Abstract:
A clogged filter detection system has an optical transmitter ( 32 ) aligned to pass light through an air filter ( 14 ) of an HVAC air flow plenum, a receiver ( 34 ) including a sensor and positioned to receive the transmitted light ( 48 ) directly or from a reflector ( 42 ), a processing assembly ( 35 ) receiving signals from the sensor and communicating a visual ( 104 ) or audible ( 106 ) indication when an accumulated level of obscuration exceeds a predetermined level and a component support structure. The support structure uses a “U” or “L” shaped bracket ( 60,63 ) attached to peripheral structure of a filter receptacle frame ( 12 ) and one or more side plates ( 24,26 ) of the bracket are connected to an arm ( 74,76 ) on which components are carried. The transmitter ( 32 ) and receiver ( 34 ) may be located on opposite sides of the filter ( 14 ) or on the same side, with a retroreflector ( 46 ) on the other side. The processing assembly includes a microprocessor ( 94 ) and related devices. Measures are also provided for counteracting the presence of fluffy material near the filter, for sensitivity control of sensors and for an aural signal device activated at a predetermined time after a visual indication.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of Application PCT/US02/24760, filed 1 Aug. 2002, which in turn claims the benefit of Provisional Application Ser. No. 60/310,377, filed Aug. 6, 2001, now abandoned, by the present applicant. 

   FIELD OF THE INVENTION 
   This invention relates to air filters and more particularly to devices for determining when air filters have become clogged. 
   BACKGROUND OF THE INVENTION 
   Filters for heating, ventilation and air conditioning systems (HVAC) play an important role by preventing cooling coils and heating surfaces from becoming coated with dust particles, which would result in a loss of efficiency and waste of energy. After a period of operation, dust accumulates on the filters, causing them to become clogged and requiring additional energy consumption. At some point it becomes cost-effective to remove the filter and either clean it or replace it with a new one. HVAC equipment manufacturers typically state their warranties so that the user, not the manufacturer, is responsible for equipment failures due to neglect in maintaining the system, with emphases on the system dust filter. A need exists for a device capable of monitoring the extent of accumulated dust on such filters and for providing a perceptible indication when obscuration of the filter reaches a predetermined level correlated with an optimum replacement time. 
   Various clogged air filter detection devices based on differential pressure monitoring have been developed in prior art for use in higher air velocity commercial systems, but devices of this type are ineffective for low velocity residential and commercial HVAC systems. Optical devices based on measurement of light passed through or reflected from air filters are also disclosed in certain prior patents. U.S. Pat. No. 5,141,309 discloses an optical system for detecting clogging of a specific type of vacuum cleaner dust filter which employs a pleated fabric filter. This patent shows a forked light unit having one arm carrying a light emitter and the other carrying a receiver so that a pleat of the filter may be placed between the arms in position for the light to be passed through the filter twice. In this device both arms are located on the same side of the filter so that access to the opposite side of the filter is not required. This patent also disclosed a forked light device with one arm disposed on one side of a flat filter and the other arm on the opposite side so that light passes through the filter and another embodiment wherein both the emitter and the receiver are located on the same side of a flat filter and light is directed against the filter surface to obtain a measure of filter loading with dust based on changes in the intensity of the reflected beam. U.S. Pat. No. 3,985,528 discloses an optical system incorporated as a control means for an “automatic roll-type filter” assembly, with a “photocell” indication when to advance clean filter material across the air duct. Placement of arms of a support element on opposite sides of the filter material for passing of light through the material is also disclosed. 
   The patents discussed above fail to recognize or provide a solution to certain problems presented in developing practical detectors using optical components for HVAC systems. Many of the filters deployed in these systems are located in a position such that very little space is available for mounting of necessary optical components. Filters are commonly placed adjacent to return air grills either touching or almost touching the grills so that no space is available for supporting a transmitter or receiver at any significant distance away from the filter on one side. In addition the nature of such filters imposes requirements for flexibility in placement of the transmitter and receiver. Both faces of the filter may be covered by a thin framework of containment sheets of metal foil provided with large circular openings or a cardboard lattice arrangement allowing air flow but restraining the filter material in place. In order for the transmitter, receiver and reflector to function properly they must be capable of being aligned with one another so as to provide an unobstructed path for passage of a beam of light through the openings. 
   An additional requirement arises from a need to place the transmitter and receiver at an offset angle with respect to one another so as to avoid partial blocking of flow through the filter at the point being monitored. 
   Peripheral framework for these filters typically comprises a U-shaped border made of flexible cardboard strips which would not provide enough rigidity to allow component support arms to be supported by this framework in a fixed position, as is required once the components are placed in proper alignment. A more stable base is therefore needed for securing of supports to which monitoring components are attached. 
   In addition to providing for the effective determination of a level of clogging in filters, detector systems including other features and characteristics would be desirable. In particular, such features would include use of miniaturized electronic components to compensate for the limited availability of space and providing programmable chips for performing such functions as calibrating the system and preparing a schedule for monitoring services. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to clogged filter detection systems comprising an optical transmitter adapted to transmit a beam of light through the body of a filter at least once, a receiver including a sensor and positioned to receive the transmitted light directly or from a reflector, a processing assembly for receiving signals from the sensor and communicating a perceptible indication when the level of obscurant reaches a predetermined value and a component supporting structure comprising support elements secured to brackets disposed at the periphery of the filter and engaged with filter receptacle framework. The systems may also include additional components and measures taken to avoid adverse effects of reflection of light by the surface of the filter and to align components of the system in a manner such as to obtain measurements which are not unduly effected by shadowing of portions of the filter at the point being monitored. 
   The invention may take the form of several embodiments which vary from one another in placement of the transmitter and receiver and in inclusion of other components required for a specific embodiment, in particular, a reflector, polarizing filter and quarterwave retarder. 
   In a first embodiment an optical transmitter and a receiver are situated on the same side of the filter, and a reflector is placed on the opposite side. Light from the transmitter is directed to the reflector and passes through the filter twice, once on the way to the reflector and a second time after being reflected and directed to the receiver. Upon reaching the receiver and the processing assembly, the reflected light is used to obtain a signal dependent upon an obscurant level in the filter. While other types of reflectors may be used, a retroreflector coupled to a pair of polarizing filters and a polarization rotating element such as a quarter wave plate is preferred to avoid adverse effects of reflection of light from the surface of the filter, especially fore more dense filters. 
   In a second embodiment the transmitter and receiver are positioned on opposite sides of the filter in alignment with one another so that the light beam passes through the filter only once. The receiver and processing assembly in this case may be carried in a common housing, while the transmitter is carried in a separate housing with a wiring interconnect. 
   In each of these embodiments, opposing components placed in aligned positions across from one another are preferably offset angularly away from an alignment parallel to air flow in order to avoid shadowing of the area being monitored. Placement of components straight across from one another would result in deposition of a non-representative amount of dust at the area of interest, particularly when one or more of the components is placed in close proximity to the filter. 
   Component support structure for systems of the invention may comprise a bracket in the form of a thin but rigid base member conforming to a plate of a filter receptacle against which a side face of a border strip of the peripheral framework of the filter is removably positioned, the receptacle also having a restraining ledge integral with and at a right angle to the plate, the ledge securing filter framework from moving in the direction of air flow. The bracket has a first side portion connectible to the ledge with a clip or the like as well as to a first arm and in some cases, a second side portion available for attachment to a second arm. The arms are adapted to support components including transmitters, receivers, processing assemblies and reflectors as appropriate. At least one of the arms may be pivotally and/or flexibly mounted on the ledge with an outer end free to be moved away from the middle portion of the filter in order to allow insertion and removal of the filter from the receptacle and to allow for movements necessary to obtain proper alignment of components. 
   Owing to flexibility of peripheral framework in which replaceable filters for HVAC systems are contained and their relatively loose fit in filter receptacles, placement of brackets as described above between the filter edge and the filter receptacle plate does not interfere with installation or retention of filters in their required positions. 
   It is therefore an object of this invention to provide a device for monitoring the extent of obscuration of air filters in HVAC systems. 
   Another object is to provide a clogged filter detector which includes means for determining and giving a perceptible indication when obscuration of the filter reaches a predetermined level. 
   A further object is to provide a support structure for components of a clogged filter detection system wherein the support structure is connected to elements of a filter receptacle. 
   Other objects and advantages of the invention will be apparent from the following detailed description and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded view of an HVAC plenum with a grill, filter and filter receptacle arranged for alignment. 
       FIG. 2  is a schematic of an optical assembly transmitting light through a filter and receiving light redirected back through the filter by a reflector assembly towards the receiver. 
       FIG. 3  is a view similar to  FIG. 2 , but with a simplified reflector assembly. 
       FIG. 4  is an exploded view of an “U” shaped bracket which fits within a filter receptor and supports detector components. 
       FIG. 5  is an end view of a bracket as in  FIG. 4  “U” with detector components and filter shown. 
       FIG. 6  is an end view of a bracket as in  FIG. 1  showing a transmitter and a receiver placed on opposite sides of a filter. 
       FIG. 7  is an exploded view of a filter receptacle arrangement in use in certain HVAC systems. 
       FIG. 8  is an end view showing placement of both the transmitter and the receiver spaced apart from the filter where more space is available. 
       FIG. 9  is a view as in  FIG. 8  wherein a laser is used as the light source. 
       FIG. 10  is an exploded partial view of a detector in which a reflector is supported adjacent to the filter on a frame attached to a grill structure. 
       FIG. 11  is an exploded view of an “L” shaped bracket for use in combination with a component support as in  FIG. 10 . 
       FIG. 12  is an end view of a detector wherein a component support member is attached directly to a grill. 
       FIG. 13  is a perspective view showing a reflector component attached directly to a mesh extending across a filter. 
       FIG. 14  is a schematic view showing operation of an electrical network for a transmitter/receiver assembly. 
       FIG. 15  is an end view of the detector built with and sharing parts with the filter receptacle. 
       FIG. 16  is an end view of the detector in which the transmitter is cantilevered away from the filter, the receiver is adjacent the filter and a spring depresses filter material at the receiver. 
       FIG. 17  is a schematic view showing operation of an electrical network for a transmitter/receiver assembly with an external sensitivity adjustment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1  of the drawings there is shown a terminal portion of a HVAC plenum  10  (in dotted lines) in which a filter receptacle  12  and a grill  16 , connected to the receptacle by means not shown, are aligned for being placed over a filter  14 . The filter has an external frame  18  supporting the filter body  20  which is made up of fiber glass or the like and is either self supporting or held in place by mesh  22  on both sides. Receptacle  12  has four side plates  24  with inner surfaces parallel to the direction of airflow and ledges  26  integral with and disposed perpendicular to the plates. Upon insertion of the filter into the receptacle, side edges  28  of the filter fit against inner surfaces of plates  24  and downstream side border faces  30  come into contact with ledges  26 , which restrain the filter from moving downstream. 
     FIG. 2  schematically depicts a clogged air filter detection system  11  positioned to monitor the obscuration level in filter  14  and to provide an indication when the level exceeds a predetermined value, signaling that the time for replacement or cleaning of the filter has come. The system includes a light transmitter  32  such as a Vishay model TLCR 5100 light emitting diode (LED) aimed toward a reflector assembly  42  placed on the side of the filter opposite from the transmitter so that a beam  48  of light passes through the filter and then strikes a retroreflector  46 , typically a 3M model 3990 retroreflector. Reflected light is then directed to a receiver  34 , such as a Vishay BPV 10 photodiode, which is coupled to a processing assembly  35  wherein a signal responsive to obscuration level of the filter is obtained. The receiver and transmitter may be located in a common housing  36 , along with components of the processing assembly. 
   As shown in  FIG. 2 , housing  36  is positioned to direct the beam of light through the filter  14  at an acute angle with respect to the plane of the filter. This results in passage of light through an area  50  of the filter which is offset from reflector assembly  42  and is not subjected to shadowing or obstruction of air flow  49  through the portion of the filter being monitored. Placement of the transmitter straight across from the reflector would result in use of a non-representative sample for detection of clogging, especially where optical components are located in close proximity to the filter. Placement of the transmitter and reflector at an angle of 30 to 50 degrees with respect to the filter is preferred. 
   Randomly polarized radiation is linearly polarized (in this case, perpendicular to the plane of the paper, and represented by the dot  54 ) via a polarization filter  38 , such as a 3M HN38 filter, as it departs the transmitter  32 . A linear polarization filter  40 , again a 3M HN38, in front of the receiver  34  is oriented to receive polarization normal to that of the transmitted radiation (i.e., polarized in the plane of the paper and represented by the two headed arrow  56 ) such that any radiation reflected from the filter surface  58  is highly attenuated (˜30 dB) before it reaches the receiver  34  photo diode (or phototransistor). The major portion of the radiation which passes through the HVAC filter to the reflector  46  and back is rotated 90 deg via the reflector/quarter-wave-retarder assembly  42 . This is accomplished in one of two methods: (a) by the corner cube retroreflector itself (&lt;100% efficient), or (b) by a combination of quarter-wave-retarder  44 , such as an Edmun Optics L54-542 retarder, and reflective surface  46 . The quarter wave retarder  44  converts the linear polarized radiation  54  to circular polarization which is converted to reverse circular as it is reflected from the reflector  46 , (i.e., from right hand circular to left hand circular or visa versa). As it passes back through the retarder  44 , the reverse circular radiation is converted back to linear, but with a 90 deg (i.e., flipped) orientation  56  with respect to its original  54  polarization. This radiation is now of the same polarization orientation  56  as the orientation of the polarization filter  40  in front of the receiver  34 . Thus its intensity, as modified by the optical density of the dust filter material, is accurately monitored at the receiver  34  and provides an accurate indication of the degree of filter dust contamination. This polarization assembly can be established as depicted, or can be reversed with polarization of transmitted radiation  48  lying in the plane of the paper and the received radiation  52  perpendicular to the plane of the paper. 
     FIG. 3  shows an embodiment as in  FIG. 2  except that no polarization filters are used at the transmitter and receiver, and a retroreflector  46  is placed directly against the filter  14  without an intervening quarter wave retarder or other means for rotation of the polarity of the reflected light. This approach is adequate for less dense (and therefore less costly) filters as are commonly used in residential HVAC systems. 
     FIG. 4  shows a bracket  60  of U-shaped cross section, having a bottom strip  62  and side strips  64  and  66  to which arms for supporting system components may be connected. Side strip  64  is secured to ledge  26  of the filter receptacle  12  by means of clips  68  which are hooked over the ledge. Side  64  has an aperture  70  placed to receive a bolt or rivet extending through the arm. Side strip  66  also has an aperture  72  to enable connection to a second arm. 
   In  FIG. 5  the bracket  60  is shown in position with other components of the system. The bracket is secured to the ledge  26  of the filter receptacle by clips,  68  and the lower end of a first arm  74  is fixedly connected to side strip  64 . Arm  74  extends outward at an angle away from the filter, and at its upper end is connected to housing  36 , in which transmitter  32  and receiver  34  are carried. A second arm  76  is pivotally connected at its lower end to side strip  66  of the bracket  60  and at its upper end supports a retroreflector assembly (typically retroreflecting tape)  46  aligned for being contacted with light  48  from transmitter  32  and reflecting it back to receiver  34 . 
     FIG. 6  shows an embodiment wherein transmitter  32  and receiver  34  are placed on opposite sides of the filter and transmitted light passes through the filter only once. Upon contacting the optical receiver, light is converted to an electrical signal, which is carried to a processing assembly in housing  36  by means of wire  78 . The wire may be placed between the bracket and receptacle plate as shown. No reflector is included in this embodiment. 
     FIG. 7  shows a filter receptacle  12  located at some distance away from an end of a duct or plenum  10 . Access to this type receptacle is provided by a slot  80  replacing a side plate so that the filter may be inserted from the side instead of from a downstream end position. Both arms of the sensor may extend out angularly away from the filter as shown in  FIG. 8  to take advantage of the space available on both sides and minimize shielding of the filter from air flow and entrained dust. The sensor shown in  FIG. 8  has a transmitter  32  on one side of the filter and a receiver  34  on the other side, with a wire  78  extending underneath the filter as shown in  FIG. 6 . 
   The embodiment shown in  FIG. 9  is similar to that of  FIG. 8 , except that the transmitter  32  in this instance is a laser, which produces a much narrower beam  48 . 
     FIGS. 10 ,  11  and  12  show an embodiment wherein a retroreflector assembly  42  is carried on an upper corner of a rectangular metallic (or plastic) frame  82  which is attached by magnets  84  to the frame  88  of louvered  86  grill  16 . The outer frame  88  of the grill  16  is shown in position to be attached by fastener  90  by engaging a flange  92  extending downward from the filter receptacle plate  24 . Arm  74 , which supports transmitter  32 , receiver  34  and housing  36 , is connected to the side strip  64  of L-shaped bracket  63  at aperture  70 . The bracket in turn is secured to ledge  26  of the filter receptacle by clips  68 . In this instance the magnetically supported frame which supports the reflector is not connected to the bracket as in other embodiments. 
     FIG. 13  depicts the retroreflector  42  attached to the filter support material  22  and not requiring a separate support structure. This attachment can be performed by either the filter manufacturer or in the field with adhesive. Attachment is typically upstream  49  of the sensor in a prealigned position, such that the sensor alignment operation is required only during initial sensor installation. 
   Operation of clogged filter detection of this invention is shown schematically in  FIG. 14  for a filter in use. Elements of the processing assembly  35  are carried in housing  36  supported by one of the arms. An electrical signal is generated at microprocessor  94 , controlled by driver  96 , and converted to optical at transmitter  32 . The resulting radiation beam  48  traverses a subject filter  14  body  20  and mesh  22  on either side thereof, impinges upon the retroreflector  42  and is reflected  52  back toward the optical receiver  34 . At the optical receiver it is converted back to electrical form and fed into an analog amplifier  97  assembly. The amplified signal is digitized via analog-to-digital (A-to-D) converter  98 , entered into the microprocessor  94  and stored in digital memory  100 . This provides a relative measurement of the optical transmittance through the optical network including the filter body  20  and mesh  22 . 
   Immediately after filter replacement (i.e., with the clean filter), a digital calibration signal from the A-to-D converter  98  output is stored in memory  100  upon command by manual activation of momentary external switch  102 . This provides a mechanism for sensor operation with many different types of filters, each with its own specific optical properties when clean. The microprocessor is programmed to compare the stored digital calibration signal with a pre scheduled daily measurement of the optical transmittance through the filter mesh. An indication is broadcast optically  104 , aurally  106  and/or otherwise locally and/or transmitted  108  to a remote location when the A-to-D output drops to a preprogrammed level. This preprogrammed level indicates that the filter  14  has clogged with dust, etc., and the optical transmittance through the filter has degraded to a pre-selected level which is a preprogrammed amount below the initial clean filter calibration level. For more complex HVAC systems, the digital value of the A-to-D output may be transmitted ( 108 ) on a scheduled basis to the remote location. 
     FIG. 15  depicts the detector manufactured simultaneously with the filter receptacle  12 , thereby reducing the number of parts. First arm  74  is attached directly to ledge  26 . Side strip  66  is attached directly to side plate  24  by means such as welding. 
   The transmitter, receiver and reflector can be mounted on either the upstream or downstream sides of the filter. More than one transmitter and/or receiver can be used to provide more versatility in eliminating obstructions due to the filter support structure. A corner structure may be added to the detector support bracket  62  for additional structural support. If the ledge  26  of the receptacle  12  is sufficiently strong to maintain alignment without any bottom strip  63 , side plates  64  may be attached directly to ledge  26  without strip  63 . 
   The embodiment shown in  FIG. 16  is similar to that of  FIG. 6 , except that the transmitter  32  is cantilevered away from the filter  14  and the receiver  34  is adjacent the filter. In cases with spun glass filters, the filter material is very fluffy and sometimes presents an uneven surface  110 , which tends to enwrap the receiver  34  and reduce dust or lint accumulation within the transmitter/receiver pathway  48 . In  FIG. 16  a spring  112  attached to the sensor frame  24  depresses the fluffy material  114  and allows dust or lint to enter the space near the receiver  34 . 
   The embodiment shown in  FIG. 17  is similar to that of  FIG. 14 , except that an external sensor sensitivity control  118  is added, enabling the user to adjust sensitivity for the type filter and environment encountered. Pleated filters tend to obscure more rapidly than non-pleated filters for similar reductions in airflow speed. 
   In an embodiment of the sensor, the microprocessor  94  of  FIG. 17  is programmed to provide a visible indicator  104  that the filter needs changing for a period of time (typically several days) prior to activating the aural  106  signal. 
   While the invention is described above in terms of specific embodiments, it is not to be understood as so limited, but is limited only as indicated in the appended claims.