Abstract:
An electronic system for monitoring the condition of the filter in an HVAC unit includes a controller which issues at least one command to a circuit for the motor of the fan producing airflow through the filter. The command to the driver circuit is associated with a desired run speed for the fan. The controller is operative to compute the speed of the fan following issuance of the command to the motor drive circuit. This computed speed is compared to a predetermined fan speed that would normally occur when the filter is dirty. The electronic system is operative to generate a warning in the event that the computed fan speed does not compare favorably with the predetermined fan speed.

Description:
BACKGROUND OF INVENTION 
     This invention relates to monitoring the condition of a component in a heating, ventilating and air conditioning unit and, in particular, to monitoring the condition of an air filter. 
     A heating, ventilating and air conditioning unit (HVAC unit) typically includes a filter which filters the air being drawn into the unit. The filter will lose its ability to properly filter the incoming air over time due to the build up of airborne particles captured by the filter. It is usually difficult to visually inspect the condition of the filter for such a build up without dismantling the unit itself. In order to provide an automatic indication of air filter condition that does not require visual inspection, systems have been developed that track the accumulated run time of a fan that draws air into the unit. When the accumulated run time exceeds a certain limit, a warning is given that the filter needs to be replaced. This way of detecting a dirty filter is not all that accurate because it does not take into account the quality of the air being processed through the unit. In this regard, relatively clean air will allow for a longer filter lifetime than if lower quality air is being processed through the unit. What is needed is a more accurate measurement of the condition of the filter itself. 
     The present invention provides an HVAC unit with an electronic system for detecting the condition of the filter in the unit regardless of the environment in which the unit is operating in. The electronic system includes a microprocessor control, which defines at least one command for a driver circuit for the motor of the fan producing airflow through the filter. The command to the driver circuit is preferably associated with a desired run speed for the fan. The computer control is operative to thereafter compute speed of the fan following issuance of the command to the motor drive circuit. This computed speed is compared to a predetermined fan speed that will occur when the filter is dirty. The electronic system is operative to generate a warning in the event that the computed fan speed does not compare favorably with the predetermined fan speed. 
     In a preferred embodiment, the computer control has the ability to issue more than one command to the driver circuit for the fan motor. Each such command is associated with a stored fan speed that can be used to determine whether a computed fan speed indicates a dirty filter. In a further preferred embodiment, the electronic system is operative to learn and define the one or more operating speeds indicative of a dirty filter condition for the particular unit. These values are stored to for use by the electronic system in determining whether a clean or dirty filter is present in the unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the invention, reference should now be made to the following detailed description taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 illustrates an indoor air conditioning unit, which provides either heated or cooled air to a space; 
     FIG. 2 illustrates the flow of air through the air conditioning unit of FIG. 1; 
     FIG. 3 is a flow chart of the process used by a controller to monitor the speed of a fan governing the flow of air through the air conditioning unit of FIG. 1; 
     FIG. 4 illustrates the storage of values used in the process of FIG. 3 to monitor the speed of the fan; and 
     FIG. 5 is a flowchart of a process used to define the table of values in FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an air conditioning unit  10  is depicted wherein air is brought in through an air intake  12  and conditioned before exiting at an outlet  14 . The air may be filtered before exiting at the outlet  14  or it may be filtered in conjunction with being heated or cooled by the air conditioning unit  10 . The unit  10  may include a keypad  16  for selecting various operating modes of the unit including the selection of a fan speed for the fan within the unit  10 . The unit  10  may also include other devices for entering information on operating modes and fan speeds. These could include a remote control device with a keypad or other form of selective entry or the unit could receive information over a hard wired bus connected to a higher level control The unit  10  also contains a display  18  which can consist of light emitting diodes, each having an assigned significance or a display screen such as a liquid crystal display that displays particular messages. 
     Referring now to FIG. 2, the air flow path through the air conditioning unit  10  is illustrated in schematic form. Air is drawn through the intake  12  and through a filter  20  by a fan  22 . The filtered air is heated or cooled by coils such as  24 , if heating or cooling is required. The conditioned air exits at the air outlet  14 . 
     A processor  26  issues commands to a motor control  28  which controls a fan motor  30  that drives the fan  22 . It is to be appreciated that the particular motor drive command issued by the processor will depend on the type of motor control  28  and fan motor  30  associated therewith. For instance, the drive command could be a particular phase-delay command to a phase fired triac which drives an inductance motor. It may also be a particular frequency command to a variable frequency motor control or it may be a particular command for a sequence of pulses to a stepping motor. In any of these cases, the particular type of motor drive command results in the motor driving the fan  22  at a particular speed. 
     The processor  26  is also connected to the key pad entry  16  as well as to the display  18  of the air conditioning unit  10 . As will be explained hereafter, the processor  26  is operative to take various selections of fan speed from the keypad entry  16  and issue appropriate command signals to the motor control  28  so as to thereby energize the motor  30  to rotate the fan  22 . The processor is furthermore operative to sense the speed of the driven fan  22  by a sensor  32  which preferably senses the rotations of the fan drive shaft. The sensed rotations of the fan drive shaft are used by the processor to compute the speed of the fan, which is thereafter used to determine whether or not the filter  20  is clean or in need of replacement. 
     Referring now to FIG. 3, a flowchart of the process executed by the processor  26  so as to determine the condition of the filter  20  is illustrated. The process begins with a step  40  wherein the processor inquires as to whether a fan speed has been selected. A fan speed will have been selected if an entry has been made on the keypad  16  and noted by the processor  26 . The selected fan speed, as indicated by the keypad  16 , is preferably a fan speed level such as low, medium, high or still further speed levels. In the event that one of the above entries has been made, the processor will duly note that the same has occurred when executing step  40 . 
     The processor will proceed to a step  42  and select a current fan speed index corresponding to the read fan speed level. The fan speed index may be a numerical index wherein an index value of one might correspond to a low fan speed level. If the selected fan speed were medium, then the fan speed index might be two, and if the fan speed index were for instance high, then the fan speed index might be three. 
     The processor proceeds to a step  44  and selects a motor drive command for the value of the current fan speed index. This step is preferably accomplished by the processor accessing a table of stored drive values in association with fan speed indexes, such as is illustrated in FIG.  4 . Assuming that the current fan speed index is one, then the drive value D 1  will be selected from the table of associated drive values. It is to be appreciated that the drive values D 1 -D n  will be appropriate values that cause the motor control  28  to drive the motor  30  at the particular speed level required by the current fan speed index. The processor proceeds to a step  46  and issues the selected motor drive command D i  to the motor control  28 . As has been previously noted, the particular motor drive command will depend on the type of motor control  28  and motor  30  associated therewith. The processor proceeds in a step  48  to time out a delay before reading the sensor  32  in a step  50 . The delay that is timed out in step  48  is preferably set so as to allow the motor  30  to reach a stable fan drive speed. The processor preferably takes a number of readings of the sensor  32  in step  50  so as to allow for a computation of the speed of the fan  22  in step  52 . 
     The processor proceeds to a step  54  and reads a fan speed threshold C i  for the current fan speed index “i”. This is preferably done by accessing the fan speed threshold for a dirty filter condition corresponding to the particular fan speed index “i” in a table such as shown in FIG.  4 . Assuming that the fan speed index is one as a result of a selected low level of fan speed, then the processor will select C 1  from the stored table of FIG. 4 in step  54 . A different fan speed threshold would be selected for a medium, high, or other fan speed with a corresponding fan speed index. 
     The processor proceeds to a step  56  and compares the computed fan speed with the fan speed threshold C i  defined in step  54 . In particular, the processor will inquire as to whether the computed fan speed is greater than the fan speed threshold C i . In this regard, it has been determined that the fan speed will increase when the filter  20  has captured a sufficient amount of air borne particles. This is due to a drop in the volume of air being moved by the fan downstream of the filter. In the event that the computed fan speed exceeds the fan speed threshold, then the processor proceeds to a step  58  and issues a dirty filter signal to the display  18 . Depending on the configuration of the display  18 , either an appropriate light or flashing light will be triggered or in the case of a message display, then an appropriate message will be displayed on a screen such as a liquid crystal display. The processor will proceed back to step  40  after an appropriate delay in a step  60 . The processor will again inquire as to whether a fan speed has been selected. In the event that a fan speed has not be selected, the processor will proceed along the no path out of step  42  to step  50  wherein the sensor  32  will again be read and the fan speed thereafter computed in step  52 . The computed fan speed will again be compared with a read fan speed threshold for the current fan speed index, which will not have changed if a fan speed has not been selected in step  40 . The computed fan speed will be compared with the fan speed threshold, C i  in a step  56  before proceeding to either step  58  or a step  62  in the event that the computed fan speed is no longer greater than the fan speed threshold C i . It is to be appreciated that this latter event may occur if the filter  20  is not sufficiently contaminated so as to continually produce the same effect on the fan  22  or if the filter  20  has been replaced. In the latter event, the filter will be clean so as to thereby produce an airflow to the fan  22  so as to produce a computed fan speed which will not be greater than the fan speed threshold in step  56 . 
     Referring to FIG. 4, it is to be noted that there is both a speed indicated for a clean filter and a fan speed indicated for a dirty filter. The speed for the dirty filter condition is preferably a positive incremental speed above that of the speed for the clear filter condition. This may either be set in a factory calibration or it may be done in accordance with the process of FIG.  5 . 
     Referring to FIG. 5, the processor inquires in a step  70  as to whether the filter  20  has been replaced in the air conditioning unit  10 . It is to be appreciated that this could be provided to the processor by an entry on the keypad  16  or by an electrical contact at the location of the filter. The electrical contact when disengaged and thereafter again engaged would indicate that a filter has been removed and thereafter replaced with another one. If the processor notes that a filter has been replaced, then the processor will proceed to a step  72  and set the current fan speed index “i” equal to one. The processor will proceed to select a motor drive command D i  corresponding to the current fan speed index. Since the fan speed index will initially be one, the motor drive command D 1  will be selected in step  74 . The processor will proceed in a step  76  to issue the selected motor drive command to the motor control  28 . The processor will implement a delay in step  78  before making a number of readings of the sensor  32  in a step  80 . The readings of the sensor are used compute a fan speed “R i ” in a step  82 . It is to be appreciated that the computed fan speed in step  82  will be the fan speed for a clean filter at the selected fan speed level. The processor proceeds in a step  84  to compute a dirty fan speed C i  by adding an amount of speed Δ to the computed fan speed R i . The amount, Δ, is preferably a difference in fan speed that has been previously derived empirically through observation as to when a filter becomes dirty in a particular flow path in a particular air conditioning unit. It is to be noted that Δ is a constant in the preferred embodiment but could be a variable amount, depending on the particular speed level that is being used to compute the relative values of C i  and R i . In the latter event, there would be a particular Δ for each speed level used in step  84 . A processor proceeds in step  86  to store both the fan speeds R i  and C i  in association with the current fan speed index “i”. The processor proceeds to step  88  and inquires as to whether the index “i” is equal to “n”. In the event that the fan speed index is not equal to “n”, then the processor will proceed in a step  90  to increment the fan speed index before returning to step  74  and selecting a motor drive for the fan speed index. The processor will again issue the appropriate motor drive command and compute the fan speed R i  and thereafter compute the fan speed C i  for a dirty filter before storing both fan speeds R i  and C i  in association with the then current fan speed index. These steps will be repeated until the fan speed index has been incremented to “n” and the corresponding fan speeds computed for that value of the fan speed index. At this time, the processor will exit from step  88  and proceed to step  92  and exit the process of FIG.  5 . 
     It is to be appreciated that the fan speed thresholds C i  for each fan speed index could also be established by merely inserting a dirty filter into the air conditioning unit and noting the corresponding fan speed for such a dirty filter. These fan speed thresholds could be stored in memory so as to be available for use in the process of FIG. 3 for depicting dirty filter conditions. 
     It is to be appreciated that a preferred embodiment for automatically detecting a dirty air filter within an air conditioning unit has been disclosed. Alterations and modifications to the thus disclosed system and process may occur without departing from the scope of the invention. For example, the fan could be positioned upstream of the filter so as to cause a different comparison of fan speed to predetermined speed in step  56 . In particular, the inquiry in step  56  would be whether computed fan speed is less than the predetermined fan speed in this situation. Accordingly, the foregoing description of the preferred embodiment is to be limited only by the following claims and equivalents thereto.