Abstract:
A defrost control system for a self-defrosting refrigerator is configured to monitor a compressor load, determine whether at least a first defrost cycle is required based on the compressor load, execute at least one defrost cycle when required; and regulate the defrost cycle to conserve energy. A controller is operatively coupled to a compressor, a defrost heater, and a refrigeration compartment temperature sensor. The controller makes defrost decisions based on temperature conditions in the refrigeration compartment in light of other events, such as refrigerator door openings, completed defrost cycles, and power up events. Defrost cycles are automatically adjusted as operating conditions change, thereby avoiding unnecessary energy consumption that would otherwise occur in a fixed defrost cycle.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to refrigerators and, more particularly, a method and apparatus for controlling refrigeration defrost cycles.  
           [0002]    Known frost free refrigerators include a refrigeration defrost system to limit frost buildup on evaporator coils. An electromechanical timer is used to energize a heater after a pre-determined run time of the refrigerator compressor to melt frost buildup on the evaporator coils. To prevent overheating of the freezer compartment during defrost operations when the heater is energized, in at least one type of defrost system the compartment is pre-chilled. After defrost, the compressor is typically run for a predetermined time to lower the evaporator temperature and prevent food spoilage in the refrigerator and/or fresh food compartments of a refrigeration appliance.  
           [0003]    Such timer-based defrost systems, however are not as energy efficient as desired. For instance, they tend operate regardless of whether ice or frost is initially present, and they often pre-chill the freezer compartment regardless of initial compartment temperature. In addition, the defrost heater is typically energized without temperature regulation, and the compressor typically runs after a defrost cycle regardless of the compartment temperature. Such open loop defrost control systems, and the accompanying inefficiencies are undesirable in light of increasing energy efficiency requirements.  
           [0004]    While efforts have been made to provide defrost on demand systems employing limited feedback, such as door openings and compressor and evaporator conditions, for improved energy efficiency of defrost cycles, an adaptive defrost on-demand system is desired to alter defrost operation to conserve energy in light of refrigerator operating conditions.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    In an exemplary embodiment of the invention, a defrost control system for a self-defrosting refrigerator is configured to monitor compressor load, determine whether at least a first defrost cycle is required based on the compressor load, execute at least one defrost cycle when required; and regulate the defrost cycle to conserve energy.  
           [0006]    More specifically a controller is provided for a refrigerator including a compressor, a defrost heater, at least one refrigeration compartment and a temperature sensor thermally coupled to the refrigeration compartment. The controller is operatively coupled to the compressor, the defrost heater, and the temperature sensor, and makes defrost decisions based on temperature conditions in the refrigeration compartment in light of other events, such as refrigerator door openings, completed defrost cycles, and power up events. Defrost cycles are automatically adjusted as operating conditions change, thereby avoiding unnecessary energy consumption that would otherwise occur in a fixed defrost cycle. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a perspective view of a refrigerator;  
         [0008]    [0008]FIG. 2 is a block diagram of a refrigerator controller in accordance with one embodiment of the present invention;  
         [0009]    [0009]FIG. 3 is a block diagram of the main control board shown in FIG. 2;  
         [0010]    [0010]FIG. 4 is a block diagram of the main control board shown in FIG. 2;  
         [0011]    [0011]FIG. 5 is a defrost state diagram executable by a state machine of the controller shown in FIG. 2;  
         [0012]    [0012]FIG. 6 is a sealed system/defrost system block diagram;  
         [0013]    [0013]FIG. 7 is a defrost algorithm flow chart;  
         [0014]    [0014]FIG. 8 is a state diagram for sensor based on-demand defrost; and  
         [0015]    [0015]FIG. 9 is a state diagram for implicit defrost control. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    [0016]FIG. 1 illustrates a side-by-side refrigerator  100  in which the present invention may be practiced. It is recognized, however, that the benefits of the present invention apply to other types of refrigerators, freezers, and refrigeration appliances wherein frost free operation is desirable. Consequently, the description set forth herein is for illustrative purposes only and is not intended to limit the invention in any aspect.  
         [0017]    Refrigerator  100  includes a fresh food storage compartment  102  and a freezer storage compartment  104 . Freezer compartment  104  and fresh food compartment  102  are arranged side-by-side.  
         [0018]    Refrigerator  100  includes an outer case  106  and inner liners  108  and  110 . A space between case  106  and liners  108  and  110 , and between liners  108  and  110 , is filled with foamed-in-place insulation. Outer case  106  normally is formed by folding a sheet of a suitable material, such as pre-painted steel, into an inverted U-shape to form top and side walls of case. A bottom wall of case  106  normally is formed separately and attached to the case side walls and to a bottom frame that provides support for refrigerator  100 . Inner liners  108  and  110  are molded from a suitable plastic material to form freezer compartment  104  and fresh food compartment  102 , respectively. Alternatively, liners  108 ,  110  may be formed by bending and welding a sheet of a suitable metal, such as steel. The illustrative embodiment includes two separate liners  108 ,  110  as it is a relatively large capacity unit and separate liners add strength and are easier to maintain within manufacturing tolerances. In smaller refrigerators, a single liner is formed and a mullion spans between opposite sides of the liner to divide it into a freezer compartment and a fresh food compartment.  
         [0019]    A breaker strip  112  extends between a case front flange and outer front edges of liners. Breaker strip  112  is formed from a suitable resilient material, such as an extruded acrylo-butadiene-styrene based material (commonly referred to as ABS).  
         [0020]    The insulation in the space between liners  108 ,  110  is covered by another strip of suitable resilient material, which also commonly is referred to as a mullion  114 . Mullion  114  also preferably is formed of an extruded ABS material. Breaker strip  112  and mullion  114  form a front face, and extend completely around inner peripheral edges of case  106  and vertically between liners  108 ,  110 . Mullion  114 , insulation between compartments, and a spaced wall of liners separating compartments, sometimes are collectively referred to herein as a center mullion wall  116 .  
         [0021]    Shelves  118  and slide-out drawers  120  normally are provided in fresh food compartment  102  to support items being stored therein. A bottom drawer or pan  122  partly forms a quick chill and thaw system (not shown) and selectively controlled, together with other refrigerator features, by a microprocessor (not shown in FIG. 1) according to user preference via manipulation of a control interface  124  mounted in an upper region of fresh food storage compartment  102  and coupled to the microprocessor A shelf  126  and wire baskets  128  are also provided in freezer compartment  104 . In addition, an ice maker  130  may be provided in freezer compartment  104 .  
         [0022]    A freezer door  132  and a fresh food door  134  close access openings to fresh food and freezer compartments  102 ,  104 , respectively. Each door  132 ,  134  is mounted by a top hinge  136  and a bottom hinge (not shown) to rotate about its outer vertical edge between an open position, as shown in FIG. 1, and a closed position (not shown) closing the associated storage compartment. Freezer door  132  includes a plurality of storage shelves  138  and a sealing gasket  140 , and fresh food door  134  also includes a plurality of storage shelves  142  and a sealing gasket  144 .  
         [0023]    In accordance with known refrigerators, refrigerator  100  also includes a machinery compartment (not shown) that at least partially contains components for executing a known vapor compression cycle for cooling air. The components include a compressor (not shown in FIG. 1), a condenser (not shown in FIG. 1), an expansion device (not shown in FIG. 1), and an evaporator (not shown in FIG. 1) connected in series and charged with a refrigerant. The evaporator is a type of heat exchanger which transfers heat from air passing over the evaporator to a refrigerant flowing through the evaporator, thereby causing the refrigerant to vaporize. The cooled air is used to refrigerate one or more refrigerator or freezer compartments via fans (not shown in FIG. 1). Collectively, the vapor compression cycle components in a refrigeration circuit, associated fans, and associated compartments are referred to herein as a sealed system. The construction of the sealed system is well known and therefore not described in detail herein, and the sealed system is operable to force cold air through the refrigerator subject to the following control scheme.  
         [0024]    [0024]FIG. 2 illustrates a controller  160  in accordance with one embodiment of the present invention. Controller  160  can be used, for example, in refrigerators, freezers and combinations thereof, such as, for example side-by-side refrigerator  100  (shown in FIG. 1).  
         [0025]    Controller  160  includes a diagnostic port  162  and a human machine interface (HMI) board  164  coupled to a main control board  166  by an asynchronous interprocessor communications bus  168 . An analog to digital converter (“A/D converter”)  170  is coupled to main control board  166 . A/D converter  170  converts analog signals from a plurality of sensors including one or more fresh food compartment temperature sensors  172 , a quick chill/thaw feature pan (i.e., pan  122  shown in FIG. 1) temperature sensors  174  (shown in FIG. 8), freezer temperature sensors  176 , external temperature sensors (not shown in FIG. 2), and evaporator temperature sensors  178  into digital signals for processing by main control board  166 .  
         [0026]    In an alternative embodiment (not shown), A/D converter  170  digitizes other input functions (not shown), such as a power supply current and voltage, brownout detection, compressor cycle adjustment, analog time and delay inputs (both use based and sensor based) where the analog input is coupled to an auxiliary device (e.g., clock or finger pressure activated switch), analog pressure sensing of the compressor sealed system for diagnostics and power/energy optimization. Further input functions include external communication via IR detectors or sound detectors, HMI display dimming based on ambient light, adjustment of the refrigerator to react to food loading and changing the air flow/pressure accordingly to ensure food load cooling or heating as desired, and altitude adjustment to ensure even food load cooling and enhance pull-down rate of various altitudes by changing fan speed and varying air flow.  
         [0027]    Digital input and relay outputs correspond to, but are not limited to, a condenser fan speed  180 , an evaporator fan speed  182 , a crusher solenoid  184 , an auger motor  186 , personality inputs  188 , a water dispenser valve  190 , encoders  192  for set points, a compressor control  194 , a defrost heater  196 , a door detector  198 , a mullion damper  200 , feature pan air handler dampers  202 ,  204 , and a quick chill/thaw feature pan heater  206 . Main control board  166  also is coupled to a pulse width modulator  208  for controlling the operating speed of a condenser fan  210 , a fresh food compartment fan  212 , an evaporator fan  214 , and a quick chill system feature pan fan  216 .  
         [0028]    [0028]FIGS. 3 and 4 are more detailed block diagrams of main control board  166 . As shown in FIGS. 3 and 4, main control board  166  includes a processor  230 . Processor  230  performs temperature adjustments/dispenser communication, AC device control, signal conditioning, microprocessor hardware watchdog, and EEPROM read/write functions. In addition, processor  230  executes many control algorithms including sealed system control, evaporator fan control, defrost control, feature pan control, fresh food fan control, stepper motor damper control, water valve control, auger motor control, cube/crush solenoid control, timer control, and self-test operations.  
         [0029]    Processor  230  is coupled to a power supply  232  which receives an AC power signal from a line conditioning unit  234 . Line conditioning unit  234  filters a line voltage which is, for example, a 90-265 Volts AC, 50/60 Hz signal Processor  230  also is coupled to an EEPROM  236  and a clock circuit  238 .  
         [0030]    A door switch input sensor  240  is coupled to fresh food and freezer door switches  242 , and senses a door switch state. A signal is supplied from door switch input sensor  240  to processor  230 , in digital form, indicative of the door switch state. Fresh food thermistors  244 , a freezer thermistor  246 , at least one evaporator thermistor  248 , a feature pan thermistor  250 , and an ambient thermistor  252  are coupled to processor  230  via a sensor signal conditioner  254 . Conditioner  254  receives a multiplex control signal from processor  230  and provides analog signals to processor  230  representative of the respective sensed temperatures. Processor  230  also is coupled to a dispenser board  256  and a temperature adjustment board  258  via a serial communications link  260 . Conditioner  254  also calibrates the above-described thermistors  244 ,  246 ,  248 ,  250 , and  252 .  
         [0031]    Processor  230  provides control outputs to a DC fan motor control  262 , a DC stepper motor control  264 , a DC motor control  266 , and a relay watchdog  268 . Watchdog  268  is coupled to an AC device controller  270  that provides power to AC loads, such as to water valve  190 , cube/crush solenoid  184 , a compressor  272 , auger motor  186 , a feature pan heater  206 , and defrost heater  196 . DC fan motor control  266  is coupled to evaporator fan  214 , condenser fan  210 , fresh food fan  212 , and feature pan fan  216 . DC stepper motor control  266  is coupled to mullion damper  200 , and DC motor control  266  is coupled to one of more sealed system dampers.  
         [0032]    Processor logic uses the following inputs to make control decisions:  
         [0033]    Freezer Door State—Light Switch Detection Using Optoisolators,  
         [0034]    Fresh Food Door State—Light Switch Detection Using Optoisolators,  
         [0035]    Freezer Compartment Temperature—Thermistor,  
         [0036]    Evaporator Temperature—Thermistor,  
         [0037]    Upper Compartment Temperature in FF—Thermistor,  
         [0038]    Lower Compartment Temperature in FF—Thermistor,  
         [0039]    Zone (Feature Pan) Compartment Temperature—Thermistor,  
         [0040]    Compressor On Time,  
         [0041]    Time to Complete a Defrost,  
         [0042]    User Desired Set Points via Electronic Keyboard and Display or Encoders,  
         [0043]    User Dispenser Keys,  
         [0044]    Cup Switch on Dispenser, and  
         [0045]    Data Communications Inputs.  
         [0046]    The electronic controls activate the following loads to control the refrigerator:  
         [0047]    Multi-speed or variable speed (via PWM) fresh food fan,  
         [0048]    Multi-speed (via PWM) evaporator fan,  
         [0049]    Multi-speed (via PWM) condenser fan,  
         [0050]    Single-speed zone (Special Pan) fan,  
         [0051]    Compressor Relay,  
         [0052]    Defrost Relay,  
         [0053]    Auger motor Relay,  
         [0054]    Water valve Relay,  
         [0055]    Crusher solenoid Relay,  
         [0056]    Drip pan heater Relay,  
         [0057]    Zonal (Special Pan) heater Relay,  
         [0058]    Mullion Damper Stepper Motor IC,  
         [0059]    Two DC Zonal (Special Pan) Damper H-Bridges, and  
         [0060]    Data Communications Outputs.  
         [0061]    The foregoing functions of the above-described electronic control system are performed under the control of firmware implemented as small independent state machines  
         [0062]    [0062]FIG. 5 is a defrost state diagram  300  illustrating a state algorithm executable by a state machine of controller  160  (shown in FIGS.  2 - 4 ). As will be seen, controller  160  adaptively determines an optimal defrost state based upon effectiveness of defrost cycles as they occur, while accounting for power losses that may interrupt a defrost operation.  
         [0063]    By monitoring evaporator temperature over time, it is determined whether defrost cycles are deemed “normal” or “abnormal.” More specifically, when it is time to defrost, i.e. after an applicable defrost interval (explained below) has expired, the refrigerator sealed system is shut off, defrost heater  196  is turned on (at state  2 ), and a defrost timer is started. As the evaporator coils defrost, the temperature of the evaporator increases. When evaporator temperature reaches a termination temperature (60° F. in an exemplary embodiment) defrost heater  196  is shut off and the elapsed time defrost heater was on (Δt de ) is recorded in system memory. Also, if the termination temperature is not reached within a predetermined maximum time, defrost heater  196  is shut off and the elapsed time the defrost heater was on is recorded in system memory.  
         [0064]    The elapsed defrost time Δt de  is then compared with a predetermined defrost reference time Δt dr  representative of, for example, an empirically determined or calculated elapsed defrost heater time to remove a selected amount of frost buildup on the evaporator coils that is typically encountered in the applicable refrigerator platform under predetermined usage conditions. If elapsed defrost time Δt de  is greater than reference time Δt dr , thereby indicating excessive frost buildup, a first or “abnormal” defrost interval, or time until the next defrost cycle, is employed If elapsed defrost time Δt de  is less than reference time Δt dr , a second or “normal” defrost interval, or time until the next defrost cycle is employed. The normal and abnormal defrost intervals, as defined below, are selectively employed, using Δt dr  as a baseline, for more efficient defrost operation as refrigerator usage conditions change, thereby affecting frost buildup on the evaporator coils.  
         [0065]    More specifically, the following control scheme automatically cycles between the first or abnormal defrost interval and the second or normal defrost interval on demand. When usage conditions are heavy and refrigerator doors  132 ,  134  (shown in FIG. 1) are opened frequently, thereby introducing more humidity into the refrigeration compartment, the system tends to execute the first or abnormal defrost interval repeatedly. When usage conditions are light and the doors opened infrequently, thereby introducing less humidity into the refrigeration compartments, the system tends to execute the second or normal defrost interval repeatedly. In intermediate usage conditions the system alternates between one or more defrost cycles at the first or abnormal defrost interval and one or more defrost cycles at the second or normal defrost interval.  
         [0066]    Upon powerup, controller  160  reads freezer thermistor  246  (shown in FIG. 3) over a predetermined period of time and averages temperature data from freezer thermistor  146  to reduce noise in the data. If the freezer temperature is determined to be substantially at or below a set temperature, thereby indicating a brief power loss, a defrost interval is read from EEPROM memory  236  (shown in FIG. 3) of controller  160 , and defrost continues from the point of power failure without resetting defrost parameters. Periodically, controller  160  saves a current time till defrost value in system memory in the event of power loss. Controller  160  therefore recovers from brief power loses and associated defrost cycles due to resetting of the system from momentary power failures are therefore avoided.  
         [0067]    If freezer temperature data indicates that freezer compartment  104  (shown in FIG. 1) is warm, i.e., at a temperature outside a normal operating range of freezer compartment, humid air is likely to be contained in freezer compartment  104 , either because of a sustained power outage or opened doors during a power outage. Because of the humid air, a defrost timer is initially set to the first or abnormal defrost interval. In one embodiment the first or abnormal defrost interval is set to, for example, eight hours of compressor run time. For each second of compressor run time, the first defrost interval is decremented by a predetermined amount, such as one second, and the first defrost interval is generally unaffected by any other event, such as opening and closing of fresh food and freezer compartment doors  134 ,  132 . In alternative embodiments, a first or abnormal defrost interval of greater or lesser than eight hours is employed, and decrement values of greater or lesser than one second are employed for optimal performance of a particular compressor system in a particular refrigerator platform.  
         [0068]    When the first defrost interval has expired, controller  160  runs compressor  272  (see FIG. 3) for a designated pre-chill period or until a designated pre-chill temperature is reached (at state  1 ). Defrost heater  196  (shown in FIGS.  2 - 4 ) is energized (at state  2 ) to defrost the evaporator coils. Defrost heater  196  is turned on to defrost the evaporator coils either until a predetermined evaporator temperature has been reached or until a predetermined maximum defrost time has expired, and then a dwell state is entered (at state  3 ) wherein operation is suspended for a predetermined time period.  
         [0069]    Upon completion of an “abnormal” defrost cycle after the first or abnormal defrost interval has expired, controller  160  (at state  0 ) sets the time till defrost to the second or normal pre-selected defrost interval that is different from the first or abnormal time to defrost. Therefore, using the second defrost interval, a “normal” defrost cycle is executed. For example, in one embodiment, the second defrost interval is set to about 60 hours of compressor run time. In alternative embodiments, a second defrost interval of greater or lesser than 60 hours is employed to accommodate different refrigerator platforms, e.g., top-mount versus side-by-side refrigerators or refrigerators of varying cabinet size.  
         [0070]    In one embodiment, the second defrost interval, unlike the first defrost interval, is decremented (at state  5 ) upon the occurrence of any one of several decrement events. For example, the second defrost interval is decremented (at state  5 ) by, for example, one second for each second of compressor run time. In addition, the second defrost interval is decremented by a predetermined amount, e.g., 143 seconds, for every second freezer door  132  (shown in FIG. 1) is open as determined by a freezer door switch or sensor  242  (shown in FIG. 3). Finally, the second defrost interval is decremented by a predetermined amount, such as 143 seconds in an exemplary embodiment, for every second fresh food door  134  (shown in FIG. 1) is open. In an alternative embodiment, greater or lesser decrement amounts are employed in place of the above-described one second decrement for each second of compressor run time and 143 second decrement per second of door opening. In a further alternative embodiment, the decrement values per unit time of opening of doors  132 ,  134  are unequal for respective door open events. In further alternative embodiments, greater or fewer than three decrement events are employed to accommodate refrigerators and refrigerator appliances having greater or fewer numbers of doors and to accommodate various compressor systems and speeds.  
         [0071]    When the second or normal defrost interval has expired, controller  160  runs compressor  272  for a designated pre-chill period or until a designated pre-chill temperature is reached (at state  1 ). Defrost heater  196  is energized (at state  2 ) to defrost the evaporator coils. Defrost heater  196  is turned on to defrost the evaporator coils either until a predetermined evaporator temperature has been reached or until a predetermined maximum defrost time has expired. Defrost heater  196  is then shut off and the elapsed time defrost heater  196  was on (Δt de ) is recorded in system memory. A dwell state is then entered (at state  3 ) wherein operation is suspended for a predetermined time period.  
         [0072]    The elapsed defrost time Δt de  is then compared with a predetermined defrost reference time Δt dr . If elapsed defrost time Δt de  time is greater than reference time Δt dr , thereby indicating excessive frost buildup, the first or abnormal defrost interval is employed for the next defrost cycle If elapsed defrost time Δt de  is less than reference time Δt dr , the second or normal defrost interval is employed for the next defrost cycle. The applicable defrost interval is applied and a defrost cycle is executed when the defrost interval expires. The elapsed defrost time Δt de  of the cycle is recorded and compared to reference time Δt dr  to determine the applicable defrost interval for the next cycle, and the process continues. Normal and abnormal defrost intervals are therefore selectively employed on demand in response to changing refrigerator conditions.  
         [0073]    Because the defrost function introduces heat to the system and the sealed system provides cold air, it is desirable that the sealed system and defrost system do not negatively interact. Therefore, a defrost system/sealed system interaction algorithm  310  is defined as follows, and as illustrated in FIGS. 6 and 7.  
         [0074]    Defrost algorithm  300 , as described above, determines when it is time to begin the defrost process, and in one embodiment further includes a defrost cycle hold-off or delay. In an exemplary embodiment, refrigerator compartment doors  132 ,  134  (shown in FIG. 1) are to be closed for at a least a predetermined time period, such as two hours, before freezer compartment pre-chill is initiated prior to actual defrost. If the predetermined door closed time, e.g., two hours, is not satisfied, the hold-off will wait until the door closed condition is satisfied, up to a predetermined maximum time, such as, for example, sixteen hours after the originally desired pre-chill entry time determined by defrost algorithm  300 . When either the door closed condition is satisfied or when the predetermined maximum time has expired, pre-chill operation is entered Hold-off timing values, including but not limited to the above-described values, may be stored in ROM, EEPROM  236  (shown in FIG. 3), or other programmable memory in order to accommodate the needs of different styles of refrigerator units.  
         [0075]    When defrost algorithm  300  requests pre-chill from sealed system  312 , sealed system  312  initiates pre-chill. When pre-chill is complete, defrost begins. Sealed system  312  then waits until the freezer temperature is above an upper set point and then turns on.  
         [0076]    More particularly, instead of checking the freezer for a lower set point to be achieved, sealed system  312  runs for a fixed pre-chill time. e.g., two hours, to keep the average temperature in the freezer from warming up too much during the defrost cycle. Upon completion of the two hour pre-chill, sealed system  312  shuts down and defrost algorithm  300  takes over. Defrost algorithm  300  runs defrost heater  196  (shown in FIGS.  2 - 4 ) until a termination temperature or a time out occurs. Defrost algorithm  300  then goes into a dwell period (five minutes in an exemplary embodiment) that holds the sealed system and defrost heater  196  off.  
         [0077]    Following the dwell period, compressor  272  (shown in FIG. 3) and condenser fan  210  (shown in FIGS.  2 - 4 ), in one embodiment, are started for a period of time during which controller  160  keeps evaporator fan  214  (shown in FIGS.  2 - 4 ) and fresh food fan  212  (shown in FIGS.  2 - 4 ) off and mullion damper  200  (shown in FIGS.  2 - 4 ) closed. Once the period ends, or when evaporator temperature achieves a threshold temperature via operation of compressor  272  and condenser fan  210 , mullion damper  200  is opened, and evaporator fan  214  and fresh food fan  212  are started in their high speed. Control is then returned to sealed system  312  for normal cooling operation.  
         [0078]    In an alternative implementation of an on-demand defrost system, two temperature sensors (thermistor  248  shown in FIG. 3 and another like thermistor) capable of measuring a temperature differential across the evaporator are utilized in conjunction with a current sensor on the compressor motor, freezer compartment sensor  246 , and a state machine algorithm, such as algorithm  320  illustrated in FIG. 8. State algorithm  320  may be used in a stand-alone defrost system or in combination with aspects of state algorithm  300  (shown in FIG. 5), such as, for example, to determine initiation of either the normal or abnormal defrost cycles. A defrost decision can then be made by comparing the relative loads of the evaporator and compressor  272 .  
         [0079]    A relationship exists between the evaporator and the compressor load such that compressor  272  experiences a largest load when the refrigerant is wholly in a liquid state and must be converted to a gas state. In this instance, liquid refrigerant in the evaporator closest to compressor  272  vaporizes before liquid refrigerant that is farther away from compressor  272 , producing a large temperature differential between a first sensor, such as thermistor  248  located on one end of the evaporator close to compressor  272  and a second sensor located on a second end of the evaporator away from compressor  272 . Further, when most of the refrigerant is converted, the temperature differential between the ends of the evaporator will reduce because the entire evaporator approaches a substantially uniform temperature (i.e., the vapor temperature of the refrigerant) as the refrigerant is converted.  
         [0080]    Therefore, at each refrigerant cycle, when compressor startup is demanded  322 , power to compressor  272  is delayed  324  by a fixed predetermined period. Following fixed time delay  324 , a temperature differential across the evaporator (ΔT) is measured  326 , compressor load current which is proportional to the condenser load is measured  328 , and a defrost decision may be made.  
         [0081]    If the compressor current indicates a light compressor load and the temperature differential across the evaporator is large, a fault condition is established  330  and an error flag is set.  
         [0082]    If the compressor current indicates a light compressor load and the temperature differential across the evaporator is small, most of the refrigerant is vaporized, the system is operating normally, and a normal refrigerant cycle continues to execute  332 .  
         [0083]    If the compressor current indicates a heavy compressor load and the temperature differential across the evaporator is large, most of the refrigerant is liquified, the system is operating normally, and a normal refrigerant cycle continues to execute  334 .  
         [0084]    If, however, the compressor current measurement indicates a large compressor load, but the differential temperature measurement across the evaporator is small, it is likely that that frost or ice is causing a uniform temperature gradient across the surface of the evaporator. A need for a defrost cycle is therefore indicated. Before initiating a defrost, a temperature of freezer compartment  104  (shown in FIG. 1) is determined  336 . If freezer temperature is at or above a predetermined point, a pre-chill cycle is executed  338  as described above, and defrost heater  196  (shown in FIGS.  2 - 4 ) is turned on  340  after the pre-chill cycle completes.  
         [0085]    If freezer compartment temperature is below a predetermined point, a pre-chill cycle is not executed, therefore saving energy the pre-chill cycle would have otherwise used, and defrost heater  196  is turned on  340 .  
         [0086]    In one embodiment, defrost heater  196  is controlled with PID (Proportional, Integral, Derivative) control or other suitable closed loop control to create and execute an optimal heat profile that defrosts the evaporator coils without unnecessarily warming freezer compartment  104 , thereby producing further energy savings.  
         [0087]    Upon completion of a defrost heater cycle, freezer compartment temperature is again measured to  342  to determine whether a cooling cycle is required for optimal food preservation. If freezer temperature is at or above a predetermined point, sealed system  312  is turned on to lower the temperature of freezer compartment  104 , thereby chilling  344  freezer compartment  104 . A normal refrigeration cycle is thereafter maintained  346 . If, however, freezer temperature is below a predetermined point, a normal refrigeration cycle is maintained  346  without chilling  344  of freezer compartment  102 .  
         [0088]    In an alternative embodiment, instead of using two temperature sensors to measure the differential temperature across the evaporator, a known thermal time constant of the evaporator is used with a single sensor, such as thermistor  248  on the evaporator. Data acquired from the single sensor, i.e., rate of change data, is combined with the known characteristics of the evaporator coil to determine the temperature differential.  
         [0089]    Referring to FIG. 9, another defrost system state machine or state algorithm  360  is realized using switches or sensors  242  (shown in FIG. 30) on refrigerator doors  132 ,  134  (shown in FIG. 1) to determine when the doors are opened, and temperature sensors  244 ,  246  (shown in FIG. 3) in the cooling cavities or compartments  102 ,  104 . State algorithm  360  may be used as a stand-alone defrost system or in combination with aspects of state algorithm  300  (shown in FIG. 5), such as, for example, to determine initiation of either the normal or abnormal defrost cycles.  
         [0090]    In one embodiment, the normal refrigeration cycle measures refrigeration compartment temperature, and more specifically, freezer compartment  104  temperature to determine operation of sealed system  312 . When refrigeration compartment temperature rises above a set point, compressor  272  (shown in FIG. 30) is turned on  362  to initiate cooling, and a timer is set  364  to measure elapsed compressor on time. This cooling cycle continues until the refrigeration compartment temperature falls below a lower threshold set point and compressor is shut down. As the compressor is shut down, the timer is stopped and the elapsed compressor run time (ΔT) is recorded  366  in controller memory.  
         [0091]    Two implicit measurements determine whether defrost is required, namely the amount of time that compressor  272  takes to cool the refrigeration compartment and the cumulative amount of time a door  132 ,  134  has been open since the last defrost cycle. Since frost buildup is a result of humidity entering refrigeration compartments when the doors are open there is no need to expend energy executing defrost cycles if the door has not been opened or has only been opened for a short period of time.  
         [0092]    A primary indicator for defrost is the length of time (ΔT) that compressor  272  runs to cool the compartment. If the system measures ΔT during the first cooling cycle after a defrost cycle, it can be determined if the time to cool the compartment is increasing thereafter. Because ΔT is a function of compressor load, a threshold time differential ΔT t  is established during the first cooling cycle that can be used to determine when defrost is required thereafter. In an alternative embodiment, a fixed, pre-programmed ΔT t  value is employed in lieu of establishing a baseline ΔT t  during the first cooling cycle.  
         [0093]    Thus, when sealed system  312  is shut down and a measured compressor run time ΔT m  is recorded  366  for that cooling cycle, ΔT m  is compared to the threshold ΔT t . If ΔT m  is less than or substantially equal to ΔT t , defrost is not needed and a normal cooling cycle continues to execute  368 .  
         [0094]    If ΔT m  is greater than the threshold ΔT t , a need for defrost is indicated. Before initiating a defrost, a temperature of freezer compartment  104  (shown in FIG. 1) is determined  370 . If freezer temperature is at or above a predetermined point, a pre-chill cycle is executed  372  as described above, and defrost heater  196  (shown in FIGS.  2 - 4 ) is turned on  374  after the pre-chill cycle completes.  
         [0095]    Upon completion of a defrost heater cycle, freezer compartment temperature is again measured to  376  to determine whether a cooling cycle is required for optimal food preservation. If freezer temperature is at or above a predetermined point, sealed system  312  is turned on to lower the temperature of freezer compartment  104  and chill  378  the freezer compartment. A normal refrigeration cycle is thereafter maintained  380 . If, however, freezer temperature is below a predetermined point, a normal refrigeration cycle is maintained  346  without chilling  378  the freezer compartment.  
         [0096]    A fail safe maximum door open time to trigger defrost is also included in the event that there have been several door openings, but no increase in cooling time has been measured.  
         [0097]    In addition, since door open and cooling times are implicit indicators of a need for defrost, a maximum time between defrost cycles is also maintained as a fail safe mechanism.  
         [0098]    Yet another implementation of an on-demand defrost system can be realized using a combination of the embodiments described above. In this embodiment, compressor on time, i.e., (ΔT) is used to determine compressor load instead of using a current sensor on the compressor.  
         [0099]    Still yet another implementation of an on-demand defrost system can be realized using any of the hardware scenarios described above but without using a state machine for making defrost decisions. Rather, Fuzzy Logic is used to make defrost decisions. Using Fuzzy inputs of compressor load (CL), evaporator temperature differential (ETD) and compartment temperature (CT) and Fuzzy outputs of defrost required (DR) and pre-chill required (PCD) a rule set can be constructed as follows:  
         [0100]    IF CL is Large and ETD is Small THEN DR is Large  
         [0101]    IF DR is Large and CT is Large THEN PCD is Large  
         [0102]    Since these are Fuzzy variables, they represent continuous overlapping values. This multivariate system produces a weighting factor (DR) that is de-fuzzied using a fuzzy impulse response to determine whether a defrost is required. The PCD variable grows as the time to defrost approaches and pre-chill begins as required. Additional rules may also be used in alternative embodiments in order to optimize defrost operation across multiple refrigerator platforms.  
         [0103]    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.