Abstract:
A dishwasher sanitation cycle includes sampling a temperature of rinse water inside a dishwasher, executing a heating cycle to keep water temperature at optimal levels, and executing a heat sum cycle to ensure that dishes are sanitized according to accepted standards.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/153,408, filed Sep. 10, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to dishwashers and, more particularly, to dishwashers having a sanitation cycle feature. 
     A dishwashing machine typically includes a water pump, spinning water jets, and a controller for executing a number of different wash cycles according to user preference, such as that disclosed in U.S. Pat. No. 4,334,143. The cleaning efficacy achieved by domestic spray-type dishwashing machines in executing those wash cycles, however, is generally determined by the manufacturer&#39;s of the machines. A public interest group known as the National Sanitation Foundation (N.S.F.) has promulgated minimum voluntary standards that have generally been accepted by dishwasher manufacturers, and that are increasingly desired by consumers. The current N.S.F. protocol (Protocol No. 95/480/05/2480) for the performance of domestic spray-type dishwashers requires that a dishwasher sanitation cycle has a time, temperature relationship that exposes dishwasher contents to a minimum amount of heat, measured in theoretical Heat Unit Equivalents (HUE). 
     However, the theoretical HUE construct is not always physically realized due to fluctuations in dishwasher system conditions, such as water temperature, during all or a portion of a dishwasher cycle. If water temperature drops, the dishwasher contents may not be sufficiently sanitized. If water temperature rises, dishes may be over-sanitized, which is energy inefficient. Consequently, the accuracy and energy efficiency of dishwasher sanitation cycles are often suspect. 
     Accordingly, it would be desirable to provide a dishwasher with an accurate sanitation cycle that minimizes the time and energy required to complete a desired degree of sanitization. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the invention, a method for sanitizing the contents of a dishwasher including a rinse water temperature sensor, a rinse water heater, and a controller coupled to the sensor and to the water heater, includes determining the temperature of the rinse water at fixed time intervals with the sensor. The sensed temperature is supplied to the controller, which compares the determined temperature of the rinse water to a minimum sanitation cycle temperature, a low sanitation cycle temperature, and a high sanitation cycle temperature. 
     The minimum sanitation cycle temperature is the lowest temperature recognized by N.S.F. that has a tabulated HUE value. For each timer interval when the determined temperature is at least the minimum sanitation cycle temperature, an HUE value corresponding to the determined temperature is indexed from a memory of the controller. HUEs are cumulatively summed at successive intervals while the determined temperature equals or exceeds the minimum sanitation cycle temperature. The process is repeated with each successive time interval until the summed HUE total equals or exceeds a minimum value according to N.S.F. protocol. 
     The low and high sanitation cycle temperatures are used to execute a heating cycle for each time interval in response to the determined temperature. The low sanitation cycle temperature is predetermined to minimize sanitation cycle time while avoiding excessive hysteresis, i.e. cycling of the heater. The high sanitation cycle temperature is determined by the crazing of glassware and dishware placed in the dishwasher. When the determined temperature is less than the low sanitation cycle temperature, the water heater is turned on. When the determined temperature is greater than a high sanitation cycle temperature, the water heater is turned off. Thus, water temperature is optimized to minimize cycle time at energy-saving temperatures. 
     Thus, an accurate dishwasher sanitation cycle is provided that conserves energy and the required time to complete a sanitation cycle while ensuring that a minimum level of sanitation is achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a dishwasher control system; and 
     FIG. 2 is a flow chart of a dishwasher sanitation cycle. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a block diagram of a dishwasher control system  10  including a controller  12  which may, for example, be a microcomputer coupled to a dishwasher user interface input  14 . An operator may enter instructions or select desired dishwasher cycles and features to be performed via user interface input  14 , and a display  16  coupled to controller  12  displays appropriate messages, indicators, a timer, and other known items of interest to dishwasher users. A memory  18  is also coupled to microcomputer controller  12  and stores instructions, calibration constants, and other information as required to satisfactorily complete a selected dishwasher cycle. Memory  18  may, for example, be a random access memory (RAM). In alternative embodiments, other forms of memory could be used in conjunction with RAM memory, including but not limited to electronically erasable programmable read only memory (EEPROM). 
     Controller  12  is also coupled to a water temperature sensor  20 , which is inputted to controller and for operating a water heater/pump  22  in response thereto as described in detail below. Analog to digital and digital to analog convertors (not shown) are coupled to controller  12  to implement the controller input from water temperature sensor  20  and the controller output to water pump/heater  22  according to known methods. Power is supplied to controller  12  by a power supply  24  configured to be coupled to a power line L. Of course, controller  12  may be used to control other dishwasher elements and functions beyond that specifically described herein. 
     In response to manipulation of user interface input  14 , controller  12  monitors various operational factors of the dishwasher, and executes operator selected functions and features according to known methods. Temperature sensor  20  is thermally coupled with water exiting the water pump to sense the temperature of the water in a dishwasher tub (not shown) and is located, for example, in a bottom of the tub, in fluid communication with the water stream discharged from a water pump inside the dishwasher, or mounted to a pipe to sense the water temperature before it exits the water pump. The construction and operation of temperature sensors are well known. 
     A signal from temperature sensor  20  is supplied to controller  12  for regulating the internal temperature of the dishwasher by processing the temperature signal from temperature sensor  20 . Controller  12  adjusts the sensed temperature, if necessary, by an empirically determined amount to compensate for temperature differences of the water at the sensed location and the theoretical test plate temperature according to N.S.F. protocol. In other words, controller  12  adjusts the sensed temperature to compensate for temperature offset at the sensed location relative to a specific position inside the wash tub that N.S.F. has selected as a reference point. A table of empirically determined temperature offsets is stored in  18  memory of controller  12 . Controller  12  indexes a temperature offset from the table based on the sensed temperature and adjusts the sensed temperature accordingly to ensure that a minimum level of sanitation is achieved. 
     FIG. 2 is a flow chart of a dishwasher sanitation cycle  90  for a dishwasher (not shown in FIG. 2) including a wash tub (not shown in FIG.  2 ), a water pump (not shown in FIG.  2 ), a water heater (not shown in FIG.  2 ), all in accordance with known dishwashers, and controller  12  (shown in FIG.  1 ). A sanitation cycle loop  90  is executed by controller  12  at an appropriate point in a controller main loop  92  during the wash cycle. First, controller  12  confirms  100  whether a sanitation cycle rinse is enabled, i.e., whether a user has selected the sanitation cycle to be performed by manipulating user interface input (shown in FIG.  1 ). If the sanitation cycle is not selected, the remainder of sanitation cycle loop  90  is by-passed by controller  12  and controller  12  returns to main loop  92 . 
     If the sanitation cycle has been selected, controller  12  confirms  101  that a dishwasher door is closed. If the door is opened, controller  12  compares  102  an elapsed time with a minimum rinse time, and either ends  104  the rinse when elapsed time exceeds the minimum rinse time, or reverts back to the main control loop  92  when elapsed time is less than the minimum rinse time. 
     When the dishwasher door is closed, controller  12  executes a one second flag and executes the following routine at one second intervals. Controller  12  compares  105  an elapsed sanitation cycle time with a maximum allowable sanitation cycle time. If elapsed sanitation cycle time is greater than or equal to the maximum allowable sanitation cycle time, then controller  12  ends  104  the sanitizing rinse, sanitation cycle loop  92  is exited and controller  12  returns to main loop  92  without setting the sanitized flag. Thus, sanitation cycle loop  90  will terminate automatically upon the expiration of a preset maximum allowable sanitation time. In a particular embodiment, for example, the maximum allowable sanitation cycle time is 60 minutes. 
     If the elapsed sanitation cycle time is less than the maximum allowable time, controller  12  samples the rinse water temperature at each 1 second interval and, depending on the position of the sensor, adjusts the sensed temperature by a temperature offset stored in controller memory  18  (shown in FIG. 1) so that actual sanitization will correspond to N.S.F. protocol. 
     Once the adjusted water temperature is determined, a heating cycle is executed based on the determined temperature. Heating cycle includes controller  12  comparing  106  the determined temperature with a low sanitation cycle temperature, and comparing  108  the determined sanitation cycle temperature with a high sanitation cycle temperature. If the determined temperature is below the low sanitation cycle temperature, heating cycle commences by the controller  12  turning on  107  the water heater. If the determined temperature is above the high cycle temperature, heating cycle concludes by controller  12  turning off  109  the water heater. 
     If the determined temperature is greater than the applicable protocol minimum temperature, i.e., the lowest temperature recognized by N.S.F. that has a tabulated HUE value, a heat sum cycle begins by comparing  110  the determined temperature with the sanitation cycle minimum temperature. In a particular embodiment, the minimum sanitation cycle temperature is 143 F. (the lowest temperature recognized by N.S.F. as having a Heating Equivalent Unit (HUE)), the low sanitation cycle temperature is 150 F., and the high sanitation cycle temperature is 155 F. The high and low sanitation cycle temperatures are selected to minimize sanitation cycle time while protecting the glassware and dishware placed in the dishwasher, and while obtaining an acceptable performance and life span of the water heater components. More specifically, the low sanitation cycle temperature is predetermined to minimize sanitation cycle time while avoiding excessive hysteresis, i.e. excessive cycling of a sequence switch (not shown) to turn the heater on and off and to maintain the water temperature between the high and low temperatures. The high sanitation cycle temperature is determined by the crazing of glassware and dishware used in the dishwasher. Of course, different temperature settings could be chosen for the low and high sanitation cycle temperatures to achieve different performance goals. 
     If the determined temperature is less than the sanitation cycle minimum temperature, controller  12  exits sanitation loop  90  and returns to main loop  92 . If the determined temperature equals or exceeds the minimum sanitation cycle temperature, controller  12  begins indexing  111  an HUE value corresponding to the determined temperature from a table of HUE values and determined temperatures stored in the controller memory. Temperatures and HUE values are provided by is N.S.F. After indexing  111  the HUE value for a particular time interval, controller  12  cumulatively sums  112  the HUE with HUE values from previous intervals in the sanitation cycle. 
     For example, the following table illustrates the operation of the heat sum cycle using the HUE values of NSF Protocol No. 95/480/05/2480: 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Time 
                 Water Temp. (° F.) 
                 HUE 
                 HUE SUM 
               
               
                   
                   
               
             
             
               
                   
                 t0  
                 142.9 
                 0.0 
                  0.0 
               
               
                   
                 t1  
                 143.0 
                 1.0 
                  1.0 
               
               
                   
                 t2  
                 143.0 
                 1.0 
                  2.0 
               
               
                   
                 t3  
                 143.0 
                 1.0 
                  3.0 
               
               
                   
                 t4  
                 143.1 
                 1.0 
                  4.0 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 t200 
                 . 
                 . 
                 250.0 
               
               
                   
                 t201 
                 144.0 
                 1.3 
                 251.3 
               
               
                   
                 t203 
                 144.1 
                 1.3 
                 252.6 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 t500 
                 . 
                 . 
                 700.0 
               
               
                   
                 t501 
                 152.0 
                 11.0  
                 711.0 
               
               
                   
                   
               
             
          
         
       
     
     Heat sum cycle also includes comparing  112  the cumulatively summed HUE value for the sanitation cycle with a desired HUE summed total. While N.S.F. Protocol No. 95/480/05/2480 domestic spray-type dishwashers require a minimum total cumulative HUE value of 2700 units, a higher HUE value may be selected by a user, or be selected as a default by controller  12 . 
     In a particular embodiment, a default HUE value for comparison  112  is automatically selected by controller  12 , such as, for example, 6500. In a further particular embodiment, a user may select another value instead of the default value. 
     If the cumulatively summed HUE value is less than the selected minimum value, the sanitation cycle loop  90  restarts and water rinse temperatures are sampled at 1 second intervals, generating a new cumulatively summed HUE value at each interval. If the cumulatively summed HUE value is greater than or equal to the selected minimum value, a sanitized flag is set  113  and the sanitizing rinse is ended  114 . Controller  12  then reverts back to main loop  92  for completing the washer cycle. In an exemplary embodiment, sanitation cycle loop is completed in about 25 minutes, and the overall wash cycle time is about 55 minutes. 
     Thus, due to close monitoring of the rinse water temperature and water heater adjustments in response thereto, the above-described dishwasher sanitation cycle achieves a specified level of sanitation accurately, consistently and efficiently, despite fluctuations in dishwasher system characteristics. The sanitation cycle is also easily adaptable to future N.S.F. standards or other applicable standards by loading the applicable data into controller memory  18 . 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.