Abstract:
An adaptive defrost control system of the present invention monitors an amount of current flowing through the defrost heater to calculate the amount of power delivered thereby. The circuit utilizes a thermistor to monitor the temperature rise of the electrical trace supplying current to the defrost heater to allow the controller to calculate an amount of power delivered thereby. A second thermistor may be used to compensate for a change in ambient temperature that might otherwise be attributed to a change in current flow through the power trace. A physical modification to the power trace to enhance the temperature rise characteristic at the point of placement of the thermistor enhances the accuracy of the calculation. A secondary current flow path around the branch of thermistor placement is also provided so as to not reduce the total current carrying capacity of the power trace.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to defrost heaters for consumer and commercial appliances, and more particularly to defrost cycle control methods to control the defrost cycle in such appliances.  
       BACKGROUND OF THE INVENTION  
       [0002]     Consumer and commercial refrigeration systems typically utilize a compressor driven refrigeration cycle to provide the cooling necessary to maintain the internal temperature of the freezer compartment of a refrigerator or freezer at a particular selected temperature. In a refrigerator, a damper door is typically utilized to allow some of the cold air from the freezer compartment to flow into the fresh food compartment to maintain the fresh food compartment at a different selected temperature. Such a compressor refrigeration system includes an evaporator positioned within or in close proximity to the freezer compartment. The evaporator is a heat exchanger through which is blown recirculated air within the freezer compartment to reduce the temperature thereof. This heat exchanger is typically a coiled arrangement of refrigeration line with fins connected thereto to increase the surface area and therefore the efficiency of the heat transfer.  
         [0003]     Unfortunately, due to the extremely cold temperature of the evaporator, any moisture in the air in the freezer compartment tends to freeze on the evaporator during operation. This frost build-up on the evaporator reduces the amount of surface area over which the air may be blown. Indeed, if this condition were allowed to continue, the frost would build to ice that would completely block off the air flow paths through the evaporator. This greatly reduces the efficiency of the refrigeration system as the heat transfer is greatly reduced. As a result, the compressor is required to run, potentially continuously, in order to try to maintain the temperature of the freezer compartment at the appropriate level. This greatly increases the cost of operating this refrigeration equipment, and reduces the owner&#39;s satisfaction as the noise generated by the continuously running compressor may well be annoying to the consumer.  
         [0004]     Recognizing these problems, many modern refrigeration systems include a defrost heater that is placed in proximity to the evaporator heat exchanger. This defrost heater is controlled by a controller, which may take the form of an electromechanical defrost timer or may utilize a microprocessor or microcontroller. In any event, the defrost heater is utilized to provide a small amount of heating of the evaporator coil after the refrigeration cycle has stopped to defrost the heat exchanger, i.e. melt the ice or frost that may have formed thereon during the refrigeration cycle. Through the use of such a defrost heater, the efficiency of the refrigeration system is maintained at a high level so as to reduce the cost of operation of the appliance and to increase the owner&#39;s satisfaction as the refrigeration cycles may be kept short due to the high efficiency energy transfer through the defrosted evaporator coils.  
         [0005]     Unfortunately, since the evaporator coils are located physically within or in thermal proximity to the interior of the freezer compartment, the heating from the defrost heater tends to increase the temperature in the freezer compartment. Since a rough rule of thumb is that twice the amount of cooling is required to remove each unit of heat from the freezer compartment, the defrost heater should be operated for the least amount of time to provide the defrosting of the evaporator coils. Otherwise, the temperature of the freezer compartment may be unnecessarily increased, which increases the amount of cooling necessary to remove this unnecessary heat in the next refrigeration cycle.  
         [0006]     Electromechanical defrost timers are used in an attempt to limit the amount of time that the defrost heater is energized. This energization time is set based on experimentations with the particular appliances to determine how long the defrost heater should operate to remove a typical amount of frost that would build up during a typical refrigeration cycle within that particular appliance. This timing is selected to optimally remove this amount of frost. If the defrost heater is run for too long, unnecessary heating of the freezer compartment occurs without any added benefit to the removal of frost from the evaporator coils. If the defrost heater is not run for a long enough period, the residual frost that remains on the evaporator coils will tend to build up over time so that eventually the benefit of the defrost cycle is completely overshadowed by the increasing accumulation of frost on the evaporator coils. Therefore, a large amount of testing and calculations go into the selection of the appropriate defrost times. While microprocessor controls may provide for adaptive defrost control, the utilization of a timed cycle is still the hallmark of the control of the defrost cycle.  
         [0007]     Unfortunately, while the defrost control cycles are carefully designed and tested to optimize the operation thereof, once the particular appliance has been installed at a consumer or commercial location, other factors that typically occur during normal operation may have a large effect on the efficiency of the defrost heater operation. Specifically, the calculation and testing to program the adaptive control to provide an optimized time for operation of the refrigeration and defrost cycle is based on a given voltage to be applied to the defrost heater. This adaptive control utilizes a temperature sensing element on the evaporator, and monitors the temperature and time of the defrost cycle. That is, after the refrigeration cycle when the defrost heater is energized, the controller monitors the temperature of the evaporator. Once the temperature reaches a predetermined point, the controller assumes that it has been fully defrosted. The time that this process takes is then used by the controller to adjust the subsequent refrigeration cycle, hence the “adaptive” nature of the controller.  
         [0008]     Since defrost heaters are typically resistive heating elements, the amount of power that is dissipated, i.e. turned into heat, through the resistive heating element is affected by variations in the line voltage. As such, a the typical controller assumes that a known amount of heating is provided during the particular defrost cycle based on a known line voltage and the value of the resistive heating element. Therefore, if all of these were to remain constant, the defrost cycle time on which the adaptive controller bases the next refrigeration cycle would operate in the field as it does in the lab.  
         [0009]     Unfortunately, the line voltage at a consumer or commercial installation may vary greatly from that of the laboratory in which the cycle time was determined, and may, in fact, vary greatly over its operational life. These variations are not symptomatic of any problem in the electrical distribution system, but are, typically, within the specifications for power delivery. Unfortunately, as the input line voltage varies, so does the amount of heat generated or power delivered by the defrost heater. This will directly affect the time it takes to raise the temperature of the evaporator to the set point under identical frost conditions.  
         [0010]     A line voltage lower than that utilized in the development of the adaptive cycles will result in less heat being delivered to melt the frost and ice from the evaporator, and therefore will result in a longer time needed to raise the temperature of the evaporator to the predetermined level. As such, the adaptive controller may well reduce the refrigeration cycle time thinking that the last refrigeration cycle time severely frosted the evaporator. Similarly, an increase in the line voltage over that utilized in the development of the adaptive cycles will result in more heat being delivered by the heater. As a result, the time to reach the predetermined temperature of the evaporator will be shorter. This will cause the adaptive controller to believe that very little frost accumulated on the previous refrigeration cycle. As a result, the controller will likely lengthen the next refrigeration cycle. These variations will increase the cost of ownership of this appliance since the adaptive control can no longer vary the cycles to their most efficient.  
         [0011]     As an example, in a typical defrost heater during a typical defrost cycle, the amount of power delivered when the line voltage is a nominal 117 volts AC is approximately 400 watts. If the line voltage increases, within its specifications, to 132 volts AC, the amount of power delivered during the same defrost cycle will increase to approximately 600 watts. Conversely, if the line voltage were to be at the low end of the specification, for example 102 volts AC, the amount of power delivered during the same defrost cycle will drop to only approximately 350 watts. As this example illustrates, the mere variation in the line voltage over its specified range results in vastly different amounts of heating that will be generated by the defrost heater, and therefore the time it takes for the defrost cycle to raise the temperature of the evaporator will vary widely.  
         [0012]     There exists, therefore, a need in the art for a defrost heater cycle control that can determine the actual amount of heating during a defrost cycle so as to properly adjust the adaptive control in order to operate in an efficient manner. This system and method of the present invention provides such an adaptive defrost heater cycle control.  
         [0013]     These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.  
       BRIEF SUMMARY OF THE INVENTION  
       [0014]     The present invention provides a new and improved defrost heater control system and method that overcomes one or more of the problems existing in the art. More particularly, the present invention provides a new and improved defrost cycle control that monitors or calculates the defrost cycle to determine an actual amount of heating during the defrost cycle regardless of variations of the input line voltage. Still more particularly, the present invention provides a new and improved defrost cycle control that adapts the cycle control based upon an amount of power delivered to the defrost heater during the defrost cycle.  
         [0015]     In one embodiment of the present invention the defrost heater control monitors an amount of current delivered to the defrost heater during the defrost cycle so that an amount of power delivered by the defrost heater may be continuously calculated to account for the length of the defrost cycle. A current sensor is used to monitor the amount of current provided to the defrost heater, which current will vary depending on the line voltage. The square of this monitored current is multiplied by the known resistance of the defrost heater to calculate an instantaneous power supplied by the defrost heater. This instantaneous power is accumulated over the defrost cycle until the defrost cycle is terminated. In one embodiment, the duration of each defrost cycle may be independently calculated during the defrost cycle itself, and takes into consideration the varying input line voltage so that only an appropriate amount of defrosting occurs during any particular cycle.  
         [0016]     In a preferred embodiment of the present invention, the current sensor is a thermistor. Preferably, the thermistor is placed on the electrical trace on the defrost heater power control board. The adaptive defrost controller monitors the temperature rise of this thermistor over time to determine the amount of current supplied therethrough. Preferably, a second thermistor is utilized off the power trace so as to allow the factors of the ambient temperature to be eliminated.  
         [0017]     Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:  
         [0019]      FIG. 1  is a simplified single line schematic diagram of one embodiment of a defrost heater control circuit constructed in accordance with one embodiment of the present invention;  
         [0020]      FIG. 2  is a simplified single line schematic diagram of an alternate embodiment of a defrost heater control circuit constructed in accordance with the teachings of the present invention;  
         [0021]      FIG. 3  is a simplified single line schematic diagram of a further alternate embodiment of a defrost heater control circuit constructed in accordance with the teachings of the present invention illustrating additionally physical features thereof; and  
         [0022]      FIG. 4  illustrates a portion of a defrost heater power trace and electrical connection pads for a thermistor that may be used in one embodiment of the present invention, such as in the circuit illustrated in  FIG. 3 . 
     
    
       [0023]     While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0024]      FIG. 1  illustrates a simplified single line schematic diagram of an adaptive defrost control circuit  10  constructed in accordance with the teachings of the present invention. Specifically, the adaptive defrost control (ADC) circuit utilizes an ADC controller  12 , which is preferably a microcontroller, microprocessor, programmable logic device, etc., that may adaptively change the refrigeration cycle time based upon how long the defrost heater  14  is energized before the evaporator temperature reaches a predetermined temperature. The ADC controller  12  controls the start and stop of a defrost cycle by closing or opening a power control switch  16 . This is well known in the art, such a power control switch  16  may take the form of an electromechanical relay, a power switching semiconductor, etc.  
         [0025]     Once the ADC controller  12  has closed the switch  16 , AC power  18 , typically from the utility, is applied to the defrost heater  14 . As discussed above, the voltage from the source  18  may vary widely during normal operation, therefore, delivering varying amounts of current to the defrost heater  14 . Preferably, the defrost heater  14  is a resistive heating element, the power delivered by which may be calculated as the amount of current provided therethrough varies over the defrost cycle. Through calculation and experimental testing in the development laboratory, the amount of power necessary to remove the frost build up from an evaporator in the freezer compartment of a particular appliance is well understood, as is the formation of such frost during the refrigeration cycle. As such, the ADC controller  12  is programmed with these values. The ADC controller  12  utilizes this information to determine the duration of the defrost cycle and adjust the refrigeration cycle to ensure continued efficient operation of the refrigeration system.  
         [0026]     In view of varying voltage from source  18  during the defrost cycle, the system of the present invention utilizes a current sensor  20  placed in circuit with the defrost heater  14  to monitor an amount of instantaneous current being supplied through the defrost heater  14 . The ADC controller  12  then calculates the total amount of power supplied by accumulating the instantaneous power during the defrost cycle. Once an appropriate amount of power has been delivered by the defrost heater  14  as typically determined by sensing the temperature of the evaporator, the ADC controller  12  commands the switching element  16  to open to stop the flow of current to the heater  14 . As such, a fixed defrost cycle time need not be used as was the case with the electromechanical timers.  
         [0027]     While any current sensing device could be utilized in the system of the present invention, in the consumer and commercial refrigeration market, cost sensitivity for individual components is high. As such, while a current transformer (CT) could be used as the current sensing device  20 , the cost of such current transformers may be prohibitive to the overall cost of the design for a consumer or commercial refrigeration unit. As such, a lower cost alternative needed to be found.  
         [0028]     Recognizing that thermistors are substantially less expensive than current transformers, on the order of 7 to 8 cents apiece and recognizing that the ADC controller  12  could calculate current based on a measured temperature rise of the power trace on the defrost heater power control board, it was decided to attempt to utilize these inexpensive thermal sensing components to determine the current flowing to the defrost heater.  FIG. 2  illustrates such a circuit utilizing a thermistor  22  to provide the current sensing function for use by the ADC controller  12 .  
         [0029]     That is, the thermistor  22  is placed on the power trace  26  for the defrost heater  14 . This power trace  26  has a known resistance, and therefore the flow of current therethrough will result in a temperature rise thereof. This temperature rise is detected by thermistor  22 . The ADC controller  12  then performs a calculation that takes into account the temperature rise over time to calculate the amount of current flowing through the trace  26 , and therefore the amount of current flowing through defrost heater  14 . This current value is then utilized to calculate the instantaneous power delivered by the defrost heater  14 , which values are accumulated until the desired temperature of the evaporator is reached, i.e. until the proper amount of power has been delivered to melt the frost and/or ice that may have accumulated on the evaporator in the prior refrigeration cycle. At such a point, the ADC controller  12  will command the switching element  16  to terminate the defrost cycle. While various types of thermistors may be used, a preferred embodiment of the present invention utilizes surface mount thermistors, such as NTC thermistors, Linear PTC thermistors, etc.  
         [0030]     To increase the accuracy of the calculation of the current flowing through the power trace  26  as sensed by the temperature rise by thermistor  22 , a second thermistor  24  is utilized to provide an indication of the ambient temperature of the control circuitry. In other words, the second thermistor  24  is used to provide a correcting factor based upon a change in the ambient temperature that may occur due to the heating caused by the defrost heater  14 . Such an increase in ambient temperature will also be sensed by thermistor  22 . If this rise in ambient temperature is not compensated, the ADC controller  12  may think that the increased temperature sensed by thermistor  22  is due to an increased current flow through the defrost heater  14  as measured on trace  26 .  
         [0031]     To prevent this erroneous situation from occurring, the adaptive defrost controller  12  compensates the temperature sensed by thermistor  22  by the temperature differential sensed by thermistor  24 . This net temperature rise, therefore, is due only to the temperature rise of the trace  26 . It is this net temperature rise that is used by the ADC controller  12  to calculate the amount of current flowing through the defrost heater  14  and the trace  26 .  
         [0032]     While the utilization of a thermistor  22  to sense the temperature rise of the power trace  26  solves both the variation in power delivered and cost of sensing current flowing the defrost heater  14 , typical power traces, and indeed wiring for power devices in general, have a very low linear resistance. This is particularly true in configurations as are typically used in such circuitry. As such, and to enhance the ability of the thermistor  22  to actually sense a temperature rise, a preferred embodiment of the present invention utilizes a physical layout that provides such enhanced sensing ability.  
         [0033]     One such circuit layout that will provide this enhanced sensitivity is illustrated in  FIG. 3 . While the circuit components remain the same, a physical configuration of the power trace  26  on which the thermistor  22  is placed in thermal contact is chosen to enhance, or increase its linear resistance so that the temperature rise to be sensed by the thermistor may be provided with better resolution. Since a typical power trace is fairly wide, it was determined that if the power trace at the point of thermistor  22  placement could be narrowed, the thermistor  22  would be able to better sense a temperature rise.  
         [0034]     However, it was also recognized that the width of the power trace is calculated based on transient and maximum current carrying capability. As such, it was important not to minimize the ability of the power circuit itself to carry such maximum currents. In view of this, a secondary current carrying path  28  was added into the circuit to provide an alternate path for current flow at the point of thermistor placement. Such an alternate path  28  insures the ability of the overall circuit to carry such high transient currents.  
         [0035]     The provision of an alternate path would also allow the current actually flowing through the defrost heater  14  to take different paths, and therefore the temperature rise sensed by thermistor  22  would not be representative of the actual current flowing through the defrost heater  14 . To overcome this problem, the length, and therefore the resistance, of the alternate path  28  was made to ensure that under normal operating conditions, the majority of the current flowing through the defrost heater  14  will flow through the power trace  26  path as opposed to the alternate path  28 . In this way, the calculation within the ADC controller  12  maintains accuracy. Indeed, in one embodiment of the present invention the current ratio flow between paths  26  and  28  may also be taken into account with the ADC  12  to ensure that the full current flow is taken into consideration.  
         [0036]      FIG. 4  illustrates an exemplary embodiment of a physical implementation of a section of the power trace  30  of  FIG. 3 . As may be seen in this  FIG. 4 , the width of the power trace  30  is greatly reduced in the branch  26  over which the thermistor will be positioned to sense temperature rise thereacross. This  FIG. 4  also illustrates the alternate path  28  that is provided to ensure that the overall current carrying capability of the trace  30  is not lessened by the modification to provide an enhanced thermal sensing capability through branch  26 . As may also be seen in this  FIG. 4 , attachment pads  32 ,  34  are provided on either side of branch  26  for surface mount placement of the thermistor  22  (see  FIG. 3 ). the electrical connection from attachment pad  34  is jumpered over branch  26  to trace  36 . The actual configuration of the alternate branch  28  may vary widely, but preferably provides an increased resistance to current flow so as to maximize the ability of the thermistor placed over branch  26  to sense a temperature rise. Of course, the resistance of branch  26  is known and utilized by the ADC controller  12  to perform the temperature rise to current calculation.  
         [0037]     All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.  
         [0038]     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.  
         [0039]     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.