Abstract:
An adaptive defrost control includes a microcomputer for controlling the initiation and termination of a defrost operation based, at least in part, on opening of the fresh food door and the freezer door. The control monitors the compressor run time and the fresh food and freezer door open times and adjusts the time until defrost accordingly. A pre-chill operation cools the freezer prior to defrosting the evaporator coils so that the defrost heat will have less of an affect on the maximum temperature after defrost, and enables the freezer compartment to be maintained at a temperature a few degrees higher than with known refrigerators, providing energy savings.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/104,539, filed Oct. 16, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to defrost control in a refrigerator, and more particularly to an adaptive refrigerator defrost control. 
     Defrost operations are performed in refrigerators to clean the evaporator coils and to keep the coils free from frost build up. Known defrost controls for refrigerators typically are based on fixed timer controls. More specifically, and with such known controls, a defrost operation is initiated after a fixed amount of compressor run time. After the defrost operation is initiated, the control keeps the compressor off for a second fixed period of time. Defrost heat is terminated during the second fixed time period by a thermal sensing device for sensing the temperature of the evaporator coils. 
     The compressor run time is affected, for example, by the length of time that the fresh food and freezer doors are open. Particularly, if the doors are often open and the compartments warm, the compressor runs more than if the doors are not opened very often and the compartments remain relatively cool. Although warming of the compartments may require that the compressor run more, such warming does not necessarily require that defrost operations be performed more often. Initiating defrost operations after a fixed amount of compressor run time, however, results in performing defrost operations more often. Unnecessarily performing defrost operations results in increased, and unnecessary, energy consumption. 
     In addition, during defrost operations, the temperature in the freezer compartment generally is not allowed to exceed a predetermined peak temperature, e.g., about 0.5° F. To enable completion of the defrost operation without exceeding the peak temperature, the freezer compartment normally is maintained at about −7° F., which is cooler than required for normal operations but necessary to prevent excessive warming during defrost operations. If the freezer temperature during normal operations could be increased even just a few degrees without resulting in exceeding the predetermined peak temperature during defrost operations, a potentially significant energy savings could be provided. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the invention, an adaptive defrost control includes a microcomputer for controlling the initiation and termination of a defrost operation based, at least in part, on opening of the fresh food door and the freezer door, as well as the state of a defrost heater and the compressor. The adaptive defrost control monitors both the compressor run time and the fresh food and freezer door open times. For initiating defrost, a timer of the control counts toward a Maximum Time Till Defrost, which is the sum of (i) the compressor run time, (ii) the fresh food door open time multiplied by a Fresh Food Rate (negative), and (iii) the freezer door open time multiplied by a Freezer Rate (negative). Therefore, rather than a fixed period of time before initiation of a defrost operation, the adaptive control provides that the time until defrost is adjusted based on opening of the fresh food and freezer doors. 
     Once initiated, and during a defrost operation, the adaptive control operates the refrigerator in a pre-chill state of a fixed period of time. The pre-chill state is provided so that the freezer is cooled prior to applying heat to the evaporator coils so that the defrost heat will have less of an affect on the food temperature and on the maximum temperature after defrost. Therefore, when the control has determined that a defrost operation should be initiated, the heating portion of a defrost is preceded by a Pre-Chill Time when the compressor is held on without regard to a cold control demand for cooling. The pre-chill operation enables normally maintaining the freezer compartment at a temperature a few degrees higher than with known refrigerators, which provides an energy savings. After pre-chill, the defrost heater is energized to clear the evaporator coils of ice. 
     Bimetal switches (sometimes referred to herein as terminators) are electrically connected in series with the defrost heater. The switches are responsive to the heat from the defrost heater after the coils have been cleared of frost and ice. After the control has sensed that the terminators have operated, i.e., opened, the control initiates a dwell. 
     Dwell is the time period after defrost heat is terminated and before the compressor is allowed to turn back on, i.e., before the cold control re-energizes the compressor. Although dwell time preferably is minimized, sufficient time must be provided to allow the freon pressures to equalize so that the compressor properly operates and to allow water to drip off the evaporator. To minimize dwell time, the termination of defrost heat by the external bimetal switches (i.e., the terminators) is monitored. Once the bimetal switches terminate the heating, dwell time is entered and the control holds the compressor off until the dwell time is ended. Once the defrost sequence is complete, the cold control then re-energizes the compressor while the adaptive control monitors the compressor on time. The adaptive control continues the monitoring function to determine when to reenter the defrost sequence. 
     If the defrost heater on time, i.e., the time from initiation of defrost operations to opening of the terminators, is longer than expected, the defrost is terminated by the adaptive control. When the control terminates a defrost, e.g., defrost time exceeds a Defrost Heat Time, a defrost relay is opened and the cold control is re-energized. Therefore, after a time terminated defrost, there is no dwell time. 
     If a defrost operation requires an abnormally long time, as defined by an Abnormal Defrost Delta Time, or has terminated due to a Defrost Heat Time, then the adaptive controller determines a time for a next defrost operation based only on the compressor run time with no door open adaptive features. This next defrost occurs after the compressor has run a fixed compressor run time referred to as an Abnormal Run Time. If a defrost operation is terminated by the Defrost Heat Time, initiation of the next defrost is determined by the Abnormal Run Time. 
     Also during adaptive run time, there is a minimum time between defrosts to ensure that a failed switch or a door open condition does not cause unnecessary defrost operations. For example, if a refrigerator door is left open, a refrigerator may enter into defrost operations every 2 hours. By requiring that a minimum time must elapse prior to entering defrost, such excessive defrost operations are avoided. 
     In addition to being adaptive, the control enables immediate entry into a defrost or pre-chill state for product service or test purposes. In an exemplary embodiment, by manually depressing the fresh food light switch a preselected number (e.g., 3) times within a preselected time period (e.g., 5 seconds), an immediate (no pre-chill) defrost is initiated unless the control is already in the defrost or dwell states. Once in the defrost state, and if the fresh food light switch is depressed a preselected number (e.g., 3) times within a preselected time period (e.g., 5 seconds), then the defrost state is exited. The pre-chill state can be entered by manually depressing the fresh food light switch a different preselected number (e.g., 6) times within the preselected time period (e.g., 5 seconds). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a defrost control; 
     FIG. 2 is a circuit schematic illustration of the defrost control printed wire board shown in FIG. 1; 
     FIG. 3 is a state diagram showing the expected states of this control; 
     FIG. 4 is a timing diagram representing typical operation; 
     FIG. 5 (FIGS. 5A and 5B collectively) is a flow chart illustrating a sequence of process steps executed by the microprocessor of the defrost control board; 
     FIG. 6 is a flow chart illustrating the sequence of process steps for the abnormal subroutine referenced in FIG. 5; 
     FIG. 7 is a flow chart illustrating the sequence of process steps for the first cycle subroutine referenced in FIG. 5; 
     FIG. 8 is a flow chart illustrating the sequence of process steps for the pull down subroutine referenced in FIG. 5; 
     FIG. 9 is a flow chart illustrating the sequence of process steps for the pre-chill subroutine referenced in FIG. 5; 
     FIG. 10 is a flow chart illustrating the sequence of process steps for the dwell subroutine referenced in FIG. 5; 
     FIG. 11 is a flow chart illustrating the sequence of process steps for the defrost subroutine referenced in FIG. 5; 
     FIG. 12 is a flow chart illustrating the sequence of process steps for the compressor subroutine referenced in FIG. 5; and 
     FIG. 13 (FIGS. 13A through 13E collectively) is a flow chart illustrating the sequence of process steps which may be implemented in firmware of the of the control board microcontroller. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An exemplary embodiment of defrost control is described below in detail in connection with a household refrigerator. Such household refrigerators are commercially available, for example, from General Electric Company, Louisville, Ky., 40225, and such refrigerators can be modified to incorporate the defrost control. The defrost control, of course, can be used in many other models and types of refrigerators, and is not limited to any one particular refrigerator type. Also, and although the present invention is described herein in the context of a household refrigerator, the invention can be used in connection with many other types of cooling apparatus in which defrost operations are performed. 
     Referring now specifically to the figures, FIG. 1 is a block diagram of a defrost control  20  in accordance with one embodiment of the present invention. Defrost control is electrically coupled to components of a refrigerator  22  to control defrost operation. More particularly, refrigerator  22  includes a freezer door and a fresh food door, and respective switches  24  and  26  generate signals indicative of whether the respective doors are open or closed. A freezer lamp  28  is connected in series with freezer door switch  24 , and a fresh food lamp  30  is connected in series with fresh food switch  26  so that when either door is open and the respective switches  24  and  26  close, the appropriate lamp is energized and illuminates the fresh food or freezer compartments. Control  20  is electrically coupled to switches  24  and  26  and receives signals at pins FFD and FZD indicative of the state of each switch. 
     Refrigerator  22  also includes a compressor  32  connected in series with a cold control switch  34 . When refrigerator  22  requires cooling, cold control switch  34  closes so that compressor  32  is energized. Refrigerator  22  also includes a defrost heater  36  and bimetal terminators  38 . As described below in more detail, terminators  38  control de-energization of defrost heater  36 . 
     Compressor  32  may, for example, be an induction compressor motor energized by a 230V, 50/60 Hz source, and having a 2.0 A full load current at 230V/50 Hz+/−15%, and a 15 A locked rotor current. Condenser and evaporator fans (not shown) may be shaded pole or ECM fan motors energized at 230V, 50/60 Hz. Defrost heater  36  may be energized as a resistance load at 230V, 50/60 Hz, a 4.5 A rated current, and 10,000 operations in 20 year life. 
     FIG. 2 is a circuit schematic illustration of defrost control  20  implanted on a printed wire board and configured for controlling refrigerator defrost operations. Generally, control  20  includes a microcontroller  40 , sometimes referred to herein as a microcomputer or processor, U 1  which controls relays K 1  and K 2 . Relays K 1  and K 2  control energization of compressor  32  and defrost heater  36 . Exemplary values for the components of control  20  are set forth below. 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Capacitors 
                   
                   
               
               
                   
                 C1: 
                 0.18 
                 uF/400 V 
               
               
                   
                 C2: 
                 2200 
                 uF/16 V 
               
               
                   
                 C3: 
                 0.1 
                 uF 
               
               
                   
                 C4: 
                 0.01 
                 uF 
               
               
                   
                 C5: 
                 0.1 
                 uF 
               
               
                   
                 C6 
                 0.1 
                 uF 
               
               
                   
                 C7: 
                 0.01 
                 uF 
               
               
                   
                 C8: 
                 0,01 
                 uF 
               
               
                   
                 C9: 
                 22 
                 uF/250 V 
               
               
                   
                 C10: 
                 22 
                 uF/250 V 
               
               
                   
                 C11: 
                 0.01 
                 uF 
               
               
                   
                 C12: 
                 0.01 
                 uF 
               
               
                   
                 Resistors 
               
               
                   
                 R1 
                 39K, 
                 1/2 W 
               
               
                   
                 R2: 
                 470 
                   
               
               
                   
                 R3: 
                 3K, 
                 3 W 
               
               
                   
                 R4: 
                 3K, 
                 3 W 
               
               
                   
                 R5: 
                 220, 
                 1/2 W 
               
               
                   
                 R6: 
                 10K 
                   
               
               
                   
                 R7: 
                 33K 
                   
               
               
                   
                 R8: 
                 33K 
                   
               
               
                   
                 R9: 
                 4.7K 
                   
               
               
                   
                 R10: 
                 4.7K 
                   
               
               
                   
                 R11: 
                 470K, 
                 1/2 W 
               
               
                   
                 R12: 
                 470K, 
                 1/2 W 
               
               
                   
                 R13: 
                 470K, 
                 1/2 W 
               
               
                   
                 R14: 
                 470K, 
                 1/2 W 
               
               
                   
                 R16: 
                 10K 
                   
               
               
                   
                 R17: 
                 470K, 
                 1/2 W 
               
               
                   
                 R18: 
                 470K, 
                 1/2 W 
               
               
                   
                 R19: 
                 470K, 
                 1/2 W 
               
               
                   
                 R20: 
                 470K, 
                 1/2 W 
               
               
                   
                 R21: 
                 10K 
                   
               
               
                   
                 R22: 
                 10K 
                   
               
               
                   
                 R23: 
                 10K 
                   
               
               
                   
                 R24: 
                 10K 
                   
               
               
                   
                 R25: 
                 10K 
                   
               
               
                   
                 R26: 
                 10K 
                   
               
               
                   
                 R27: 
                 10K 
                   
               
               
                   
                 R28: 
                 10K 
                   
               
               
                   
                 Diodes 
               
               
                   
                 CR1: 
                 1N4007 
                   
               
               
                   
                 CR2: 
                 1N4007 
                   
               
               
                   
                 CR3: 
                 1N4007 
                   
               
               
                   
                 CR4: 
                 1N4007 
                   
               
               
                   
                 CR5: 
                 1N4007 
                   
               
               
                   
                 Zener Diodes 
               
               
                   
                 ZD1 
                 1N5231 
                   
               
               
                   
                 ZD2: 
                 1N5231 
                   
               
               
                   
                 ZD3: 
                 1N5231 
                   
               
               
                   
                 ZD4: 
                 1N5231 
                   
               
               
                   
                 ZD5: 
                 1N5231 
                   
               
               
                   
                 ZD6: 
                 1.8V, 
                 1N4678 
               
               
                   
                 Transistors 
               
               
                   
                 Q1: 
                 MPS0A44 
                   
               
               
                   
                 Q2: 
                 MPS-A44 
                   
               
               
                   
                 Q3: 
                 MMBT-4403 
                   
               
               
                   
                 Transformers 
               
               
                   
                 X1: 
                 4 
                 MHz 
               
               
                   
                 Metal Oxide Varistor 
               
               
                   
                 MOV1: 
                 300 V 
                 MOV 
               
               
                   
                 Fuse Link 
               
               
                   
                 F1 
               
               
                   
                 Relays 
               
               
                   
                 K1: 
                 Relay-24 V, 
                 15 Amp/275 V 
               
               
                   
                 K2: 
                 Relay-24 V, 
                 15 Amp/275 V 
               
               
                   
                 Processor 
               
               
                   
                 U1: 
                 PIC16C54/XT 
                   
               
               
                   
                   
               
             
          
         
       
     
     The present invention is not limited to implementation with the specific components described above, and many variations are possible. 
     FIG. 3 is a state diagram showing the expected states of control  20 , and FIG. 4 is a timing diagram illustrating the timing associated with each state. Each state, and the associated timing, are described below. 
     Compressor State 
     In this state, compressor  32  is controlled by cold control  34 . Defrost control  20  sums the compressor run time by monitoring the compressor voltage at pin CR (FIG.  1 ). Defrost control  20  also monitors the Fresh Food and Freezer door open times and reduces the compressor run time by a factor multiplied by the door open times. 
     Pre-Chill State 
     Compressor  32  is controlled by C relay (FIG.  1 ). C relay is on during the Pre-Chill Time period. No adaptive inputs affect the timing during Pre-Chill. 
     Defrost State 
     Compressor  32  is held off and defrost heater  36  is turned on by operating D relay (FIG.  1 ). During this time, control  20  monitors defrost terminator  38  to determine when terminators  38  open. If terminators  38  open during the defrost time, heater  36  is turned off and control  20  immediately enters the dwell state. If terminator  38  do not open during the defrost time, defrost operation is terminated by time and the dwell state is bypassed. No adaptive inputs affect the Defrost timing. 
     Dwell State 
     During the Dwell time, relay D (FIG. 1) is energized and maintains compressor  32  off. No adaptive inputs affect the Dwell timing. 
     First Cycle State 
     After a power failure, control  20  monitors the compressor run time. If control  20  determines that compressor  32  has turned off during the Normal Power Up Time period, control  20  enters the Pre-Chill state and a normal defrost condition. If control  20  determines that compressor  32  has not turned off during the Normal Power Up Time, control  39  enters the Pull Down state. 
     Pull Down State 
     In the Pull Down state, control  20  monitors the compressor run time for the Pull Down Time and immediately enters defrost operation without a pre-chill at the end of the Pull Down Time. 
     Abnormal State 
     The Abnormal state is entered when the defrost time has been longer than the abnormal defrost time as defined by Abnormal Defrost Delta Time, and/or when the defrost state is time terminated. When in the Abnormal state, control  20  enters the next defrost operation after a fixed amount of compressor run time, referred to herein as the Abnormal Run Time. 
     To achieve the state control and timing described above, control  20  operates in accordance with the process steps illustrated in FIGS. 5-12. The flow charts set forth in FIGS. 5-12 are provided to set forth a general overview of the operation of microcontroller  40 . A flow chart illustrating one possible microcontroller firmware implementation is set forth in FIG.  13 . 
     More specifically, FIG. 5 illustrates process steps  50  executed by controller  40  upon initiation, or start  52 , of operation, i.e., power-up. Upon power up  52 , controller  40  may execute factory tests (not shown). These tests may be useful in both the factory and in the field in the event that a problem has been detected. In any event, after power up, controller  40  sets State equal to First Cycle and New State equal to True  54 . Controller  40  then reads 56 inputs, e.g., FFD, FZD, CR and DT, which takes about 20 msec, and determines whether the defrost service has been requested, e.g., one press of switch S 1 ,  58 . If yes, controller  40  checks whether the State equals Defrost or State equals Dwell  60 . If no, then controller  40  sets State equal to Defrost and New State equal to True  62 . If yes, then controller  40  sets State equal to First Cycle and New State equal to True  63 . 
     After completing steps  60 - 63 , or if the defrost service was not requested, controller  40  checks whether pre-chill service has been requested  64 , e.g., two presses of switch S 1 . If yes, then State is set equal to Pre-Chill and New State is set equal to True  66 . 
     As an alternative to switch SW 1 , an existing switch in the refrigerator may be used, e.g., the fresh food compartment light switch, to request defrost service and pre-chill service. For example, by manually depressing the fresh food light switch a preselected number (e.g., 3) times within a preselected time period (e.g., 5 seconds), an immediate (no pre-chill) defrost is initiated unless the control is already in the defrost or dwell states. Once in the defrost state, and if the fresh food light switch is depressed a preselected number (e.g., 3) times within a preselected time period (e.g., 5 seconds), then the defrost state is exited. The pre-chill state can be entered by manually depressing the fresh food light switch a different preselected number (e.g., 6) times within the preselected time period (e.g., 5 seconds). Of course, many other variations are possible. 
     Referring again to FIG. 5, and after completing step  66 , or if pre-chill service is not requested, then controller  40  determines whether New State is equal to True  68 . If yes, then state constraint are defined  66 . After defining state constraints, or if New State is not equal to True, then controller  40  determines whether the current state equals Abnormal  72 , First Cycle  74 , Pull Down  76 , Pre-Chill  78 , Dwell  80 , or Defrost  82 . Depending upon the current state, controller  40  then executes Abnormal  84 , First Cycle  86 , Pull Down  88 , Pre-Chill  90 , Dwell  92 , or Defrost  94  subroutines. If the state is not one of the identified states, then controller  40  executes a compressor  96  subroutine. Subsequent to executing the appropriate subroutine, then processor delays processing for about 50 msec. and returns to step  56 . 
     Set forth below is a description of each subroutine. More particularly, FIG. 6 is a flow chart illustrating the sequence of process steps for the abnormal subroutine  84  referenced in FIG.  5 . Generally, abnormal defrost control provides a faster defrost if the last defrost continued in the heating stage for a greater time than expected. By defrosting sooner, it is expected that any icing condition would clear up. 
     Specifically, if the defrost requires an abnormally long time, defined by the Abnormal Defrost Delta Time, or has terminated due to the Defrost Heat Time, then the next defrost is controlled by the compressor run time only with no door open adaptive features. The next defrost occurs after compressor has run a fixed compressor run time referred to herein as the Abnormal Run Time. The abnormal defrost time is less than the Defrost Heat Time. Therefore, if defrost is terminated by the Defrost Heat Time, the next defrost is determined by the Abnormal Run Time. Since the abnormal defrost time is limited by the Defrost Heat Time and since there are jumpers which change the Defrost Heat Time as described below in more detail, the Abnormal Defrost Delta Time is defined as the Defrost Heat Time minus the abnormal defrost time. Thus, the Abnormal Defrost Delta Time defines the time left in a Defrost Heat Time at which a flag is set which will later initiate an Abnormal Run Time instead of a normal adaptive condition. 
     Specifically referring now to FIG. 6, in the abnormal state, controller  40  decrements a counter T 1   100 , and if T 1  is less than or equal to zero  102 , then controller  40  sets State equal to Pre-Chill and New State equal to true  104 . If T 1  is greater than zero, or after controller  40  sets State and New State, controller  40  returns  106  back to main routine  50  illustrated in FIG.  5 . 
     FIG. 7 is a flow chart illustrating the sequence of process steps for the first cycle subroutine  108  referenced in FIG.  5 . More particularly, first cycle defrost control facilitates ensuring that if the refrigerator is warm, a sufficient time is provided to pull down the compartment temperatures before a defrost occurs, and also provides for an early defrost if the refrigerator is already cold. 
     Since no knowledge about the last defrost is carried through a power failure of more than, for example, 1.4 seconds, a special power up sequence is provided. Specifically, after power is reapplied from a power failure, the compressor continuous on time is monitored. If the compressor turns on (almost immediately) and remains continuously on for a time equal to Normal Power Up Time, then the refrigerator is assumed to be in a pull down condition and a defrost is delayed until the Pull Down Time has elapsed. Otherwise, if the compressor shuts down before the Normal Power Up Time, then it is assumed that the refrigerator is already cold and that a defrost could be needed. Therefore, a defrost sequence is initiated at the Normal Power Up Time. The Pull Down Time and Normal Power Up Time may be different for different controls. 
     More specifically referring to FIG. 7, and with respect to achieving the above described control, in first cycle subroutine  86  controller  40  checks whether the compressor is on  108 . If no, then Compressor 100% is set equal to false  110 , and processing returns  112  back to main routine  50 . If yes, timer Ti is decremented  114  and controller  40  checks whether timer T 1  has a value less than or equal to zero  116 . If no, then processing  112  returns to main routine  50 . If yes, then controller  40  determines whether Compressor 100% is set equal to true  118 . If no, the State is set equal to Pre-Chill and New State is set equal to True  120 . If yes, then controller  40  sets State equal to Pull Down and New State equal to True  122 . Processing then returns to the main routine  50 . 
     FIG. 8 is a flow chart illustrating the sequence of process steps for pull down subroutine  88  referenced in FIG.  5 . More particularly, and as with first cycle defrost control, pull down control facilitates ensuring that if the refrigerator is warm, a sufficient time to pull down of the compartment temperatures is provided before a defrost occurs, and also provides for an early defrost if the refrigerator is already cold. 
     More specifically referring to FIG. 8, and with respect to achieving the above described control, in Pull Down subroutine  88 , controller  40  checks whether compressor is on  124 . If no, then processing returns  126  back to main routine  50 . If yes, then timer T 1  is decremented  128  and controller  40  checks whether timer T 1  has a value less than or equal to zero  130 . If no, then processing returns  126  to the main routine  50 . If yes, then controller  40  sets State equal to Defrost and New State is set equal to True  132 . Processing then returns  126  to main routine  50 . 
     FIG. 9 is a flow chart illustrating the sequence of process steps for pre-chill subroutine  90  referenced in FIG.  5 . More specifically, pre-chill control provides for cooling the freezer compartment prior to a defrost to decrease the potential for the food to warm too much during a defrost operation. That is, pre-chill is intended to cool the freezer prior to applying heat to the evaporator coil so that the defrost heat will have less of an effect on the food temperature and on the maximum temperature after defrost. Therefore, when the control has determined that a defrost should be initiated, the heating portion of a defrost will be preceded by a Pre-Chill Time when the compressor is held on without regard to the cold control demand for cooling. Pre-Chill Time may be different for different controls. 
     Referring specifically to FIG. 9, once Pre-Chill operations  90  are initiated, controller  40  decrements the value of timer T 1   134 . Controller  40  then checks whether timer T 1  has a value less than or equal to zero  136 . If no, the processing returns  138  to main routine  50 . If yes, then controller  40  sets State equal to Defrost and New State equal to true  140 . Processing then returns  135  to main routine  50 . 
     FIG. 10 is a flow chart illustrating the sequence of process steps for dwell subroutine  92  referenced in FIG.  5 . More particularly, fixed time dwell control controls the elapsed time after a heated defrost prior to re-energizing the compressor. The Dwell state is as short as possible to reduce food heating but must allow the freon to equalize so that the compressor properly operates. 
     The intent of the Dwell state is to minimize the dwell time as compared to known controls. Therefore, the termination of defrost heat by the bimetal switches is monitored. As soon as the bimetal switches terminate the heating step of defrost, a Dwell time is entered which holds compressor off until the Dwell time is ended. Dwell Time may be different for different controls. 
     Referring now specifically to FIG. 10, once Dwell operations  92  are initiated, controller  40  decrements the value of timer T 1   142 . Controller  40  then checks whether the value of timer T 1  is less than or equal to zero  144 . If no, then processing returns  146  to main routine  50 . If yes, then controller  40  checks whether the abnormal flag has been set  148 . If yes, then controller  40  sets State equal to Abnormal and New State is set equal to True  150 . If no, controller  40  sets State equal to Compressor and New State equal to True  152 . Processing then returns  146  to main routine  50 . 
     FIG. 11 is a flow chart illustrating the sequence of process steps for defrost subroutine  94  referenced in FIG.  5 . The defrost state initiates a defrost which cleans the evaporator coils and keeps them free from frost build up. 
     During normal defrost operations, the following operations are performed. 
     1) Pre-chill for a fixed amount of time, 
     2) Heated defrost terminated by temperature, and 
     3) Dwell for a fixed amount of time. 
     During abnormal defrost operations, the following operations are performed. 
     1) Pre-chill for a fixed amount of time, and 
     2) Heated defrost terminated after a fixed amount of time. 
     When the defrost sequence is complete, controller  40  allows the cold control to operate the compressor while monitoring the compressor on time, and other factors for an adaptive defrost control, to determine when to reenter the defrost sequence. 
     Defrost heater  36  is turned off by bimetal switches  38  which are located at the top of the evaporator. Switches  38  sense heat from defrost heater  36  after the coils have cleared of frost and ice. After controller  40  has senses that terminators  38  have operated, a dwell is initiated. 
     The defrost head may in some cases last longer than expected. In the case when the defrost heat has lasted longer than the Defrost Heat time, the defrost heat is terminated by controller  40 . When controller  40  terminates a defrost, it opens the defrost relay which reenergizes the cold control. Therefore, after a time terminated defrost, there will be no dwell time. Defrost Heat Time may be different for different controls. 
     Referring now specifically to FIG. 11, once Defrost operations  94  are initiated, controller  40  decrements the value of timer T 1   154 . Controller  40  then checks whether the value of timer T 1  is less than or equal to zero  156 . If yes, then controller  40  sets State equal to Abnormal and New State equal to True  158 , and processing returns  160  to main routine  50 . If no, then controller  40  checks whether timer T 1  has a value less than or equal to the Abnormal Delta Time  162 . If yes, then the abnormal flag is set  164 . If no, or after setting the abnormal flag, controller  40  determines whether the terminator is open  166 . If no, processing returns  160  to main routine  50 . If yes, then State is set equal to Dwell and the New State is set equal to True  168 . Processing then returns  160  to main routine  50 . 
     FIG. 12 is a flow chart illustrating the sequence of process steps for compressor subroutine  96 referenced in FIG.  5 . Generally, controller  40  adapts the defrost time depending upon the door open time of both the fresh food and freezer. This adaptive defrost control is implemented by monitoring both the compressor run time and the fresh food and freezer door open times. More specifically, a timer counts toward the Maximum Time Till Defrost, and this count is the sum of: 
     compressor run time, 
     fresh food door open time (i.e., the Fresh Food Rate), and 
     freezer door open time (i.e., the Freezer Rate). 
     Referring now more specifically to FIG. 12, controller  40  checks whether the fresh food door is open  170 , and if yes, then timer T 1  is set to equal T 1 -FF Rate  172 . After setting timer T 1 , or if the fresh food door is not open, controller  40  checks whether the freezer door is open  174 . If yes, then timer T 1  is set to equal T 1 -FZ rate  176 , and after setting timer T 1 , or if the freezer door is not open, controller  40  determines whether the compressor is on  178 . If no, processing returns  180  to main routine  50 . 
     If yes, then timers T 2  and T 1  are decremented  182 , and controller  40  checks whether timer T 1  has a value less than or equal to zero  184 . If no, processing returns  180  to main routine  50 . If yes, then controller  40  checks whether timer T 2  has a value less than or equal to zero  186 . If no, processing returns  180  to main routine  50 . If yes, then State is set to equal Pre-Chill and New State is set to equal True  188 . Processing then returns  180  to main routine  50 . 
     The value of timer T 1  is the minimum time until a defrost operation is initiated. The value of timer T 2  is the maximum time until a defrost operation is initiated. The minimum time between defrosts ensures, for example, that a failed switch or a door open condition does not cause unnecessary defrost operations. For example, if a refrigerator door is left open, a refrigerator may enter into defrost operations every 2 hours. By requiring that a minimum time must elapse prior to entering defrost, such excessive defrost operations are avoided. 
     FIG. 13 is a flow chart illustrating the sequence of process steps which may be implemented in firmware of the control board microcontroller  40 . More specifically, and after initiating, or starting operations  200 , controller  40  sets State equal to First Cycle and New State equal to True  202 . Then, controller  40  reads  204  inputs, e.g., FFD, FZD, CR, and DT, which takes about 30 msec. Controller  40  then determines whether the service press button  206  has been pressed, e.g., one press of switch S 1 , and if yes, checks whether the State equals Defrost or State equals Dwell  208 . If no, then controller  40  sets State equal to Defrost and New State equal to True  210 . 
     After completing steps  208 - 210 , or if the service press button was not pressed, controller checks whether the test press button  212  has been pressed, e.g., two presses of switch S 1 . If yes, then State is set equal to Pre-Chill and New State is set equal to True  214 . 
     After completing steps  214 , or if the test press button was not pressed, then controller  40  determines whether New State is equal to True  216 . If yes, then state constraints are defined  218 . After defining state constraints, or if New State is not equal to True, then controller  40  determines whether the current state equals Abnormal  220 , Defrost  222 , Dwell  224 , Pre-Chill  226 , First Cycle  228 , or Pull Down  230 . If the current state is not set to any of these states, then controller  40  determines whether the fresh food door is open  232 , and if yes, sets timer T 1  equal to T 1 -FF Rate  234 . If the fresh food door is not open, or after setting timer Ti, then controller  40  checks whether the freezer door is open  236 . If the freezer door is open, then controller  40  sets timer T 1  equal to T 1 -FZ Rate  238 . 
     Controller  40  then proceeds to determine whether the compressor is on  240 . Controller  40  also proceeds to determine whether the compressor is on  240  if the State is equal to First Cycle  228  or Pull Down  230 . If the compressor is not on, then controller  40  checks whether State is equal to First Cycle  242 . If yes, controller  40  sets Comp 100% equal to false  244 . After setting Comp 100%, or if State is not equal to First Cycle, then controller  40  enters a 50 mSec delay  246  and returns to step  204 . If the compressor is on, then controller  40  decrements  248  timer T 2 . 
     After decrementing timer T 2 , or if State is equal to Abnormal, Defrost, Dwell, or Pre-Chill, then controller  40  decrements  250  timer T 1 . If timer T 1  has a value greater than zero  252 , then controller  40  checks whether State equals Defrost  254 . If no, then controller  40  enters into a delay cycle  246 . If yes, then controller  40  checks whether timer T 1  has a value less than or equal to abnormal delta time  256 . If yes, then the abnormal flag is set  258 . After setting the abnormal flag, or if timer T 1  has a value greater than the abnormal delta time, then controller  40  checks whether the terminator is open  260 . If yes, then State is set equal to Dwell and New State is set equal to True  262 . After setting State and New State, or if the terminator is not open, then controller  40  enters the delay cycle  246  and processing returns to step  204 . 
     If timer T 1  has a value less than or equal to zero  252 , then processing proceeds in accordance with the current state. That is, controller  40  proceeds with processing based on the current state in the sequence illustrated in FIG.  13  and as described below. This sequence is utilized because it controls selection of the next state. More specifically, if State equals Defrost  264 , then controller  40  State is set equal to abnormal and New State is set equal to True  266 . If State equals Pre-Chill  268 , then State is set equal to Defrost and New State is set equal to True  270 . If State is equal to Pull Down  272 , then State is set equal to Defrost and New State is set equal to True  274 . If State is set to Dwell  276 , and if the abnormal flag is not set  278 , then State is set equal to Compressor and New State is set equal to True  280 . If State is set to Dwell  276 , and if the abnormal flag is set  278 , then State is set equal to Abnormal and New State is set equal to True  282 . 
     If State is equal to First Cycle  284 , and if Compressor 100% is not set equal to True  286 , the State is set equal to Pre-Chill and New State is set equal to True  288 . If State is equal to First Cycle  284  and Compressor 100% is set equal to True  286 , then State is set equal to Pull Down and New State is set equal to True  290 . If State is set equal to Abnormal  292 , then State is set equal to Pre-Chill and New State is set equal to True  294 . 
     If the current state is not equal to Defrost  264 , Pre-Chill  268 , Pull Down  272 , Dwell  276 , First Cycle  284 , or Abnormal  292 , then controller  40  checks whether timer T 2  has a value less than or equal to zero  296 . If yes, then State is set equal to Pre-Chill and New State is set equal to True  298 . After setting State and New State, or if timer T 2  has a value greater than zero, then processing proceeds to delay cycle  246 , and then to step  204 . 
     As explained above, the processing described in connection with FIG. 13 may be controlled by firmware of microcontroller  40 . Such control is believed to facilitate efficient defrost operations. 
     In addition, four microcontroller inputs are used to change the Pre-Chill and Defrost Heat Times. The times may be selected to allow a single control to be used with different refrigerator models. In addition, a push button (i.e., Switch S 1 ) is used to force the control into a Defrost or a Pre-Chill state. Therefore, for product service and testing, a defrost operation can be initiated upon request. By depressing Switch S 1  once, an immediate (no pre-chill) defrost is initiated unless the control is already in the Defrost or Dwell States. After the defrost is initiated, the normal defrost sequence follows and the normal rules as described above determine the start of the next defrost. Further, by double pressing switch S 1 , a pre-chill state is initiated from any state. The normal defrost sequence follows and the normal rules as described above determine the start of the next defrost. Terminate Defrost can be entered from the Pre-Chill state and then renews the Pre-Chill state to the initial conditions. 
     Exemplary constant values, as well as minimum and maximums for each constant, are set forth below. 
     
       
         
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Constant 
                 Value 
                 Min/Max 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Pre-Chill Time 
                   
                 Jumpers to set 0.5, 
                 0 to 4 
                 hours 
               
               
                   
                   
                 1.0, 1.5, or 2.0 
               
               
                   
                   
                 hours 
               
               
                 Defrost Heat Time 
                   
                 Jumpers to set 24, 
                 15 to 60 
                 min 
               
               
                   
                   
                 35, 40, or 45 
               
               
                   
                   
                 minutes 
               
               
                 Dwell Time 
                 5 
                 minutes 
                 0 to 10 
                 min 
               
               
                 Normal Power Up 
                 2 
                 hours 
                 0 to 16 
                 hours 
               
               
                 Time 
               
               
                 Pull Down Time 
                 5 
                 hours 
                 0 to 16 
                 hours 
               
               
                 Maximum Time 
                 60 
                 hours 
                 6 to 80 
                 hours 
               
               
                 Till Defrost 
               
               
                 Minimum Time 
                 8 
                 hours 
                 0 to 16 
                 hours 
               
               
                 Till Defrost 
               
               
                 Fresh Food Rate 
                 143 
                   
                 0 to 240 
                   
               
               
                 Abnormal Defrost 
                 15 
                 minutes 
                 0 to 
                 Defrost 
               
               
                 Delta Time 
                   
                   
                   
                 Heat Time 
               
               
                 (=Defrost Heat Time- 
               
               
                 abnormal time) 
               
               
                 Abnormal Run Time 
                 8 
                 hours 
                 0 to 16 
                 hours 
               
               
                   
               
             
          
         
       
     
     Many variations of the above described operations are possible. For example, and with respect to pre-chill operations as described above, when the control has determined that a defrost should be initiated, the heating portion of a defrost is preceded by a Pre-Chill Time when the compressor is held on without regard to the cold control demand for cooling. Rather than a set period of time for holding the compressor on during pre-chill, the Pre-Chill Time could be adjusted based on whether the compressor was on when pre-chill is requested. For example, if the compressor is not on when pre-chill is requested, compressor operation would be delayed until the temperature in a refrigerator compartment reaches a preselected temperature. Once the preselected temperature is reached, the compressor is then energized for a fixed period of time, such as 2 hours. 
     If, however, the compressor is on when pre-chill is requested, then the amount of time that the compressor remains on for pre-chill is adjusted based on how long the compressor had been on in the cycle when the pre-chill request was received. For example, if the compressor had been on for 15 minutes when the pre-chill request was received, then the compressor would remain on for 1 hour and 45 minutes, which provides a total compressor on time of 2 hours. Without such adjustment based on how long the compressor had been on when the pre-chill request was received, the compressor would be on for 2 hours and 15 minutes. In the foregoing example, by operating the compressor for a total of 2 hours rather than for 2 hours and 15 minutes, the desired cooling is achieved and energy savings are provided since the compressor does not operate more than a total of 2 hours during pre-chill even if the pre-chill request is received while the compressor is on. 
     From the preceding description of various embodiments of the present invention, it is evident that the objects of the invention are attained. Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is intended by way of illustration and example only and is not to be taken by way of limitation. Accordingly, the spirit and scope of the invention are to be limited only by the terms of the appended claims.