Abstract:
Control of defrost cycle initiation in a refrigeration apparatus is based on the difference between the last two defrost times, taking into account the sign of the difference so that the increase or decrease in the time between defrosts corresponds to the trend in defrost times rather than on any particular defrost time or the average of defrost times. A variety of limits are included to ensure that the time between defrosts is reactive to sudden aberrational changes in defrost times, and does not exceed minimum or maximum values.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an adaptive defrost control system for an automatically defrosting refrigeration apparatus. 
     2. Description of the Related Art 
     In a conventional refrigeration apparatus, the refrigerant evaporator accumulates frost at a rate which depends on a number of conditions. These conditions include the number of times the refrigeration apparatus is accessed, the ambient humidity, and the total accumulated compressor run time. Although these conditions are variable, in the conventional non-adaptive system, the defrost cycle is initiated a fixed period of time after the previous defrost cycle has ended, regardless of the actual frost buildup. 
     In order to increase efficiency and thereby reduce energy costs, as well as increase the quality of products being stored in the refrigeration apparatus, it has previously been proposed to base the initiation of the defrost cycle on need, i.e., to operate the defrost heater only when frost build-up becomes excessive. Because measurement of the actual frost accumulation is difficult, it has also been proposed to initiate the defrost time based on an estimated rather than actual frost accumulation. 
     This type of adaptive defrost system was disclosed, for example, in U.S. Pat. No. 3,111,894, which proposed that the rate of frost accumulation be estimated based on an assumed inverse relationship between the frost accumulation and the time required for the defrost heater to raise the evaporator temperature to a predetermined temperature during a previous defrost cycle, with the period between defrosts being controlled accordingly. 
     The inverse relationship method of estimating frost accumulation was also used in the system described in U.S. Pat. No. 4,156,350. This patent discloses a digital timer circuit for calculating the interval between defrost cycles, rather than the heat-absorbing body and analog circuitry disclosed in the earlier U.S. Pat. No. 3,111,894, but the method used to calculate the assumed frost accumulation period is otherwise the same in both prior patents, i.e., it is based on a direct inverse relationship between the previous defrost time and the frost accumulation period or time between defrost cycles. 
     While the adaptive defrost control system of U.S. Pat. Nos. 3,111,894 and 4,156,350 offers improved efficiency in some situations, the assumed inverse relationship is not necessarily optimal. For example, in situations where successive defrost times fluctuate significantly, the sign of the change in the frost accumulation period will lag the actual change, and the assumed and actual frost accumulation periods will thus rarely converge, resulting in an interval between defrosts which is shorter or longer than necessary. 
     To solve this problem, the interval between defrost cycles could be based on the inverse of an average of more than one previous defrost time, rather than on the inverse of a single previous defrost time. While this would reduce the effect of widely fluctuating defrost times, the resulting prediction would still not be optimal, as illustrated by the following example: 
     EXAMPLE OF WHY INVERSE RELATIONSHIP IS NOT OPTIMAL EVEN WHERE PREVIOUS TIMES ARE AVERAGED 
     If one assumes the following inverse relationship between the defrost time (dt) and the time between defrost operations (tbf): 
     
         dt=9 minutes→tbf=12 hours 
    
     
         dt=10 minutes→tbf=11 hours 
    
     
         dt=11 minutes→tbf=10 hours 
    
     
         dt=12 minutes→tbf=9 hours, 
    
     then for the situation in which the last three defrost times, in order, beginning with the earliest defrost time, change as follows (for example, due to a season change in ambient humidity): 
     
         dt=12 minutes 
    
     
         dt=11 minutes 
    
     
         dt=10 minutes. 
    
     Using just the last defrost time would give a time between defrosts of 
     
         tbf=11 hours, 
    
     while using the average of the last three defrost times would give a time between defrosts of 
     
         tbf=10 hours. 
    
     The latter result would clearly be contrary to the trend of decreasing defrost times (12 min.→11 min.→10 min.) Consequently, using the average of the previous defrost times as the basis for the inverse relationship would actually give a worse result that just using the last defrost time, while using the last defrost time would also be inaccurate if the clear trend of decreasing frost accumulation were to continue. 
     In order to solve this problem, a new system would be desirable which takes into account the direction as well as the magnitude of changes in the previous defrost time. 
     In order to further improve the predictive accuracy of an adaptive defrost system, it would also be desirable to depart from the strict inverse relationship concept of the systems described in U.S. Pat. Nos. 3,111,894 and 4,145,350 by monitoring the compressor run cycles during a frost accumulation period and evaluating the run times based on a variable standard which takes into account the trends in the defrost interval. While U.S. Pat. No. 4,156,350 discloses monitoring of a total compressor run time during a frost accumulation period, individual cycles in the prior system are not compared to a variable standard for optimal efficiency. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a principal objective of the invention to improve the predictive accuracy of an adaptive defrost system for a refrigeration apparatus by initiating defrost operations based on defrost time trends rather than on an assumed inverse relationship between the frost accumulation period or interval between defrost cycles and the previous defrost time(s), in order to increase the incidence of convergence of the predictions with the actual frost accumulation. 
     It is also an objective of the invention to even further improve the accuracy of the frost accumulation prediction by continuously monitoring the compressor behavior during an interval between defrosts, and varying the defrost interval if, during a refrigeration cycle, the compressor run time exceeds a calculated maximum. 
     These objectives are achieved by providing an automatically defrosting refrigeration apparatus of the type which includes a refrigerant evaporator, a heater for defrosting the evaporator, defrost initiation means for initiating a defrost operation and timer means for measuring a defrost time required to carry out the defrost operation, in which the accumulated compressor run time interval between defrost operations is controlled based on a difference between two successive defrost times, rather than on just the previous defrost time or an average of previous defrost times, and in which the sign of the difference as well as the magnitude is taken into account. 
     In an especially preferred embodiment of the invention, the interval between defrost operations is decreased, subject to a predetermined minimum interval, by an amount equal to the difference if the difference between defrost times is less than zero or the most recent defrost time is greater than or equal to a predetermined defrost safety limit, and the interval is increased, subject to a predetermined maximum interval, by an amount equal to a sum of the difference and a constant time period offset if the defrost times have increased or stayed the same and the most recent defrost time is less than a predetermined defrost safety limit. 
     Upon start up of the preferred system, the initial interval between defrost cycles is preferably set to a minimum value, the refrigeration system is allowed to run until an accumulated compressor run time is greater than or equal to the initial interval, whereupon the defrost heater is turned on and the defrost time is stored, the next interval is set to the initial interval, and a defrost cycle initiated after the next interval in order to provide the two defrost time values necessary to begin the difference determination. 
     In addition to controlling the interval between defrost cycles based on a defrost time difference, the current continuous compressor run time is also preferably monitored, and the interval between defrost operations is set to a minimum value if the current continuous compressor run time is greater than the first continuous compressor run time, i.e., the continuous run time during the initial refrigeration cycle after a defrost cycle, which in turn cannot be greater than a variable based on the current interval between defrost cycles without also causing the interval between defrosts to be set to the minimum value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a refrigeration apparatus constructed in accordance with the principals of a preferred embodiment of the invention. 
     FIG. 2(a) and 2(b) form a flowchart illustrating the manner in which the interval between defrost is controlled by the circuit of FIG. 1 in accordance with the principles of the preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The implementation shown in FIG. 1 is a defrosting device which replaces a standard defrost timer on household refrigerators. The refrigeration apparatus includes a conventional compressor 1, cold control switch 2, defrost heater 3 for removing frost, and power supply 4. The defrosting device includes a relay switch 5 for preventing compressor operation and turning on heater 3 to initiate a defrost operation, and a conventional bi-metal type thermostat 6 which automatically shuts off the defrost operation when a predetermined temperature is sensed. A control circuit 7, preferably in the form of a microprocessor chip with an internal RAM and ROM is connected to control the relay coil 8 via a standard relay control circuit 9. The relay coil 8 is positioned to move the relay switch 5 to the defrost mode when energized, the relay normally allowing compressor operation. 
     The defrost time is monitored in this embodiment by monitoring the voltage to the defrost heater 3 via voltage detection circuit 10. In addition, a second voltage detection circuit 11 is preferably connected to the compressor power supply in order to monitor compressor run time, the compressor run time being controlled by operating switch 2 in a conventional fashion. A timer 12 which is connected to reset the microprocessor via OR gate 13 as necessary. The microprocessor also includes a conventional power line cycle driven clock 14 for providing all timing functions and a reset switch circuit 15 is connected to the reset terminal of microprocessor 7 via OR gate 13. 
     As shown in FIGS. 2(a) and 2(b), upon start-up, the controller begins with a power-up sequence (steps 100-120) which sets the compressor run time between defrosts variable (tbf) to a minimum value (minv) and clears the previous defrost time memory upon initial start-up. The refrigeration system is allowed to run in a normal fashion (steps 130-210, described in more detail below) until the accumulated compressor run time (ct) is greater than or equal to the tbf variable, at which time a defrost flag is set and the defrost subroutine is called (steps 210 and 220) on the next compressor off cycle. After initiation of the defrost cycle, the system waits for the defrost heater to be energized and then proceeds to monitor the defrost thermostat. From the time of defrost heater energization until the defrost thermostat opens or a maximum defrost time maxdt is reached, the defrost time variable dt is incremented, after which the frost accumulation or time between defrosts variable tbf is set according to the difference between the defrost time variable dt and a previous defrost time variable pdt stored in the microprocessor&#39;s RAM. 
     Following the first defrost cycle after a power-up condition, tbf is not altered. On subsequent defrost cycles, the difference between the two values is used to modify the time between defrosts (tbf) variable according to the following procedure, implemented in step 290, 295,300, 305, and 310, and based on the stored previous defrost time (pdt), the most recent defrost time (dt), a preset defrost safety limit (ds), a maximum defrost time (maxdt), a minimum time between defrosts (minv), and a maximum time between defrosts (maxv): 
     1. If ((pdt-dt)&lt;0 or (dt&gt;ds), then tbf=tbf+60(pdt-dt). 
     2. If ((pdt-dt)&gt;0) and (dt&lt;ds), then tbf=tbf+60(pdt-dt)+1. 
     3. If (dt&gt;maxdt), then tbf=minv. 
     4. If (tbf&lt;minv), then tbf=minv. 
     If (tbf &gt;maxv), then tbf=maxv. 
     The first condition indicates that for an increase in defrost times, or where the previous defrost time is greater than or equal to safety value ds, the time between defrosts is altered by the difference in defrost times. For a decreasing or steady defrost time in which the difference term is greater than or equal to zero, and so long as the defrost time is less than the safety value, the defrost time is increased by sum of the difference and one hour. Except for the constant 1, which is in units of hours, the defrost times are in units of minutes. The value of tbf is also compared to the limits minv and maxv such that if tbf is greater than maxv, tbf is set equal to the maximum value, and if tbf is less than minv, then tbf if set equal to the minimum value. After completing the defrost subroutine (steps 220-330) the refrigeration system is again allowed to operate in a normal fashion, with a timer accumulating the compressor run time (ct) until it is greater than or equal to tbf, at which time another defrost cycle is initiated. 
     In order to take into account actual conditions during the interval between defrosts, the preferred system takes into account compressor run times during individual refrigeration cycles. If any refrigeration cycle is excessively long, such that frost builds up at a rate greater than would be indicated by recent trends in the time between defrosts, the current time between defrosts is set to a minimum value. For example, in this embodiment, during the initial refrigeration cycle after a defrost cycle, tbf is set to minv whenever the condition exists where the initial continuous compressor run time cctinit exceeds the value of (1+29/tbf). During subsequent refrigeration cycles, the current continuous compressor run time (cct) is monitored and, if the condition exists where cct exceeds the value of cctinit, the tbf variable is also set to minv. This portion of the control routine (steps 190-200) effectively overrides the above-described method of setting the time between defrosts variable tbf, where actual compressor running conditions have changed sufficiently to require such an override. 
     Having thus described a particularly preferred embodiment of the claimed invention, it will be appreciated by those skilled in the art that the basic concepts described above admit numerous variations, all of which are intended to be included within the scope of the invention. For example, the controller could take into account defrost times prior to the most recent two defrost times, and thereby obtain a more extensive chart of trends, with appropriate weights given to the most recent trends, and, for example, provision for eliminating aberrational jumps in the trends. Accordingly, the above description and drawings should not be read as limiting in any way, but rather the invention should be defined solely by the appended claims.