Adaptive defrost control system for a refrigerator

An adaptive demand defrost control system controls the length of an interval between defrost operations in accordance with the number and duration of compartment door-openings, the duration of a previous defrost operation as corrected by the temperature of the evaporator prior to defrost, and the length of time the compressor has been energized. A count is stored which is varied according to a decrementing schedule, with the decrementing schedule in turn being based upon a comparison of the corrected defrost duration with either a desired defrost duration or a range of desired defrost durations. A defrost operation is initiated when the count reaches a predetermined value. An alternative embodiment of the invention develops an indication of the ambient humidity and controls humidity-dependent apparatus in accordance with the indication.

BACKGROUND OF THE INVENTION 
This invention relates to defrost controls for a refrigerator, and more 
particularly, to an adaptive demand defrost control system which provides 
a variable interval between defrost operations which is based upon several 
factors, including the amount and duration of door openings and the length 
of previous defrost operations. 
In general, in a refrigerator it is desirable to defrost only as often as 
is necessary to maintain an efficient cooling system. This objective 
dictates that a balance be struck between the competing considerations of 
system operation with a frosted evaporator, the energy consumed in 
removing a frost load from the evaporator and the acceptable level of 
temperature fluctuation within the refrigerated compartments caused by a 
defrosting operation. 
A successful attempt at meeting this objective is shown and described in 
U.S. patent application Ser. No. 155,154, now U.S. Pat. No. 4,327,557, 
filed May 30, 1980, entitled "Adaptive Defrost Control System" and 
assigned to the assignee of this application. The system disclosed therein 
takes into account the number and duration of freezer and fresh food door 
compartment openings, the duration of the previous defrosting operation, 
and the total accumulated compressor run time since the previous defrost 
operation. In general, defrosting is provided at variable intervals as 
determined by a weighted accumulation of the number and duration of 
freezer and fresh food door openings, with the weighting functions being 
adaptably controlled as a function of the time required to perform the 
previous defrost operation. 
The control disclosed in the above application stores a count which is 
decremented by the weighting functions during a door-open interval. The 
count is decremented at a first constant rate during a first predetermined 
period of time that the fresh food door is open, and is decremented at a 
second constant rate thereafter. The count is decremented at a third 
constant rate during an initial predetermined period of time that the 
freezer door is open, and a fourth constant rate thereafter. 
The rates of decrementing the count are determined by comparing the 
measured length of a defrosting operation against a desired defrost 
length. In many instances, the comparison of the measured defrost length 
with the desired defrost length operates to change the length of the 
interval before the next defrost operation, in turn forcing the next 
succeeding defrost length toward the desired value. 
While the defrost control described above has been successful in 
implementing efficient control of a defrost heater, it has been found that 
efficiency can be further increased if, in addition to the factors 
utilized by the above described defrost control, the evaporator 
temperature is considered as a factor in determining the length of a 
defrost interval. 
Generally, it has been found that there is little or no correlation between 
the duration of a defrost operation and the amount of frost which has 
actually been removed from the evaporator during the defrost operation. 
This is due to the fact that the measured length of a defrost operation is 
not only dependent upon the amount of frost on the evaporator coil, but is 
also strongly dependent upon the temperature of the evaporator at the time 
the defrost operation is initiated. Since the defrost control disclosed in 
the above-mentioned patent utilizes the length of a defrost operation as a 
factor in determining the duration of the next defrost interval, the 
defrost control may provide less-than-optimal defrost operation if the 
temperature of the evaporator is not considered. 
Moreover, it has been found that the decrementing of the count at constant 
rates during the time the fresh food door is open does not result in an 
entirely accurate representation of the amount of frost which has formed 
on the evaporator due to the moisture introduced into the refrigerator 
while the door is open. Again, this may result in a less-than-optimal 
defrost interval. 
Furthermore, it has been found desirable to incorporate control of a 
humidity-dependent apparatus, such as an anti-sweat heater, in accordance 
with the ambient humidity to which the refrigerator is exposed. Reliable 
humidity sensors are, however, relatively expensive and impractical for 
use on household refrigerators and the like. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a defrost control system for a 
refrigerator provides a defrost operation at the end of a variable 
interval referred to as a defrost interval, that is a function of the 
number and duration of compartment door openings using an adaptive control 
scheme that is dependent upon the measured length of the previous defrost 
operation, as corrected by a measure of the evaporator temperature prior 
to the initiation of the defrost operation. 
In many refrigerators air is discharged from an evaporator directly into a 
freezer compartment, and the temperature within the freezer compartment 
therefore provides an accurate indication of the relative temperature of 
the evaporator. In such refrigerators, the measured defrost length can be 
corrected as a function of the measured temperature of the freezer 
compartment, rather than the measured temperature of the evaporator. This 
eliminates the need for a separate temperature sensor connected directly 
to the evaporator, and a single sensor can be used to measure the freezer 
temperature and provide a relative measure of the evaporator temperature. 
In the illustrated embodiment of the invention, a count is stored 
representing the interval before which a defrost is initiated. The count 
is varied according to a decrementing schedule which varies as a function 
of time. Specifically, the decrementing schedule is arranged such that the 
count is varied by different amounts for each second of a predetermined 
interval that the fresh food door is open. Following the predetermined 
interval, the count is varied at a first constant rate. For each second 
that the freezer door is open, the count is varied at a second constant 
rate which is greater than the first constant rate. In particular, the 
count is decremented by an integer multiple of a factor W, with the 
integer factor being a function of the door which is opened, and in the 
case of the fresh food door, the length of time the door is open. 
Once the count has been varied to a predetermined value, a defrost 
operation is initiated. It has been found that the decrementing schedule 
noted above allows for a close approximation of the manner in which frost 
actually builds up on the evaporator in response to door openings. 
Consequently, the correlation between the defrost interval and the actual 
frost load on the evaporator is improved and, hence, refrigerator 
operation efficiency is enhanced. 
The factor W is calculated in accordance with a comparison of the measured 
defrost length with a desired defrost length, the measured defrost length 
being corrected as a function of the measured freezer temperature prior to 
the defrost operation. It has been found that correcting the measured 
defrost length in this manner is particularly important in providing a 
high degree of correlation between the defrost length and the amount of 
frost actually removed from the evaporator coils during the defrost 
operation. Consequently, this corrected defrost duration allows the factor 
W to be calculated in such a way that the defrost operations are initiated 
in an efficient manner. 
A first alternative embodiment of the defrost control operates to compare 
the measured defrost length against an optimum defrost length which is 
varied as a function of the measured freezer temperature during a defrost 
interval. It will be appreciated that because of the variations which are 
usually encountered in refrigerator components, there is a range of 
optimum defrost lengths rather than one particular desired defrost length. 
The control operates to vary the decrementing factor W when the actual 
defrost length is outside a predetermined range of values surrounding the 
optimum defrost length, with no change being made to W if the measured 
length is within the range of optimum defrost length values. 
A second alternative embodiment of the invention develops an indication of 
the ambient humidity within which the refrigerator is operating. A 
humidity factor is calculated which is a function of the amount of frost 
formed on the evaporator, as indicated by the length of a defrost 
operation, and the usage encountered by the refrigerator, as indicated by 
the length of time that the refrigerator doors have been open. If the 
humidity factor exceeds a predetermined maximum, then a humidity-dependent 
device, such as an anti-sweat heater, may be energized to reduce the 
condensation of moisture on the exterior of the refrigerator. In this way, 
a reliable indication of humidity is obtained without the need for 
expensive humidity-sensing apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, there is illustrated a conventional refrigerator 
20 in conjunction with a block diagram of the defrost control system of 
the present invention. The refrigerator 20 includes a cabinet 22 which in 
turn includes an internal compartment separator 24 separating a freezer 
compartment 26 from a fresh food compartment 28. A freezer door 30 seals 
off the freezer compartment 26 from the outside and a fresh food door 32 
encloses the fresh food compartment. 
The fresh food and freezer compartments are cooled by passing refrigerated 
air into the compartments. The air is refrigerated as a result of being 
passed in heat exchange relationship with a conventional evaporator 34 and 
is forced by an evaporator fan 36 into the refrigerated compartments 
26,28. The refrigeration apparatus includes a compressor 38 and a 
condenser (not shown) interconnected with the evaporator 34 in a 
conventional manner to effect the flow of refrigerant thereto. A defrost 
heater 40 is positioned adjacent the coils of the evaporator 34 and is 
periodically energized by the defrost control of the present invention to 
defrost the evaporator 34. The defrost heater 40 may be a conventional 
resistive heater that is energized directly from an AC line by means of a 
relay or triac. 
A conventional bimetal temperature sensor 42 is located on or adjacent the 
coils of the evaporator 34 so as to sense a predetermined temperature 
thereof. The bimetal sensor 42 operates to terminate a defrost operation 
in a manner to be described below. 
A freezer door switch 44 having an actuator 44a is mounted on the cabinet 
22 so that the actuator 44a contacts the closed freezer door 30. 
Similarly, a fresh food door switch 46 having an actuator 46a is mounted 
on the cabinet 22 with the actuator 46a in contact with the closed fresh 
food compartment door 32. The actuators 44a,46a, are spring-loaded so that 
when one of the doors 30,32 is opened, the corresponding actuator 44a,46a 
moves outwardly out of contact with the corresponding door 30,32 thereby 
causing the contacts of the switches 44,46 to close. 
The freezer temperature is sensed by a freezer thermistor 50 positioned 
within the freezer compartment 26. A thermistor 52 is disposed within the 
fresh food compartment 28 to sense the temperature therein. 
Disposed along the front face of compartment separator 24 is an anti-sweat 
or mullion heater 54 which is utilized to reduce moisture condensation, as 
will be described in greater detail below. 
The defrost control of the present invention shown in block diagram form in 
FIG. 1 may be implemented by using discrete digital logic or through the 
use of a microcomputer. In the preferred embodiment illustrated, a single 
chip microcomputer 58 is used to implement the defrost control. The 
microcomputer integrated circuit may be a conventional, single chip device 
and may include on the chip, a 2048.times.8 bit program read-only memory, 
or ROM 60, and a 128 word random access memory, or RAM 62. The 
microcomputer 58 also includes a central processing unit, or CPU 64, which 
performs the various computations used in the defrost control process. The 
ROM 60 contains the control program, the control logic, and the constants 
used during control execution. The RAM 62 contains registers 66 (shown 
more particularly in FIG. 2A) which store the several variables used in 
the control program. Also included in the RAM 62 are a seconds timer 68, a 
compressor minute timer 69, a compressor run timer 70, a freezer door 
timer 72, a fresh food door timer 74, a defrost length timer 76, a drip 
time timer 78, a defrost flag register 80 and an adaptive mode flag 
register 81. While for purposes of clarity, the RAM 62 has been 
illustrated as containing separate storage registers for each variable, it 
is to be understood that each storage register may contain the value of 
several variables over the course of a program execution. 
In the illustrated embodiment, microcomputer 58 is implemented by using a 
COPS 444 microcomputer manufactured by National Semiconductor Corp., which 
has 21 input/output ports and serial input/output capability. 
The inputs to the microcomputer 58 include the freezer door switch 44, the 
fresh food door switch 46, the bimetal sensor 42, and the thermistors 
50,52 via an analog to digital converter 82. The state of the bimetal 
sensor 42 is inputted to the microcomputer 58 through a relay K2. Another 
input to the microcomputer 58 is from clock pulse circuitry 84 which 
provides a reference signal for measuring real time events, such as the 
length of a defrost operation. 
Outputs from the microcomputer 58 are coupled to energize the defrost 
heater 40, the compressor 38, the mullion heater 54 and the evaporator fan 
36 through relays K1, K3, K4 and K5, respectively. 
The defrost control system of the present invention utilizes various data 
to determine when a defrost operation should be initiated. These data 
include the number and duration of freezer and fresh food compartment door 
openings, the duration of the previous defrosting operation as corrected 
by the temperature existing within the freezer prior to the defrost 
operation, and the total accumulated compressor run time since the 
previous defrosting operation. The number and duration of compartment door 
openings are detected by monitoring the door switches 44,46 associated 
with the two compartment doors 26 and 28. The actual duration of the 
defrost operation is determined by monitoring the bimetal sensor 42 and 
measuring the amount of time it takes from the start of the defrosting 
operation until the evaporator 34 reaches a predetermined temperature, as 
indicated by the opening of the bimetal sensor 42. 
The defrost heater 40 is energized at variable intervals as determined by a 
weighted accumulation of the number and duration of freezer and fresh food 
door openings. The microcomputer 58 stores a number or count that must be 
decremented to zero before a defrost operation is initiated. This count, 
referred to as TBND (time before next defrost), is decremented by 
different amounts for each second of the first five seconds that the fresh 
food compartment door 32 is open, and is thereafter decremented at a 
constant rate. The count TBND is decremented by a constant amount during 
each second of a defrost interval that the freezer door 30 is open, 
regardless of the amount of time the door is open. 
A weighting or decrementing factor, designated W, is established and is 
utilized to decrement the count TBND according to the following weighting 
schedule, shown in graphic form in FIG. 9: 
______________________________________ 
Second of Fresh Food 
Door 32 Opening Decrement TBND by 
______________________________________ 
First 16 .times. W 
Second 8 .times. W 
Third 4 .times. W 
Fourth 2 .times. W 
Fifth 1 .times. W 
Each Additional 1 .times. W 
Each Second of Freezer 
5 .times. W 
Door 30 Opening 
______________________________________ 
This weighting schedule is based on test data and closely approximates the 
manner in which frost develops on the evaporator of a conventional 
side-by-side refrigerator (illustrated generally in FIG. 1) in response to 
compartment door openings. 
The count TBND is also decremented by one count for each second of 
compressor 38 run time. 
The weighting factor W is updated, when necessary, by adding to it a 
correction factor, designated CORR, which is derived by adding the 
contents of the defrost timer 76 with a term equal to ten times the 
freezer temperature (in degrees Fahrenheit) occurring during the defrost 
interval prior to the defrost operation and by comparing this corrected 
defrost length with a desired defrost length designated DESDEF. 
Normally, once the count TBND has been decremented to zero, the defrost 
heater 40 is energized. However, the compressor run time timer 70 actuates 
inhibiting means to prevent the initiation of a defrost operation if the 
count TBND reaches zero before a predetermined minimum amount of 
compressor run time has been accumulated. The control checks for minimum 
compressor run time when the count TBND is decremented to zero to 
determine whether the defrost indication is due to abnormal condition, 
such as an excessive number of door openings during a defrost interval. 
Under this condition, the adaptive portion of the control technique is 
disabled to prevent the control from adaptively varying the decrementing 
factor W. 
A first alternative embodiment of the invention compares the actual defrost 
length against a range of values surrounding the optimal defrost length 
and varies the weighting factor W in accordance therewith. The optimal 
defrost length against which the measured defrost length is compared is 
varied as a function of the measured freezer temperature during defrost, 
thereby varying, under the same circumstances, the newly derived door 
weighting function and, hence, varying the rate at which the count is 
decremented during the next defrost interval. In effect, the temperature 
within the freezer compartment 26 prior to a defrost operation is 
considered in determining the weighting factor W and, hence, the next 
defrost interval. 
A second alternative embodiment of the invention considers door-open 
information as well as the duration of a defrost operation to develop a 
measure of the ambient humidity in which the refrigerator 20 is operated. 
This measure of ambient humidity is used to control anti-sweat heaters 
associated with the refrigerator cabinet, such as the mullion heater 54, 
to reduce the amount of condensation occurring on the cabinet. 
Referring now to FIGS. 2A and 2B, the circuit of the adaptive defrost 
control system shown in block form in FIG. 1 is illustrated in detail. Two 
power supply inputs VCC and GRD for the microcomputer 58, FIG. 2A, are 
connected to a source of DC potential V1 and ground potential, 
respectively. The voltage V1 is developed by an AC to DC converter and 
regulator 100, shown in FIG. 2B, which receives AC line current over a 
pair of terminals 102,104. A second output from the AC to DC converter 100 
is developed on a line 106 and is coupled to an input IN0 of the 
microcomputer 58. The signal on the line 106 is a 60 hertz square wave 
signal which provides a time base for the seconds and minute timers 68 and 
69, shown in FIG. 1. 
A clock input CKI of the microcomputer 58 receives a 200 kilohertz signal 
from the clock circuit 84, seen in FIGS. 1 and 2B, over a line 110. The 
signal from the clock circuit 84 establishes the time base for program 
execution performed by the microcomputer 58. 
A power-on reset circuit, or POR 111 provides a reset signal to an input 
RESET of the microcomputer 58 for a short time period following the 
application of power thereto to prevent an erroneous energization of 
outputs thereof during the startup procedure. Circuit 111 also shuts off 
the microcomputer when the DC input voltage falls below a predetermined 
level. 
The door-open information is coupled to the microcomputer 58 over two input 
lines IN1 and IN2, FIG. 2A. A contact 44b of the freezer door switch, FIG. 
2B, is connected to the input IN1 through a resistor R1 and to supply 
potential V1 through a resistor R2. Similarly, a contact 46b of the fresh 
food door switch 46 is connected to the input IN2 through a resistor R3 
and to voltage supply V1 through a resistor R4. The opposite terminals of 
both switches are connected together and to ground potential. A capacitor 
C1 and diode D1 are connected between the input IN1 and ground. Likewise, 
a capacitor C2 and a diode D2 are connected between the input IN2 and 
ground. 
The determination of whether a door 30,32 is open is made by analyzing the 
signals present at the inputs IN1 and IN2. For example, if the freezer 
door 30 is open, then the switch contact 44b will be closed, thereby 
coupling a low state signal to the input IN1. This signal in turn causes 
the freezer door timer 72, shown in FIG. 1, to begin timing the period of 
the door-open interval. 
The circuitry connected to the input IN2 operates in an identical manner to 
start and stop actuation of the fresh food door timer 74, FIG. 1. 
A data input CKO is coupled to circuitry which senses the energization of 
the defrost heater 40 and the opened-closed status of the bimetal sensor 
42. When the microcomputer 58 determines that defrosting is required, a 
signal is generated at an output D1 which is coupled through a driver 
circuit 112 and which energizes a relay coil K1. A set of relay contacts 
K1a are closed by the energized relay coil K1, thereby coupling a source 
of potential V2 across the defrost heater 40 and the bimetal sensor 42. At 
this time, the bimetal sensor 42 is closed, energizing the defrost heater 
40. 
The relay K2 is coupled across the defrost heater 40 to sense the 
energization thereof. Energization of the coil K2 in turn opens relay 
contacts K2a and allows a high state signal to be coupled from the voltage 
source V1 through a resistor R5 to the input CKO. Transient protection is 
afforded by a pair of capacitors C3,C4 and a voltage-variable resistor R6. 
A resistor R7 limits the current flowing from the voltage source V1 to 
ground when the relay contacts K2a are closed. 
Additional inputs to the microcomputer 58 are provided at a series of 
inputs S0, SI, G0 and SK from the analog to digital converter 82. The A to 
D converter, in turn, receives as inputs the freezer and fresh food 
compartment thermistors 50,52, respectively. 
The A-D converter senses the voltage across the thermistors 50,52 and 
provides a digital output indicating the temperatures to which these 
thermistors are exposed. 
An output D0 of the microcomputer 58 is utilized to control the compressor 
38 via the relay coil K3 and through the driver circuit 112. The 
energization of the relay coil K3 by the output D0 closes the associated 
contacts K3a, in turn actuating the compressor 38. 
If it is desired to control the mullion heater 54 with the microcomputer 
58, then an output D2 is utilized. When a high state signal is generated 
at the D2 output, a relay coil K4 is energized via the driver circuit 112 
thereby closing the relay contacts K4a. The mullion heater is then 
connected across a voltage source V2, in turn energizing the heater 54. 
Referring specifically to FIG. 2A, the registers 66 within the RAM 62 store 
various intermediate and final results during execution of the control 
program. These registers, designated FT, ODL, CORR, W, TBND, MINDT, MAXDT 
and HF are utilized in a manner to be hereinafter described in detail. The 
RAM 62 also contains a door-open counter 220 and a freezer temperature 
timer 196 which are utilized as noted below. 
A series of registers are contained within the ROM 60 and are designated 
MAXDEF, DESDEF, MAXW, MINW and HMAX. These registers contain constants 
used during the control program. In the preferred embodiment, the contents 
of these registers are as follows: 
______________________________________ 
REGISTER CONTENTS 
______________________________________ 
MAXDEF 1260 seconds 
DESDEF 960 seconds 
MAXW 360 seconds 
MINW 60 seconds 
HMAX 33 
______________________________________ 
Referring also to FIGS. 3 and 4, the control program of the adaptive 
defrost control system will be described. The program cycle is executed 
once each second to continuously update the system condition. Moreover, 
during each program cycle, the seconds timer 68 is incremented. 
As seen in FIG. 3, following energization of the various components used in 
the control, a block 120 initializes the variables used in the control 
program. The defrost flag register 80 and the adaptive mode flag register 
81 shown in FIG. 1 are both reset. The register FT, which stores the 
freezer compartment temperature sensed by thermistor 50, is initialized to 
zero. 
The register W which stores the decrementing factor is assigned a value of 
210, which is midway between its lower limit stored in the MINW register, 
and its upper limit stored in the MAXW register. 
The register CORR, which stores the correction factor, the freezer and 
fresh food door timers 72,74, the defrost timer 76 and the seconds timer 
68 are all assigned a value of zero. 
The register TBND, which stores the time before next defrost, is assigned a 
value of 518,400, which must be decremented to zero before a defrost 
operation is initiated. The compressor run timer 70 is assigned a value of 
360 minutes (6 hours) of compressor operation before a defrost operation 
may be initiated. The minute timer 69 is assigned a value of 60. 
Following the initialization performed in block 120, a decision block 122 
determines whether the defrost heater 40 is energized by analyzing the 
signal appearing at the CKO input of the microcomputer 58, FIG. 2A. If the 
defrost heater 40 is energized, then control passes to a block 152, FIG. 
4, which is the first step of the defrost routine, to be described in 
greater detail below. 
If the block 122 determines that the defrost heater 40 is not energized, 
then a decision block 124 determines whether the control is in the 
adaptive mode. This is performed by determining whether the adaptive mode 
flag register 81 is set. If it is determined that the control is not in 
the adaptive mode, then control passes to a block 126 which determines 
whether the compressor has accumulated 6 hours of run time by checking the 
contents of the compressor run timer 70. It should be noted that the 
compressor run timer 70 is decremented by one count at the end of each 
minute of compressor operation, as indicated by the compressor minute 
timer 69, which is operative only when the compressor 38 is energized. If 
the decision block 126 determines that the compressor has accumulated 6 
hours of run time then control passes to the block 152 to initiate the 
defrost sequence. 
If the decision block 124 determines that the control is in the adaptive 
mode, then a decision block 128 determines whether the compressor minute 
timer 69 has elapsed. If the timer 69 has elapsed, then the timer 69 is 
reset and the compressor run timer is decremented by one and the register 
TBND is decremented by sixty. 
A decision block 132 then determines whether the contents of the register 
TBND have been decremented to zero. If it has not, or if the block 128 
determines that the compressor minute timer has not elapsed, then control 
passes to a block 134 which determines whether the fresh food door 32 is 
open. This is determined by analyzing the input IN2 of the microcomputer 
58, FIG. 2A, and determining whether a high state signal is present 
thereon. If the block 132 determines the count TBND has been decremented 
to zero, then control passes to the block 126. 
If the block 134 determines that the fresh food door 32 is open, then the 
count TBND is decremented by a block 136 by an amount depending on the 
contents of the fresh food door timer 74, shown in FIG. 1. If the door 32 
has been open for less than five seconds, then the count TBND is 
decremented according to the weighting schedule as represented by the 
following equation: 
EQU TBND=TBND-(16 (1/2).sup.t-1) W for 0&lt;t.ltoreq.5 
where t equals the time in full seconds that the door 32 has been open. 
If the door 32 has been open for longer than five seconds, then the count 
TBND is decremented by the current value of W. 
A block 138 follows the block 136 and determines whether the count TBND has 
been decremented to zero. If it has, then control passes to the block 126. 
If the count TBND has not been decremented to zero, then a block 140 
determines whether the freezer door 30 is open by sensing whether a high 
state signal is present on the input IN1. If the door is open, then count 
TBND is decremented according to the weighting schedule represented by the 
following formula: 
EQU TBND=TBND-5(W) 
A block 144 then determines whether the count TBND has been decremented to 
zero, and if it has, then control passes to the block 126. On the other 
hand, if the count TBND has not been decremented to zero, then a block 146 
sets the adaptive mode flag, indicating that the defrost control is in the 
adaptive mode. Control then passes to a block 148 comprising a temperature 
control routine. 
The temperature control routine is utilized to control the temperatures 
within the freezer compartment 26 and fresh food compartment 28. 
Generally, the routine senses the values of the thermistors 50,52 and 
compares the temperatures indicated thereby against user-selected set 
points. If the fresh food or freezer compartment temperatures exceed a 
range of temperatures surrounding the set points, then the compressor 38 
is energized or de-energized to bring the compartment temperatures within 
the range of temperatures. 
Control from the temperature control routine performed by block 148 then 
passes back to the decision block 122. 
If, whenever control is passed to decision block 126, it is determined that 
the compressor has not run for six hours, then a block 150 resets the 
adaptive mode flag, thereby removing the defrost control from the adaptive 
mode. This is desirable since the adaptive control has called for a 
defrost operation following an interval which is shorter than the minimum 
compressor run time due to an abnormal condition, such as a large number 
and/or duration of door openings. Therefore, the control prevents the next 
defrost interval from being adaptively varied in response to the abnormal 
condition, 
As shown, control then passes from block 150 to block 148. 
If the block 126 determines that the compressor 38 has run for six hours, 
then control passes to a block 152, FIG. 4, which initiates the defrost 
routine. The block 152 de-energizes the compressor 38 by providing a low 
state signal at the output D0 at the microcomputer 58, energizes the 
defrost heater 40 by energizing the output D1 of the microcomputer 58 and 
sets the defrost flag register 80, FIG. 1, indicating that defrost is 
occurring. 
A decision block 154 then determines whether the bimetal sensor 42 is open 
by analyzing the input CK0 to the microcomputer 58. If a low state signal 
is coupled to the input CKO, indicating that the bimetal 42 has opened, 
then the contents of the drip timer 78, FIG. 1, are decremented by one, 
and control passes to a decision block 158. 
It should be noted that the drip timer 78, initialized to 30 seconds by the 
block 120, FIG. 3, is utilized to prevent re-energization of the 
compressor 38 for a 30 second period of time following a defrost operation 
to allow water to drip off the evaporator coils 34 to prevent re-icing 
thereof. 
The decision block 158 then determines whether the drip timer 78 has 
elapsed. If it has not, then control passes back to the temperature 
control routine performed by the block 148, FIG. 3. 
If the drip timer 78 has elapsed, then a block 160 determines whether the 
control is in the adaptive mode by checking the contents of the adaptive 
flag register 81. If this register is not set, indicating that the control 
is not in the adaptive mode, then control passes to a block 162, which 
sets the adaptive mode flag and re-initializes the count TBND to its 
original value. The next defrost operation will then take place once the 
count TBND has been decremented to zero unless the compressor timer 70 has 
not elapsed, as described above in connection with FIG. 3. 
If the block 160 determines that adaptive mode flag has been set, then 
control passes to a block 164 which calculates the value stored in the 
CORR register shown in FIG. 2A. The value stored in this register is 
calculated as follows: 
EQU CORR=[ACTDEF+(FT)(10)]-DESDEF 
EQU CORR=0 if: 930&lt;[ACTDEF+(FT)(10)]&lt;990 
where ACTDEF is the actual defrost length measured by the defrost timer 76, 
DESDEF is a constant representing the desired or optimum defrost length 
and stored in the ROM 60, FIG. 2A, FT is the freezer temperature (in 
degrees Fahrenheit) measured during the temperature control routine 
performed by block 148. 
As seen by the above equations, the actual defrost length, measured by the 
control and stored in the register ACTDEF, is corrected as a function of 
the freezer temperature occurring during the temperature control routine. 
This temperature is multiplied by 10 for scaling purposes. 
It should also be noted that if the corrected defrost time, represented by 
the summation of the actual defrost time and the freezer temperature 
multiplied by 10, is within a particular range of time, such as between a 
lower limit of 15.5 minutes (i.e. 930 seconds), and an upper limit of 16.5 
minutes (i.e. 990 seconds), then the value stored in the CORR register is 
set equal to zero. This feature is included in the defrost control 
technique to account for the manufacturing tolerances of the bimetal 
sensor 42, which may have a switching point up to 3.degree.-4.degree. F. 
on either side of its nominal rating. Consequently, a defrost length 
within this range of time is considered to be of optimal duration and, 
hence, no correction is required. 
The following chart illustrates the manner in which the defrost operation 
duration ACTDEF is corrected in response to changes in the measured 
freezer temperature prior to defrost. The following chart also illustrates 
the manner in which the correction factor CORR for the variable W varies 
in response to the corrected defrost operation duration. 
______________________________________ 
FREEZER CORRECTED 
ACTDEF TEMP. DEFROST LENGTH CORR 
______________________________________ 
840 sec 15 .degree.F. 
990 sec 30 
840 10 940 0 
840 5 890 -70 
840 0 840 -120 
840 -5 790 -170 
______________________________________ 
Where corrected defrost length=ACTDEF+FT(10) 
Following the block 164 is a block 166 which adds the value stored in the 
CORR register with the value stored in the W register and assigns this 
result to the W register. 
A decision block 168 then determines whether the newly calculated value of 
W is between the upper and lower limits MINW and MAXW, respectively. As 
previously noted, the value MINW is equal to 60 and the value MAXW is 
equal to 360. If it is determined by the block 168 that the newly 
calculated value of W is between these limits, then control passes to the 
block 162. If W is not within this range, then a block 170 changes the 
value of W to put it within the range between MINW and MAXW. For example, 
if W is less than MINW, then the block 170 stores in the W register a 
value equal to MINW, and conversely, if the value of W is greater than 
MAXW, then the MAXW value is stored in the W register. Control from the 
block 170 then passes to the block 162. 
Following the block 162 is a block 172 which de-energizes the defrost 
heater 40 by de-energizing the output D1 of the microcomputer 58. The 
block 172 also resets the defrost flag 80, reinitializes each of the 
timers 69, 70, 72, 74, 76 and 78, and delays the evaporator fan 36 
reenergization for a short delay period. This is to insure that the 
evaporator 34 has been cooled somewhat following a defrost operation to 
prevent the reintroduction of warm air into the refrigerated compartments 
26,28 when the evaporator fan 36 is energized. 
If the block 154 senses a high state signal at the input CKO of the 
microcomputer 58, indicating that the bimetal sensor 42 is not open, then 
a block 174 reinitializes the drip timer 78 to 30 seconds. A block 176 
then increments the defrost timer 76 by one minute when 60 seconds of 
defrost heater 40 operation have elapsed. 
A block 178 then checks to determine whether the defrost operation duration 
ACTDEF stored in the defrost register 76 is greater than a maximum 
duration MAXDEF, stored in the ROM 60, FIG. 2A. As before noted, the value 
of MAXDEF is equal to 21 minutes. If the defrost operation duration has 
not exceeded this upper limit, the control passes to the block 148, FIG. 
3, which cycles the refrigerator 20 through the temperature control 
routine. 
If the block 178 determines that the defrost operation duration has 
exceeded the upper limit MAXDEF, then the defrost control is taken out of 
the adaptive mode by a block 180, and the register W, FIG. 2A, is assigned 
the value stored in the MAXW register in the ROM 60. This will result in 
the next defrost operation being initiated after six hours of accumulated 
compressor run time. By assigning the value MAXW to the W register, the 
control will, depending on the amount of usage the refrigerator receives, 
tend to initiate the next adaptive defrost operation after a relatively 
short defrost interval. This is desirable since the current defrost length 
duration has been exceedingly long, indicating a severe buildup of ice on 
the evaporator coils 34. 
Control from the block 180 then passes to the block 172 and from there to 
the block 148 which performs the temperature control routine. 
First Alternative Embodiment--Variable Optimum Defrost Length 
Referring now to FIG. 5, there is illustrated a block diagram of a process 
which may be used in lieu of the blocks 164 and 166 shown in FIG. 4. The 
process shown in FIG. 5 is utilized to compare the actual defrost length 
against a variable optimum defrost length, designated ODL, as opposed to a 
fixed desired defrost time (DESDEF in the previous embodiment). The 
process shown in FIG. 5 utilizes two registers in the RAM 66, designated 
MINDT and MAXDT, representing the minimum desired defrost length and the 
maximum desired defrost length, respectively. The range between these two 
desired defrost lengths represents the range of possible values for the 
optimum defrost length ODL. 
The registers MINDT and MAXDT are initialized by the initialization block 
120, FIG. 3, immediately following energization of the system to 
predetermined desired values, such as 8 minutes and 20 minutes, 
respectively. 
Following the block 160, FIG. 4, a block 190 determines whether the freezer 
temperature was greater than 20.degree. F. during the previous defrost 
operation. This is performed by analyzing the contents of the register FT, 
FIG. 2A, which stores periodic readings of the freezer temperature during 
the defrost operation. If the freezer temperature was not above 20.degree. 
F., then the optimum defrost length ODL is incremented by adding a small 
value, such as 60 seconds, to the contents of the register ODL, which will 
tend to increase the length of subsequent defrost operations. 
If the block 190 determines that the freezer temperature was greater than 
20.degree. F., then a block 194 determines whether this temperature was 
exceeded for a time greater than 10 minutes. This is accomplished by 
analyzing the contents of the freezer temperature timer 196, FIG. 2A, 
which measures the length of time the freezer temperature exceeded 
20.degree. F. 
If it is determined that the freezer temperature exceeded 20.degree. F. for 
greater than 10 minutes, then the optimum defrost length ODL is 
decremented by subtracting from the contents of the register ODL a small 
amount such as 60 seconds. The decrementing of the optimum defrost length 
ODL in turn results in a tendency of a subsequent defrost length to become 
shorter, thereby limiting the rise of temperature within the freezer 
compartment 26. 
If it is determined by the block 194 that the freezer temperature exceeded 
20.degree. F. for less than 10 minutes, then no change is made to the 
existing optimum defrost length ODL, and, hence, the contents of the 
register ODL remain unaffected. 
Following the blocks 192, 198 or 200, is a decision block 202 which checks 
to determine whether the optimum defrost period ODP is within 
predetermined limits. This is accomplished by determining whether the 
contents of register ODL are greater than or equal to the contents of the 
MINDT register and less than or equal to the contents of the MAXDT 
register. It should be noted that the particular limits of eight minutes 
and 20 minutes for MINDT and MAXDT and the freezer temperature of 
20.degree. F. illustrated in this embodiment are exemplary only and other 
numbers may be substituted therefor. 
If the block 202 determines that the optimum defrost length ODL is not 
within the range between MINDT and MAXDT, then block 204 puts the optimum 
defrost period within this range by either increasing or decreasing the 
contents of the ODL register to MINDT or MAXDT. 
If it is determined that the optimum defrost length is within the range 
between MINDT and MAXDT, then control bypasses the block 204 and proceeds 
directly to a decision block 206. 
The decision block 206 checks the contents of the register ACTDEF and 
determines whether the value stored therein is between the values stored 
in the register ODL.+-.30 sec. The .+-.30 sec. defines a range of 
acceptable values surrounding the optimum defrost length ODL and is 
included to account for performance variations due to manufacturing 
tolerances, such as the tolerance for the bimetal sensor 42. If ACTDEF is 
within this range, then control passes directly to the block 162, FIG. 4. 
If the block 206 determines that the value ACTDEF is not within .+-.30 sec. 
of the optimum defrost length, then a block 208 recalculates the value 
stored in the W register depending upon the value of ACTDEF. If the value 
ACTDEF is greater than the value stored in the ODL register, then the 
value of W is incremented by the amount that ACTDEF exceeds ODL. If ACTDEF 
is less than the value stored in the ODL register, then the value W is 
decremented by the amount that ACTDEF is less than ODL. In this way, if 
the actual defrost length was less than the minimum optimum defrost length 
value, ODL-30 sec., the recalculated value of W will tend to increase the 
next interval between defrost operations, and, hence, the next defrost 
length will tend to be increased. Conversely, the value of W will be 
incremented, and hence, the next defrost length will tend to be decreased 
if the actual defrost length was greater than the maximum optimum defrost 
length value, ODL+30 sec. 
Following the block 208, the block 168 checks to determine whether W is 
between its minimum value MINW and its maximum value MAXW, as described in 
connection with FIG. 3. Control from the block 168 then proceeds to either 
block 162 or block 170 to continue the defrost control process. 
It can thus be seen that this embodiment of the invention comprises a 
control technique in which the actual defrost length tends toward an 
optimum defrost length which can vary between predetermined limits in 
response to the temperature conditions existing within the freezing 
compartment during defrost operation. In the embodiment illustrated, the 
optimum defrost length and hence the actual defrost length will tend 
toward a value which does not allow the temperature within the freezing 
compartment to rise above 20.degree. F. for more than 10 minutes. These 
temperature and time limits are employed to minimize the potential adverse 
effects of defrost operations on the food stored in the freezing 
compartment. Other temperature and time limits could be used, if desired. 
Second Alternative Embodiment--Humidity Measurement Technique 
Referring now to FIGS. 6-8, there is illustrated a humidity measuring 
technique which may be utilized to develop a measure of the ambient 
humidity and control a humidity responsive device, such as the mullion 
heater 54, shown in FIGS. 1 and 2B. The subject matter shown in FIG. 6 is 
inserted, as shown, between the blocks 122 and 124 shown in FIG. 3, while 
the subject matter shown in FIG. 7 is inserted between the blocks 158 and 
160 shown in FIG. 4, and the subject matter shown in FIG. 8 is inserted 
immediately following the block 172, FIG. 4. 
The humidity measurement technique utilizes the register HF located within 
the RAM 66, the contents of which represent a value referred to as the 
humidity factor which is proportional to the humidity to which the 
refrigerator 20 is exposed. 
It should be noted that, for this embodiment, the register HF and a door 
open counter 220 should both be initialized to zero by the block 120, FIG. 
3 at the beginning of the control program. 
Referring to FIG. 6, if the block 122 (FIG. 3) determines that the defrost 
heater 40 is not energized, then a decision block 222 analyzes the input 
IN1 of the microcomputer 58 to determine whether the freezer door 30 is 
open. If the door 30 is open, then a block 224 increments the door open 
counter 220 by an amount X, where: 
EQU X=16(1/2).sup.t-1 for 0&lt;t.ltoreq.5 
or 
EQU X=1 for t&gt;5 
If block 222 determines that the freezer door is not open, or following the 
calculation by the block 224, control passes to a block 226 which 
determines if the fresh food door 32 is open. If the door 32 is open, then 
the door open counter is incremented by a value Y, which is equal to 5. 
It should be noted that the variables X and Y may have values other than 
those shown above based upon the amount of moisture that is normally 
caused to enter the refrigerator 20 whenever the freezer door 30 or the 
fresh food door 32 is opened. 
Following the block 228, or if the block 226 determines that the fresh food 
door 32 is not open, control passes to the block 124 (FIG. 3) to continue 
the defrost control process. It should be noted that the door open counter 
220 is incremented as shown in FIG. 6 once for each second that the 
freezer door or fresh food door is open. 
Referring now to FIG. 7, if the block 158 determines that the drip timer 78 
has elapsed, signalling the end of a defrost operation, then the corrected 
defrost length is calculated as follows: 
EQU Corrected Defrost Length=ACTDEF+(FT)(10) 
A block 232 then calculates the humidity factor HF by dividing the 
corrected defrost length by the contents of the door open counter 220. 
This result is stored in the HF register in the RAM 66. 
To ensure that the number representing a measure of the ambient humidity is 
a whole number, it may be desirable to scale up the number representing 
the corrected defrost length before it is divided by the contents of the 
door open counter 220 to obtain the humidity factor HF. Alternatively, the 
reciprocal of the humidity factor can be calculated and stored in the HF 
register within RAM 66. 
Control from the block 232 then passes to block 160 to resume the defrost 
control process. 
Thus, the humidity factor HF is calculated only at the conclusion of a 
defrost operation and, since the corrected defrost length represents a 
measure of the amount of moisture which had accumulated on the evaporator 
during the last defrost interval and the contents of the door open counter 
represent a measure of the usage the refrigerator received during that 
interval, it can be appreciated that the above defined quotient represents 
a relative measure of the ambient humidity existing during the last 
defrost interval. 
Referring now to FIG. 8, following the block 172 (FIG. 4) a block 234 
compares the value stored in the register HF with a maximum humidity level 
stored in the register HMAX contained within the ROM 60. If the value of 
HF is greater than the value HMAX, then the mullion heater 54 is energized 
by generating a signal at the output D2 of the microcomputer 58 to warm 
the mullion area of the cabinet and thereby reduce condensation thereon. 
The proper value for HMAX is best determined experimentally, and will vary 
depending on the type and size of the refrigeration apparatus involved. By 
way of example, in the illustrated embodiment HMAX may have a value of 33 
where the number representing the corrected defrost length is multiplied 
by 100 (for scaling) before calculating the humidity factor HF in block 
232. 
Due to moisture leakage paths typically associated with the cabinet 
construction and door seals of a refrigerator, frost will gradually 
accumulate on the evaporator during periods when the refrigerator doors 
are being opened infrequently or not at all. Under such usage conditions 
the humidity factor HF calculated by block 232 will tend to be very large, 
regardless of the ambient humidity, because the contents of the door open 
counter will be extremely small. An erroneous indication of high ambient 
humidity can be prevented under such conditions by incorporating means for 
checking the contents of the door open counter 220 for some predetermined 
minimum amount of door opening time, and disregarding or disabling the 
humidity factor calculation of block 232 if the predetermined minimum time 
has not been accumulated. 
It should be understood that other types of apparatus may be controlled by 
the above described humidity measuring technique, such as visual 
indicators, alarms or the like.