control unit  44  first signals the necessity of replacing the filter mat  20 , the filter mat  20  is replaced but the corresponding detectors are not properly cleaned. Therefore, any dust covering the detectors provokes the amount of detected fluorescent light to be initially below the initial level, as shown in the figure. However, since a new initial value is then stored and the current amount of detected fluorescent light is from then on compared to the newly stored initial value, the failure to clean the corresponding detectors does not result in a subsequent premature signalling of the necessity of replacing the filter mat  20  or the portion thereof exposed to airflow, as displayed in the figure. The lower row of the plot shows the situation in which the filter mat  20  is temporarily removed, for example to carry out maintenance tasks, but not replaced. Instead, the same filter mat  20  which has been removed is inserted back to continue operation and hence no significant increase in the current amount of detected fluorescent light is detected. The control unit  44  recognises that situation and does not store a new initial value. This only happens in the second of the shown vertical markings, which corresponds to a replacement of the filter mat  20 . 
       FIG. 7  schematically shows a subrack  50  of a rack for holding telecommunication equipment configured for receiving a pluggable filtering device  40 . The pluggable filtering device  40  comprises a filter mat  20 , a detector  12  and an electrical contact  17  for operatively connecting the pluggable filtering device  40  to the subrack electrical contact  52  of the subrack  50  when the pluggable filtering device  40  is disposed in the subrack  50  as shown in the figure. The subrack  50  further comprises a brush  54  disposed such that when the pluggable filtering device  40  is inserted or extracted from the subrack  50 , the detector  12  of the pluggable filtering device  40  is brushed by the brush  54 , such that any dirt deposited on the detector  12  is brushed away. The configuration shown in  FIG. 7  allows for the pluggable filtering device  40  to be sold as a complete plug-in unit comprising a low-cost detector  12  and a low-cost filter mat  20 . Additionally or alternatively, the filter mat  20  of the pluggable filtering device  40  may be replaceable. The replacement of the filter mat  20  of the pluggable filtering device  40  is eased by the drawer configuration shown in the figure, which allows for the insertion and removal of the pluggable filtering device  40  into and from the subrack  50  in an easy manner. 
     Although preferred exemplary embodiments are shown and specified in detail in the drawings and the preceding specification, these should be viewed as purely exemplary and not as limiting the invention. It is noted in this regard that only the preferred exemplary embodiments are shown and specified, and all variations and modifications should be protected that presently or in the future lie within the scope of protection of the invention as defined in the claims. 
     REFERENCE SIGN LIST 
     
         
           10  system 
           12 ,  13  detector 
           12   a  light source 
           12   b  photosensor 
           14 ,  16  additional detector 
           17  electrical contact 
           18   a ,  18   b  protected surfaces of detector 
           20  filter mat 
           21  usable portion of filter mat 
           22  part of filter mat comprising or treated with a fluorescent or reflective material and exposed to airflow 
           23  filtering portion of filter mat 
           24  part of filter mat comprising or treated with a fluorescent or reflective material and not exposed to airflow 
           25  used portion of filter mat 
           26  part of filter mat not comprising or treated with a fluorescent or reflective material and exposed to airflow 
           27  stripe of fluorescent material exposed to airflow 
           29  stripe of fluorescent material not exposed to airflow 
           30  usable mat storage region 
           32  used mat storage region 
           36   a ,  36   b  roller cylinders 
           38   a ,  38   b  ends of the filtering portion of the filter mat 
           40  filtering device 
           42  impregnator 
           44  control unit 
           46  driving mechanisms 
           50  subrack 
           52  electrical contact of subrack 
           54  brush