Abstract:
A system for monitoring a percentage of filter life remaining of a filter assembly of filter-fan devices. The system uses a single thermistor that is operated in two separate modes for monitoring the filter assembly. The thermistor is positioned in the airflow of the filter-fan adjacent to the air outlet. A first current is applied to the thermistor for determining the temperature of the airflow from a first thermistor voltage. The first thermistor voltage is responsive to the first current applied to the thermistor. A second current that is greater than the first current is applied to the heat the thermistor. The temperature of the thermistor is determined from a second thermistor voltage which is responsive to the second current applied to the thermistor. The percentage of filter life remaining of the filter assembly is determined from the temperature of the airflow and the second thermistor voltage.

Description:
This application claims priority to U.S. Provisional Application No. 60/176,354 filed on Jan. 14, 2000, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a filter monitoring system for filter-fan products and more particularly relates to a filter monitoring system using a thermistor that monitors the airflow through the product to determine ultimately when a filter requires replacement. The system can also provide an indication of percentage of filter life remaining. 
     2. Brief Description of the Prior Art 
     Filter-fan products such as some types of portable fans, air purifiers, humidifiers and dehumidifiers include filters for removing airborne particles from the homes or offices in which they operate. Such filters include fine particle high efficiency particulate air (HEPA) filters, filters for trapping relatively large particles and carbon filters to remove odors. Typically, a fan is positioned adjacent a removable filter to force air through the filter thereby trapping airborne particles therein. 
     FIG. 2 illustrates a cross section through new a filter-fan device  12  that includes a housing  14 , a fan  16 , and a filter assembly  18 . The fan  16  has a motor  20  and a fan blade  22 . In operation, the rotation of the fan blade  22  causes air to be drawn into the air inlet  24  through the filter assembly  18 . The filtered air then moves through to enter the fan  16 . The fan  16  expels the air into a scroll  26  which then exits through the air outlet  28 . This illustrates the basic operation of the filter-fan device  12  that uses replaceable filter assemblies  18 . The air inlet  24  is preferably formed as a perforated grill in a door of the filter-fan device  12 . 
     As the efficiency of these type of products depends upon the replacement of he filter when spent, the ability to easily determine when the filter is spent is important. Accordingly, it is desirable to provide such fan-filter products with a system to monitor the remaining life of a filter and to indicate when the filter should be replaced. 
     U.S. Pat. No. 4,050,291 to Nelson discloses a filter condition responsive apparatus for a stove vent system. The stove vent system consists of a duct having two filters located in series with a fan for creating the airflow. The patent indicates that the filters have similar pressure drop characteristics although the first filter is designed to collect grease and the second filter is a charcoal filter for removing odors. The filter condition responsive device includes a first bypass containing a thermistor which is constantly heated by an electrical heater. The bypass is essentially a small duct connected to the duct of the stove vent system with an inlet on one side of the grease filter and the outlet on the other side of the grease filter. The electrical heater appears to be a separate resistor that generates heat. A second bypass is included across the charcoal filter along with a thermistor which is also constantly heated by an electrical heater. The thermistors are electrically coupled and are connected to an amplifier which in turn is connected to a filter condition indicator. When the grease filter becomes clogged, the bypass flow rates through bypasses are different. This difference is measured by a bridge network circuit and indicated on the filter condition indicator. 
     U.S. Pat. No. 5,014,908 to Cox discloses a control circuit for use in a humidifier. The patent discloses using a pair of thermistors for checking the condition of a wick in a humidifier. The filter check includes a first thermistor TRM 1  located adjacent to the air inlet before the wick W and a second thermistor TRM 2  located adjacent to the air outlet AO. When the humidifier is operating properly with a good wick, the air absorbs water from the wick causing the temperature to drop. The control circuit compares the air temperatures at the air inlet and air outlet through the thermistors and provides an indication to replace the filter when there is not a substantial difference. 
     U.S. Pat. No. 5,429,649 to Robin discloses a device for the detection of the clogging of an air filter. The device includes first and second sensing means, an auxiliary air duct, and an electronic circuit. The first and second sensing means are thermistors. The first sensing means is located in the air duct for measuring the air flow velocity in proximity to the air filter. The second sensing means is located in the auxiliary air duct and measures the air flow velocity therein. The electronic circuit compares the velocities measured by the sensing means to determine whether the air filter is clogged. 
     U.S. Pat. No. 5,668,535 to Hendrix et al. discloses a filter condition sensor and indicator that includes a heated thermistor located in a “small-by-pass air flow path” and a circuit having an indicator light. The device experiences an increase in air velocity through the “small by-pass air flow path” due to clogging of the filter. The indicator light is illuminated when the thermistor is cooled to a predetermined value. The device includes means for adjusting the current through the thermistor depending upon the setting of a multi-speed fan. The patent discloses two embodiments of the “small by-pass air flow path”. In both embodiments the by-pass is “connected to the output air flow path separate from the filter.” (See Column 3, Lines 43-45). That is, the air that flows by the thermistor is not filtered by the filter. The first embodiment is a separate compartment and the second embodiment is an extension through the filter. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for monitoring a percentage of filter life remaining of a filter assembly of filter-fan device. The filter-fan device generally include a fan that generates an airflow that passes through the filter assembly and exits through an air outlet. The invention uses a single thermistor that is operated in two separate modes for monitoring the filter assembly. 
     The method of operating the thermistor within a filter-fan device to monitor a percentage of filter life remaining of a filter assembly includes positioning the thermistor in the airflow within the filter-fan adjacent to the air outlet. A first current is applied to the thermistor for determining the temperature of the airflow from a first thermistor voltage. The first thermistor voltage is responsive to the first current applied to the thermistor. A second current that is greater than the first current is applied to heat the thermistor. The temperature of the thermistor is determined from a second thermistor voltage which is responsive to the second current applied to the thermistor. The percentage of filter life remaining of the filter assembly is determined from the temperature of the airflow and the second thermistor voltage. Preferably an indication of the percentage of filter life remaining of the filter assembly is provided on a visual display. Preferably the thermistor is positioned about 3 inches from the air outlet of the filter-fan. 
     The system for monitoring the percentage of filter life remaining of a filter assembly in accordance with the present invention includes a thermistor and a controller. The thermistor is positioned in the airflow within the filter-fan adjacent to the air outlet. The controller is configured to operate the thermistor as noted above in connection with the method of the invention. Preferably the controller includes a microprocessor having a first look-up table and a second look-up table. The first look-up table correlates the first thermistor voltage with the temperature of the airflow. The second look up table correlates the temperature of the airflow and the second thermistor voltage with the percentage of filter life remaining of the filter assembly. Preferably the system includes a display for providing a visual indication of the percentage of filter life remaining. Preferably the controller is configured to account for different line voltages to the filter-fan. 
     In an alternative embodiment where the filter-fan includes a multiple speed fan, the controller is.configured to determine the percentage of filter life remaining of the filter assembly from the temperature of the airflow, the second thermistor voltage and the speed of the fan. Preferably the controller includes a microprocessor that has a plurality of second look-up tables with at least one second.look-up table being associated with each fan speed. 
     As a result of the present invention ,a method and apparatus for monitoring a percentage of filter life remaining of a filter assembly is provided that uses a single thermistor. The thermistor is located in an airflow of filtered air and is not located in a separate bypass duct. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front perspective view of a new filter-fan device that includes the present invention; 
     FIG. 2 is a cross-sectional view taken along line  2 — 2  as shown in FIG. 1; 
     FIG. 3 is a diagram of a control circuit in accordance with the present invention; 
     FIG. 4 is a block diagram for a filter-fan circuit that includes the present invention; 
     FIG. 5 is a first look-up table for use by the airflow monitoring circuit in the first mode; 
     FIG. 6 is a second look-up table for use by the airflow monitoring circuit in the second mode with the fan set on a low speed; 
     FIG. 7 is a second look-up table for use by the airflow monitoring circuit in the second mode with the fan set on a medium speed; and 
     FIG. 8 is a second look-up table for use by the airflow monitoring circuit in the second mode with the fan set on a high speed. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with the present invention the filter monitoring system is generally included in a control circuit  10  that controls the operation of a filter-fan device  12  and monitors the amount of air flowing to determine the degree to which the filter assembly is clogged. Since the airflow depends on the speed of the fan  16  as well as the degree to which the filter is clogged, the control circuit  10  preferably adjusts for the speed of the fan  16 . 
     Referring now to FIG. 3, the control circuit  10  preferably includes a power supply  30 , a microprocessor  32 , a liquid crystal display (LCD)  34 , pushbutton control switches S 1  through S 7 , triac drivers Q 3  through Q 7 , and an airflow monitoring circuit  36 . Preferably the control circuit  10  also includes an infrared remote control receiver  38 . The control circuit  10  shown in FIG. 3 is generally incorporated into the filter-fan circuitry as shown in the block diagram shown in FIG.  4 . 
     The power supply  30  uses the reactance of capacitor C 1  to drop the voltage to a low level. Preferably a capacitor is used instead of a resistor to limit the amount of heat generated by the circuit  10 . Resistor R 5  limits the surge current that flows when AC power is applied and the AC waveform is at a high value. Zener diode D 2  regulates the power supply voltage to a nominal 5.1 VDC. 
     The microprocessor  32  is preferably a Microchip PIC 16 C 72  with digital inputs and outputs plus four analog to digital converter (ADC) inputs, RA 1 , RA 2 , RA 3  and RA 5 . Resistor R 12  and capacitor C 7  set the internal clock oscillator frequency and diode D 4 , resistor R 11  and capacitor C 6  insure the microprocessor  32  is reset when power is removed and reapplied. Pressing switch Si also resets the microprocessor  32  to turn off all power to the triacs and the LCD  34 . 
     The LCD  34  has two seven segment digits that display the percentage of filter life remaining, SLEEP, HIGH, MED, LOW and OFF to indicate the speed of the fan  16 , DOOR AJAR to indicate when the door  29  has not been properly closed, and CHANGE FILTER to indicate when the filter assembly  18  has become clogged. FILTER LIFE REMAINING and % are turned on whenever the fan  16  is running. The display  34  is driven by the microprocessor  32  using resistors R 2 , R 3 , R 4 , R 8 , R 9  and R 1 O and ports RCO to RC 7  as segment drivers and RB 1  to RB 3  as backplane drivers. The LCD  34  is backlit by an electroluminescent panel. The light is turned on only in darkness using a cadmium sulfide sensor R 43  to sense light level. A triac Q 7  controls application of 120VAC power to the panel. 
     Fan speed switches S 2  through S 5  are monitored by the microprocessor  32  which controls the speed of the fan  16  by turning on triacs Q 3  through Q 6 . Switch S 6  turns the LCD backlight off and on when the light level is low. In high light levels, the backlight is turned off by a signal from the cadmium sulfide sensor R 43 . The voltage at the junction of resistor R 28  and cadmium sulfide sensor R 43  is monitored by the microprocessor  32  which only applies power to the LCD backlight when the light level is low. Switch S 6  overrides the operation of the cadmium sulfide sensor R 43  so the backlight can be turned off even at night. Switch S 7  is a microswitch that is open when the filter assembly compartment door  29  is not properly closed. The microprocessor  32  monitors the voltage at ADC input RA 3 . The voltage at ADC input RA 3  indicates which switch is pressed because the resistor values R 19 , R 21 , R 23 , R 24 , R 26  and R 30  are different. 
     When the microprocessor  32  senses that a switch S 2  through S 6  has been pushed, it applies a high voltage to the resistor that is connected to the gate of the corresponding triac Q 3  through Q 7 . The triac then allows current to flow from one side of the motor  20  of the fan  16  to ground. The other side of the motor  20  is tied to the high side of the AC line in the unit wiring as shown in FIG.  4 . 
     The airflow monitoring circuit  36  operates in two modes depending on the voltage the microprocessor  32  applies to the base of Q 8  from RA 4 . When RA 4  is low, Q 8  is turned off and the amount of current flowing through thermistor RT 1  is determined by the voltage on the cathode of diode D 5  and the value of resistor R 14 . The thermistor RT 1  is located within the airflow inside the filter-fan device  12  as shown in FIG.  2 . Preferably the thermistor RT 1  is located centrally and inside of the air outlet grill  28 , and most preferably about 3 inches below the grill  28 . Referring now to FIG. 3, the voltage at the junction of thermistor RT 1  and resistor R 14  is a measure of the resistance of thermistor RT 1 , and therefore the temperature of thermistor RT 1 . Transistor Q 2 , resistor R 13  and resistor R 41  convert the voltage to a value suitable for application to the microprocessor  32  ADC input RA 5 . Diode D 7 , capacitor C 11  and resistor R 42  rectify the voltage appearing on the collector of transistor Q 2  and apply it to ADC input RA 5 . The microprocessor  32  then reads the voltage and uses a first look-up table to determine the temperature of the air passing over thermistor RT 1 . A representative example of a first look-up table calibrated for use in the present invention is shown in FIG.  5 . In the first look-up table, T 1  is temperature in degree Celsius and Tlookup is the hexadecimal value used by the microprocessor  32  to determine temperature. If the hexadecimal value read from the thermistor RT 1  with low (non-self heating) current is less than d 5 h, the temperature is assumed to be 5 degree Celsius. If the value is between d 5 h and a 9 h, the temperature is assumed to be 10 degree Celsius, etc. The temperature values are used in connection with a second look-up table in the second mode as described below. 
     In the second mode, the airflow monitoring circuit  36  has Q 8  turned on causing more current to pass through thermistor RT 1  and resistor R 16  causing the temperature of thermistor RT 1  to increase. In still air, the temperature increases to 130 to 150 degree Celsius. Moving air cools thermistor RT 1  and increases its resistance causing the voltage on ADC input RA 5  to decrease. The speed of the air is related to the amount of cooling and therefore the voltage on ADC input RA 5 . The amount of cooling of thermistor RT 1  is also related to the ambient temperature of the air. Using the reading of ambient temperature taken in mode  1  and the current fan speed, the microprocessor uses a second look-up table and finds a value that it sends to the LCD  34 . Examples of the second look-up table calibrated for use at specific fan speeds in the present invention are shown in FIGS. 6 through 8. The second look-up tables include readings of the analog to digital converter input to the microprocessor  32  expressed in hexadecimal format. The hexadecimal readings represent the voltage on ADC input RA 5 , and therefore the temperature of RT 1 . The values in the left column of each look-up table represent percentage filter life remaining. The remaining columns contain the look-up values for an ambient temperature between the range of 5 degree Celsius to 35 degree Celsius. For example, if the fan speed is set to low, the ambient temperature is 20 degree Celsius and the analog to digital converter reading is 97 h, the filter life remaining is 30% as shown in FIG.  6 . Preferably the microprocessor is configured to account for differences in the line voltage as this will affect the fan speed. Preferably the value displayed on the LCD  34  is the percentage of filter life remaining. A value of 50% airflow reduction was assumed for 0% filter life remaining. When this value is reached, The CHANGE FILTER icon is also turned on in the LCD  34 . 
     Preferably the control circuit  10  also includes an infrared remote control receiver  38 . The receiver  38  allows the consumer to remotely control the fan speed of the filter-fan device  12 . The remote control receiver  38  preferably includes a device U 3  that receives the infrared remote signal and converts it to a logic level. The logic level data stream enters GPO of US and is decoded, checked for errors and sets a pattern of logic levels on GP 3  to GP 5 . GP 3  to GP 5  are tied to resistors and along with resistor R 40 , present a unique voltage to ADC input RA 1  of the microprocessor  32 . Depending on the voltage level, microprocessor  32  either does nothing or changes fan speed. 
     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be effected by one skilled in the art without departing from the scope or spirit of the invention.