Note: Descriptions are shown in the official language in which they were submitted.
CA 02149564 2004-12-07
"FITZZY LOGIC ADAPTIVE DEFROST CONTROL"
BACKGROUND OF THE INVENTION
The present invention generally relates to refrigeration
devices. More particularly, the present invention relates to
defrost cycle controllers for refrigerators and freezers.
As is known, refrigerator and freezer systems, especially
of the home appliance type, provide cooled air to an enclosure
in which food and the like can be stored, thereby to prolong
the edible life of the food. The enclosures, namely
refrigerators and freezers, are cooled by air blown over heat
exchangers, the heat exchangers extracting heat from the air
thereby producing cooled air. The heat exchangers generally
operate on the known cooling effect provided by gas that is
expanded in a closed circuit, i.e., the refrigeration cycle.
However, to be expanded, the gas must also be compressed and
this is accomplished by the use of a compressor.
The efficiency of the systems can be enhanced by reducing
the amount of frost that builds up on the heat exchanger, as is
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known. Modern systems are generally of the self-defrosting
type. To this end, they employ a heater specially positioned
and controlled to slightly heat the enclosure to cause melting
of frost build-up on the heat exchanger. These defrost heaters
are controlled pursuant to defrost cycle algorithms and
conf igurations .
As a result, these freezers-refrigerators undergo two
general cycles or modes, a cooling cycle or mode and a defrost
cycle or mode. During the cooling cycle, a compressor is
connected to a line voltage and the compressor is cycled on and
off by means of a thermostat, i.e., the compressor is actually
run only when the enclosure becomes sufficiently warm. During
the defrost cycle, the compressor is disconnected from the line
voltage and instead, a defrost heater is connected to the line
voltage. The defrost heater is turned off by means of a
temperature sensitive switch, after the frost has been melted
away.
Generally, there are three known ways or techniques for
controlling the operation of such a compressor and such a
defrost heater with what is referred to herein as a defrost
cycle controller. These three ways are referred to herein as
real or straight time, cumulative time, and variable time.
The real time technique involves monitoring the connection
of the system to line voltage. The interval between defrosts
is then based on a fixed interval of real time.
The cumulative time method involves monitoring of the
cumulative time a compressor is run during a cooling interval.
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The interval between defrost cycles is then varied based on the
cumulative time the compressor is run.
The variable time method is the most recently adopted
method and involves allowing for variable intervals between
defrost cycles by monitoring both cumulative compressor run
time as well as continuous compressor run time, and defrost
length. The interval between defrost cycles then~is based more
closely on the need for defrosting.
As is known, during a defrost cycle there is also dripping
of melted frost to a drip pan from which the melted frost
evaporates. This is known as the drip mode or cycle and those
terms are used herein.
Among others, the United States government has
continuously enacted more and more stringent laws and
regulations relating to the efficiency of refrigerators and
freezers, particularly as home appliances. As a result, much
research has been directed to more effective control over the
refrigeration cycles of refrigerators and freezers and,
particularly, to the defrost cycle, since in this cycle, the
effect of refrigeration is, on the one hand, counteracted by
removing cold from the enclosure, and on the other hand,
enhanced by increasing the efficiency of refrigeration by
removing insulating frost.
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Patents directed to defrost controllers include:
U.S. Pat. No. 4,156,350 Refrigeration Apparatus
Demand Defrost Control
System and Method
U.S. Pat. No. 4,411,139 Defrost Control System and
Display Panel
U.S. Pat. No. 4,850,204 Adaptive Defrost System
with Ambient Condition
Change Detector
U.S. Pat. No. 4,884,414 Adaptive Defrost System
U.S. Pat. No. 4,251,988 Defrosting System Using
Actual Defrosting Time as a
Controlling Parameter
SUMMARY OF THE INVENTION
The present application provides one or more inventions
directed to improvements in refrigeration/freezer defrost cycle
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controllers. These improvements can be provided in a single
all-encompassing unit or practiced separately.
According to the present invention, there is provided a
defrost cycle controller for modifying the total length of time
a compressor operates before a defrost cycle is initiated,
wherein the time the compressor is on between defrost cycles is
referred to herein as a frost accumulation period: The defrost
cycle controller includes a relay operatively connected to
mutually exclusively couple the compressor and a defrost heater
to a power supply. A first signal line provides a first signal
indicative of the operating status of the compressor while a
second signal line provides a second signal indicative of the
defrost heater. A microprocessor is operatively coupled to the
first and second signal lines and to the relay to control
energization of the relay and to selectively couple the
compressor and said defrost heater to the power supply. The
microprocessor includes means for deenergizing the compressor
and coupling the defrost heater to the power supply when a
continuous compressor run time exceeds a predetermined demand
defrost time. The microprocessor further includes means for
determining the time required to actually defrost the
evaporator during a defrost operation, referred to herein as a
defrost time, and means for modifying the frost accumulation
period in response to the time to complete the defrost
operation.
The present invention further provides a method for
determining the time required to actually defrost the
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evaporator during a first defrost operation, referred to herein
as a first defrost time and for determining the time required
to actually defrost the unit during a second defrost operation
wherein the second defrost operation is immediately subsequent
to the first defrost operation and referred to herein as a
second defrost time. An inference is made as to whether the
frost accumulating period should be modified before initiating
the next defrost operation in response to the first defrost
time and the second defrost time and the frost accumulating
period is modified if required.
The present invention further includes a fuzzy control
which performs fuzzy logic based inference functions on the
basis of signals representative of the defrost times as
described above such that the optimum frost accumulation period
is efficiently achieved. In particular, membership functions
according to fuzzy theory are defined for the current defrost
time and previous defrost time when defrost is initiated as a
result of the cumulative compressor run time reaching a target
compressor run time or frost accumulation time. Rules are
defined for the linguistic values resultant from mapping the
defrost times against the membership functions. Each rule is
executed using the fuzzy theory to thereby achieve an optimum "
frost accumulation time. Further, membership functions
according to fuzzy theory are defined for the defrost time when
defrost is initiated as a result of an excessively long
continuous compressor run time. Rules are defined for the
linguistic values resultant from mapping the defrost time
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against the membership functions. Each rule is executed using
the fuzzy theory to thereby achieve an optimum frost
accumulation time. These and other features of they
inventions) will become clearer with reference to the
following detailed description of the presently preferred
embodiments and accompanied drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a circuit diagram of a generic adaptive
defrost controller embodying principles of the invention(s).
FIG. 2 illustrates a schematic of a defrost controller
circuit embodying principles of the invention(s).
FIG. 3 is a flow chart of an algorithm employed in the
circuit of FIG. 2.
FIG. 4 is a flow chart of another algorithm employed in
the circuit of FIG. 2.
FIG. 5 illustrates the input membership functions for the
current defrost time and the previous defrost time.
FIG. 6 illustrates the fuzzy logic rule base for current
defrost time linguistic values and the previous defrost times
linguistic values as derived from FIG. 5.
FIG. 7 illustrates the output membership functions for
determining the change in the target compressor run time or
frost accumulating period based on the output linguistic values
from FIG. 6.
PA-7217-0-RE-USA
Patent
FIG. 8 illustrates the input membership functions for the
defrost time when defrost is initiated in response to an
excessively long continuous compressor run.
FIG. 9. illustrates the fuzzy logic rule base for the
defrost time input linguistic values as derived from FIG. 8.
FIG. 10. illustrates the output membership functions for
determining the change in the target compressor run time or
frost accumulating period based on the output linguistic values
from FIG. 9.
FIG. il. shows a lookup table reflecting output values
based on the fuzzy logic illustrated in FIGURES 8-10.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
As discussed above, there is provided a defrost controller
including one or more features that, among other things, are
particularly useful in increasing the efficiency of a
refrigerator/freezer by controlling the defrost cycle and the
frost accumulation period.
In FIG. 1 there is illustrated a defrost cycle controller
including a defrost timer module 12 that can embody
principles of the invention. As illustrated, coupled between
110 volt alternating current power lines L1 and N is the
defrost timer module 12, a defrost heater 14, and a compressor
16. The power line L1 is connected to the defrost timer module
via a connection P3 and the power line N is connected to the
defrost timer module 12 by means of a connection P6.
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The defrost heater 14 is connected between the power line
N and the defrost timer module 12 by means of a connection P5.
Additionally, the defrost heater 14 is connected to a
connection P2 via a bi-metal temperature sensitive switch T2.
Similarly, the compressor 16 is connected between the
power line N and a connection P1 of the defrost timer module
12. Additionally, the compressor 16 is connected'to a
connection P4 of the defrost timer module 12 by means of a
thermostat switch T1.
The defrost timer module 12, as will be explained below,
preferably includes a microprocessor or application specific
integrated circuit (a/k/a ASIC) or microcontroller, with inputs
and outputs connected to, among others, the compressor 16 the
defrost heater 14, the bi-metal temperature switch T2 and
thermostat T1.
As also will be described more fully below, the defrost
timer module 12 preferably is provided as a plug-in module that
can be connected to the compressor 16 and defrost heater 14
simply by plug-in connections. Thus, all components relating
to the defrost timer module 12 would be located in the plug-in
module except for the compressor 16, defrost heater 14 and
associated thermostat switch T1 and bi-metal switch T2. w
In FIG. 2 there is illustrated a schematic of a circuit
implementable as the defrost timer module 12. The module 12 is
illustrated in position for interconnecting with the defrost
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heater 14 and compressor 16 via plugs or connectors J1 and J2
formed by the individual connections P1 through P4 and P5
through P6, respectively.
As illustrated, the defrost timer module 12 can comprise a
micro-controller or microprocessor or ASIC 20 operatively
interconnected with various circuit elements to effect the
operation demand for such a module. Preferably, the
microprocessor 20 comprises a programmable integrated circuit
sold under the designation PIC16C54-RC/P by Microchip
Corporation. However, any economical microcontroller with
sufficient memory will do.
The embodiment illustrated in FIG. 2, is depicted in a
cooling mode for a freezer, i.e. wherein the circuit is not in
a defrosting cycle and the compressor 16 is allowed to run. To
this end, a control relay K1 is set accordingly with its
normally closed contact NC closed so as to supply power from L1
to the compressor 16 via connections P4 and P3 while is
normally open contact NO is open to prevent operation of the
defrost heater 14.
In operation, the microprocessor 20 senses signals
presented to it via connections P1 and P5 which inform the
microprocessor 20 about the actual running of the compressor 16
and the actual operation of defrost heater 14. The
microprocessor can then determine the cumulative and continuous
run times of the compressor and defrost heater on time, thereby
to determine how to alter the operation of those devices to
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obtain maximum efficiency and performance from the system
associated therewith.
As is known, the thermostat switch T1 will cycle the
compressor 16 on and off during the cooling period to maintain
desired temperature. Similarly, the bi-metal switch T2 will
turn the defrost heater 14 off upon completion of defrost. In
this regard, a defrost interval preferably is set~to be about
21 minutes, and the bimetal switch T2 opens at a predetermined
temperature to end the heater on time remaining in the drip
period. The bimetal switch T2 is not closed until the
compressor has been run for a duration sufficient to cool the
heater coils to a predetermined degree. However, the
microprocessor 20 controls when the compressor 16 and the
defrost heater 14 can operate, by switching between cooling and
defrost cycles.
In FIG. 2, power from the power line L1 is supplied to
connection P3 from which it is then directed to a power supply
circuit 22. Connection P4 is connected to the thermostat
switch T1 associated with the compressor 16.
Power supply 22 essentially comprises two power supplies:
a logic power supply 24 made up of resistor R3, zener diode
CR3, and capacitors C1, C3 and C4; and a relay power supply 26
which comprises resistors R1 and R5 and capacitor C2. As
illustrated, resistor R2, diode CR2, and diode CR1 are common
to both the logic power supply 24 and the relay power supply
26. Resistor R2 is a high impedance resistor having a
resistance on the order of 20 K ohms while resistors R1 and R.5
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preferably have resistances of 820 ohms. Resistor R3 is
preferably valued at about 39 K ohms.
The logic power supply 24 generates an operating voltage
VCC approximately equal to 5 volts which enables the
microprocessor to start running. Meanwhile, capacitor C2 or
relay power supply 26 charges to a value significantly higher
than rated voltage. In the presently preferred embodiment, a
charge of 55-60 volts was determined to be adequate. Resistors
R2, and the impedance from logic power supply act as a voltage
divider to limit the voltage on capacitor C2.
The relay power supply 26 provides a low-cost, low-energy
usage power supply. This power supply allows the
microcontroller 20, which typically requires a 5 volt power
supply, to drive the relay K1, which typically requires 12-48
volts, while maintaining low energy consumption.
In the embodiment illustrated in FIG. 2, diode CR2
rectifies the 110 volt alternating current line provided from
line L1 thereby to provide rectified current for the 5 volt
power supply while maintaining a charge on capacitor C2, while
the relay K1 is off. Resistor R2 and the 5 volt side of the
power supply circuit 26 create a voltage divider for proper
voltage level to the capacitor C2. Diode CR1 rectifies the 110
volt alternating current supply voltage after the relay K1
energizes and provides additional current to the relay K1. The
resistors R5 and R1 limit the current through the coil of the
relay K1 while it is energized.
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When the microprocessor 20 energizes the relay K1 by
turning on a transistor Q1 connected thereto, the relay K1 is
initially energized by a voltage across the capacitor C2. The
defrost heater 14 is connected to the normally open contact NO
of the relay 14, as illustrated. Thus, when the microprocessor
20 turns on the transistor Q1 and activates the relay K1, the
relay K1 changes state to connect its common terminal with its
normally open contact NO, thereby connecting line L1 with
connection P2 thereby to energize the defrost heater 14.
Connection line P2 is also connected to the power supply
intermediate resistors R2 and R5 through rectifier CR1.
Once relay K1 changes state to connect its common terminal
with its normally open contact NO, the alternating current line
voltage from L1 is fed to the defrost heater 14 via connection
P2 and the compressor 16 is disconnected from the line voltage
L1. The line voltage is also applied to diode CR1, thereby
bypassing the high impedance resistor R2 and energizing relay
K1 thereafter through lower impedance resistors R1 and R5.
Because the voltage required to maintain the relay K1 in
position is less than the voltage required to effect a change
of state in the relay K1, this arrangement is appropriate and
utilizes the known property of a relay to advantage. That is,
the relay power supply 26 comprising resistors R1 and R5 and
capacitor C2 is only engaged and, therefore, only dissipates
power when the relay K1 is actuated. The relay power supply 26
provides a voltage that is less than the voltage required to
actuate the relay K1.
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Thus, the high impedance circuit including resistor R2 is
employed during initial activation of the power relay K1, but
the current flow through the power relay K1 is used to employ a
lesser impedance circuit portion or segment for holding the
relay K1 in its closed position.
The microprocessor 20 is provided with two inputs via the
connections P1 and P5, as also is illustrated in FIG. 2.
Information regarding the compressor 16 is provided via the
connection P1 while information about the defrost heater 14 is
provided via connection P5.
The compressor 16 is monitored at connection P1 by means
of the low pass filter comprising the resistor R6 and the
capacitor C7 whenever the compressor is running. As should be
apparent, the input will toggle whenever the compressor is
running and not toggle whenever it is not running.
However, a possible failure mode for a defrost timing
device, based on compressor run time, is to lose the compressor
monitoring signal. If the signal is lost, for example due to a
broken wire, loose connection, etc., the refrigerator may never
be placed in a defrost mode. This could result in food loss,
customer dissatisfaction, and a service call.
The generation of a default mode is provided for such a "
failure. In this regard, the feature provides a default mode
in which a lost compressor signal is ignored and the assumption
is made that the compressor is operating 100% of the time K1 is
not energized. This assumption results in no lost refrigerator
performance, except for an increase in energy consumption.
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This default mode could also be service selectable for a back-
up mode for worse case conditions, such as extremely high
humidity areas.
To this end, voltage at connection P1 must be provided to
indicate that the compressor is on. This can be accomplished,
as illustrated by providing a pull-up resistor R19 coupled to
tie the connection P1 to the line connecting the inormally
closed contact NC of the relay K1 to the connection P4. If the
signal from the compressor is blocked from reaching the
microprocessor 20 via connection P2, i.e., connection becomes
broken, the pull-up resistor R19 will provide a voltage to the
microprocessor 20. If the compressor signal is provided, the
impedance of the compressor 16 will cancel out the effects of
the resistor R19.
It should be noted that the resistor R19 preferably is
provided on the module 12 and thus can be considered internal
to the defrost timing module 12, even though in reality, it
could be a resistor simply mounted on a circuit board. In any
event, the resistor R19 most preferably is connected internally
to the module 12, else the signal provided by the resistor R19
could also be lost if the connection P1 is broken.
The microprocessor 20 preferably includes an internal "
watch dog and an internal power on reset circuitry. There is
no need to signal condition the lines that monitor the
alternating current signal supply to the compressor 16 and the
power supplies 24 and 26 because the microprocessor 20
preferably includes a Schmitt trigger input with a built-in
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hysteresis on the line connected to the connection P1. Line
monitoring of the defrost heater 14 is treated as a direct
current (DC) signal by the inclusion of a capacitor C5 which
directs all alternating current signals on that line to
ground.
In Figure 2, the microprocessor 20 includes an input
labeled "RTCC" which is an acronym for real time clock counter.
It can be appreciated that when the compressor 16 is allowed to
run, 60 Hz signals will be provided to the microprocessor 20
via connection P1. In this state, the microprocessor 20 can
maintain track of real time and react accordingly.
Should the compressor be turned off, however, the 60 Hz
timing signal will be lost, for example, during defrost and
dripping.
Although initially it was considered necessary to monitor
the alternating current at this portion, by providing 60 Hz
timing information of the microprocessor 20 during defrosting
and dripping, this requirement has been eliminated by
performing an internal timing calibration via computer
programming of the microprocessor 20. The microprocessor 20
thus detects failure of the relay K1 if 60 Hz information
appears while the control circuit is in a defrost or drip
mode.
One feature of the inventions) is a particular way to
determine the need for a refrigerator or freezer to defrost
based upon the length of time the compressor 16 runs
continuously, described herein as the continuous compressor run
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time CCT. A maximum continuous compressor run time MCCT which
would trigger a defrost cycle, referred to as demand defrost
time DDT, can be variable based on the cumulative run time CT
of the compressor 16.
To this end, the microprocessor 20 can be configured to
include an algorithm to monitor when an extended run period
after a default compressor run period has been reached. This
information can be applied to utilize the algorithm to perform
a demand defrost routine.
Essentially, this routine would initiate a defrost cycle
when an extended continuous compressor run period CCT is
encountered which exceeds the demand defrost time value DDT.
The demand defrost time DDT would have no initial target such
as for example an initial default target of 10 hours. Instead,
demand defrost time value DDT would be set based on the
cumulative compressor run time CT. For example, if the
cumulative compressor run time CT is 10 hours, then a
continuous run time of 2 hours would trigger a defrost. As the
cumulative run time increases, the continuous run time that
would trigger a defrost cycle, the demand defrost time DDT,
would decrease. An example is shown in the following table:
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Table 1
Cumulative Compressor Continuous Run Period
Run Time i(CT~ For Trigctering Defrost Cycle ~DDT~,
0 - 10 hours Not Applicable
- 15 hours 2 hours
- 20 hours 1.5 hours
or more hours 1 hour
While this algorithm presents the risk of an increase in
the chance that frost will build up on an evaporator coil
because the initial cumulative and continuous compressor run
periods would be long, it should also be more energy efficient
because initially there generally is little frost build-up.
In a modified version of this concept, a cumulative run
time of 8 hours will set a continuous run period of 1 hour for
triggering a defrost cycle.
It is possible to configure the defrost timer module 12 as
a fixed time cumulative run timer by removing or disconnecting
the contact P5 by means of which the defrost heater 14 and bi-
metal switch T2 are monitored. In this regard, generally in "
order for the timer 12 to perform properly it must receive
input signals from the compressor 16 and the defrost heater 14.
Monitoring of the signal from the defrost heater provided by a
contact P5 informs the microprocessor 20 how long the bi-metal
switch T2 took to open once a defrost cycle had been started.
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This information then is used to predict the next run period of
the compressor 16.
If upon entering the defrost mode the microprocessor 20
does not detect that the bi-metal switch T2 is closed and then
opened, the length of the defrost period will not be available
to calculate the next run period of the compressor 16. The
microprocessor 20 will then have to revert back to a default
run setting. Therefore, to keep the microprocessor 20 at the
default run time period of the compressor 16, the feedback
provided via contact P5 from the defrost 12 should be
disconnected. This will cause the defrost time module 12 to
perform as a fixed time cumulative run timer.
As is known, certain areas of the country are prone to
frequent power outages. This can result in a malfunction of
certain types of electronic controls. Therefore, many will
include a device to maintain the memory of the controller such
as a battery or super-capacitor. If the present control system
is subjected to a series of outages, a potential frost build-up
could occur in the freezer and/or refrigerator associated
therewith.
To this end, the sensitivity of the defrost timer 12 to
frequent power outages can be reduced by modifying the power up '-
algorithm of the microprocessor. The power up routine can be
modified so that if the microprocessor 20 powers up to find the
unit is cold and the thermostat switch T1 is open, the
microprocessor 20 can perform an initial modified defrost
routine. However, if the microprocessor 20 powers up to find a
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unit with a closed thermostat switch T1, the initial compressor
run period will be reduced.
As illustrated in FIG. 3, when the controller 20 powers
up, it monitors the status of the feedback signals from the
refrigerator/freezer at P1 and P5 to determine the status of
the unit. If the refrigerator/freezer can be determined to be
cold, i.e., the bi-metal T2 is closed, and the thermostat T1 is
not calling for cold, i.e. the thermostat T1 is open, then the
controller 20 will perform a modified defrost cycle. This
modified defrost cycle will not include a drip period as
skipping such a drip period will minimize the time until the
compressor 16 begins to run. After this modified defrost
cycle, the next target compressor build time will be set to a
default value, for example such as ten hours.
However, if the unit powers up to see that the unit is
calling for cold, i.e., thermostat T1 is closed, then an
initial defrost will not occur, thus insuring that when a
customer first plugs in the unit, the compressor will run to
show that the unit is functioning, but the target compressor
build time will be set to a lower value, such as six hours.
The foregoing reduces the time window of a power outage
that could disrupt the performance of the controller. The w
value of this reduce build time is a function of expected
frequency of the power outages and the "pull down" performance
specification of the refrigerator. If the initial compressor
build time is too short, the time to cool a warm refrigerator
will be extended because a defrost will occur to soon.
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In FIG. 4 there is illustrated a flow chart of logic that
can be programmed into the microprocessor 20 to effect the
normal operation of the defrost timer 12. As illustrated,
after the microprocessor 20 has undergone an initialization
procedure, for example setting variables, etc., in a first step
100, a determination is made as to whether or not the
compressor is on in a step 102. At this juncture the
microprocessor senses whether or not a signal is present at
connection P1. If the determination is positive, i.e., the
answer is yes, then the continuous compressor run time CCT is
counted and accumulated in a step 104. If the answer is no,
then the microprocessor remains in a loop, i.e. it returns to
step 102, until such time as the compressor is turned on by the
switch T1. As illustrated by block 106, simultaneous with the
counting of the continuous compressor run time, the present
compressor run time is summed with all subsequent compressor
run times since the previous defrost period to determine a
cumulative compressor run time value CT. In a step 108, the
cumulative run time CT of the compressor 16 is compared to a
target cumulative compressor run time value TCT which may be
able to return to as a frost accumulating period. The target
cumulative compressor run time is initialized from ROM to RAM
in step 100 and is preferably contemplated to be initialized as
hours.
If the cumulative run time CT of the compressor is equal
to or greater than the target cumulative compressor run time
value TCT, then the microprocessor enters into a defrost mode
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2149~~~.
as indicated in FIG 4a. To initiate the defrost mode, the
compressor 16 is de-energized and the defrost heater is
energized as shown in steps 110 and 112 respectively. As
indicated by block 114, the defrost time DT, which is defined
as the time the defrost heater is energized, is counted until
an end of the defrost period is reached, as determined by the
opening of bi-metal temperature sensitive switch fit. The
measured defrost time DT is then stored to RAM, as indicated in
block 116. Subsequently, as shown in block 118, the previous
measured defrost time PDT and the currently measured defrost
time DT are supplied as inputs to a fuzzy logic control system,
incorporated in the microprocessor 20 and described herein as a
fuzzy logic control 20a, for modifying the target compressor
run time TCT.
As indicated by block 120, a drip time follows the defrost
time during which the melted frost is allowed to drip off the
heat exchanger. Thereafter, as indicated by block 122, the
relay K1 is de-energized and then the microprocessor returns to
step 102.
One feature of the invention(s), therefore, is the control
system for optimizing the target compressor run time value
TCT and in particular fuzzy logic control 20a. The fuzzy logic
control 20a receives as inputs the current defrost time DT and
the previous defrost time PDT and compares these inputs to an
optimum defrost time ODT. The timer module 12 defines the
optimum defrost time ODT, which is correlated with an
optimumization operation of the refrigerator wherein the energy
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efficiency of the refrigerator operation is optimized. It can
be understood that the optimum defrost time varies with,
depends upon the size of the defrost heater.
In the preferred embodiment, if the timer defrost module
is associated with a refrigerator having a 600 watt defrost
heater, the optimum time is 13 minutes, while for a
refrigerator having a 400 watt defrost heater, the optimum
defrost time is 17 minutes. Generally, measured defrost times
DT and PDT that are shorter than the optimum defrost time ODT
indicate that only a small amount of frost has accumulated on
the evaporator coils. In this case, therefore, the fuzzy
control 20a would lengthen the subsequent target compressor run
time value TCT. In a like manner, defrost times DT and PDT
that are longer than the optimum defrost time ODT indicate
excessive frost on the evaporator coils and, therefore, the
fuzzy logic control 20a would decrease the subsequent target
compressor run time value TCT.
The fuzzy control 20a, therefore , can be understood to
receive input values representing the current defrost time DT
and the previous defrost time PDT while the output of the fuzzy
control 20a is a signal representative of a change in the
target compressor run time TCT. w
The fuzzy control 20a executes three fuzzy logic stages:
(1) fuzzification, (2) rule application and (3)
defuzzification, according to the mathematics of fuzzy theory.
In the fuzzification stage, the system inputs, the current
defrost time DT and the previous defrost time PDT, are
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manipulated and mapped to linguistic values or fuzzy inputs
through predetermined membership functions. FIG. 5 shows the
set of membership functions for the input values of current
defrost time DT and the previous defrost time PDT, whereby the
same membership functions can be used for both input values.
In FIG S, the ordinate represents the degree of membership and
the abscissa represents the defrost time DT, both.current DT
and previous PDT, in 0.5 minute increments. The trapezoidally
shaped membership functions for NXB (negative extra big), NB
(negative big) and PB (positive big) and the triangularly
shaped functions NS (negative small), Z (zero) and PS (positive
small) map the range of defrost times to degrees of membership
in the fuzzy functions based on an experts knowledge of defrost
functioning. In this fashion, the defrost operation may be
controlled in an optimum fashion in accordance with the expert
knowledge as represented in the fuzzy system.
In general, in the rule application stage, logic rules are
applied to the set of linguistic values or input membership
values resultant from mapping the current defrost time DT and
previous defrost time PDT to the input membership functions.
From this application of the logic rules, a set of linguistic
output values or conclusions are derived. FIG. 6 illustrates
the fuzzy logic rule base applied to the input membership
values for determining conclusions or fuzzy outputs in the
present invention. By use of these fuzzy logic rules, an
inference may be made regarding the fuzzy input values. The
construction of the fuzzy logic rule base represents an experts
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knowledge of defrost operation based on the length of the
current and previous defrost times.
The rule application stage includes two separate -
operations: (1) rule evaluation and (2) rule aggregation.
In the rule evaluation operation, the degree to which each
rule is fired is controlled using a max-min Inference method.
In this manner, the degree of membership of the conclusions or
fuzzy output values resultant from the rules fired is equal to
the minimum degree of membership for the fuzzy input values.
In the rule aggregation operation, the set of fuzzy output
values, representing degrees of membership in the output
membership functions, are aggregated. Specifically, for each
output membership function, the rule fired with the maximum
degree of membership or maximum rule strength controls the
degree of membership.
In the defuzzification stage, the aggregated fuzzy output
values are applied to a set of output membership functions,
illustrated in FIG. 7, for determining the output of the
fuzzy controller 20a for controlling the amount of change in
the target compressor run time TCT. In the preferred
embodiment, a center of gravity method is used in the
defuzzification stage. ,_
Two sample cases are described below to demonstrate the
control system.
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2~4~~~~
Case 1
Initial Conditions:
Current defrost 9 minutes
time DT:
Previous defrost time PDT: 10.5 minutes
Fuzzification: (See Fig. 5)
Current defrost
time DT:
1.0 NB
Previous defrost time PDT:
0.3 NB; 0.3 NS
Rule Applications: (See Fig. 6)
Rule evaluation: If current defrost time is NB and previous
defrost time
is NB then
change in
compressor run time is PB (0.3 PB).
If current defrost time is NB and previous
defrost time
is NS then
change in
compressor run time is (0.3 PB).
Rule aggregation: Linguistic output membership values) -
0.3 PB
Defuzzification: (See Fig. 7)
Since the linguistic output values) is 0.3 PB, change in
target compressor run time TCT is +11 hours (utilizing the
center of gravity method).
In this case, it can be seen that both the current defrost
time (9 min.) and the previous defrost time (10.5 min.) are
much less than the optimum defrost time (13 min). From this
information, it can generally be assumed that only a small
amount of frost accumulated on the evaporator coils and the
defrost cycle was initiated prematurely. It will be desirable,
therefore, to increase the subsequent target compressor run
time TCT. Applying the fuzzy control to these inputs results
in a change in the target compressor run time of 11 hours.
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Case 2:
Initial Conditions:
Current defrost time DT: 13.5 min.
Previous defrost time PDT: 14.5 min.
Fuzzification: (See Fig. 5)
Current defrost time DT: 0.5 z, 0.5 PS
Previous defrost time PDT: 0.5 PS, 0.5 PB
Rule Application: (See Fig. 6)
Rule Evaluation: Set of linguistic output values or
conclusion: 0.5 Z, 0.5 NS, 0.5 Z and 0.5
NS
Rule Aggregation: Linguistic output values: 0.5 Z and 0.5 NS
Defuzzification: (See Fig. 7)
Since the linguistic output values are 0.5 Z and 0.5 NS,
change in target compressor run time TCT is -2.5 hours
(utilizing the center of gravity method).
In this case, it can be seen that both the current defrost
time (13.5 min.) and the previous defrost time (14.5 min.) are
greater than the optimum defrost time (13 min). From this
information, it can generally be assumed that an excessive
amount of frost accumulated on the evaporator coils and the
defrost cycle was not initiated soon enough. It will be
desirable, therefore, to decrease the subsequent target
compressor run time TCT. Applying the fuzzy control to these w
inputs results in a change in the target compressor run time of
-2.5 hours.
Referring now back to Fig. 4, and 4B, if given the
cumulative run time CT of the compressor 16 is less than the
target cumulative compressor run time TCT, as compared in step
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108, the microprocessor determines whether the cumulative run
time CT of the compressor has exceeded a minimum cumulative run
time MINCT, such as 8 hours, as shown in step 130. The minimum
cumulative run time MINCT is initialized to RAM from ROM in
block 100. If the cumulative run time CT has not exceeded the
minimum cumulative run time MINCT, then the microprocessor
loops back to step 102. However, if CT exceeds MINCT, then the
microprocessor 20 determines whether the continuous compressor
run time CCT is equal to or has exceeded a maximum continuous
run time value or demand defrost time DDT as shown in step 132.
The demand defrost time value DDT is set from ROM and may
preferably be equal to 1 hour. However, the demand defrost
time DDT may be variable based on the above described
method illustrated in table 1. If the continuous compressor
run time CCT has not exceeded the demand defrost time DDT, the
microprocessor 20 loops back to step 102. However, if the
continuous compressor run time CCT is equal to or exceeds the
demand defrost time DDT, then the microprocessor 20 initiates a
defrost cycle.
To initiate the defrost mode, the compressor 16 is
de-energized and the defrost heater is energized as shown in
steps 134 and 136 respectively. As indicated by block 138, the
defrost time DT, which is defined as the time the defrost
heater is energized, is counted until an end of the defrost
period is reached, as determined by the opening of bi-metal
temperature sensitive switch T2. Subsequently, as shown in
block 140, the currently measured defrost time DT is supplied
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as an input to a fuzzy logic control system, incorporated in
the microprocessor 20 and described herein as a fuzzy logic
control 20b, for modifying the target compressor run time TCT.
As indicated by block 142, a drip time follows the defrost
time during which the melted frost is allowed to drip off the
heat exchanger. Thereafter, as indicated by block 144, the
relay K1 is de-energized and then the microprocessor returns to
step 102.
One feature of the invention(s), therefore, is the control
system for optimizing the target compressor run time value TCT
and in particular fuzzy logic control 20b. The general
operation of the fuzzy control 20b may be described as follows.
The fuzzy logic control 20b receives as an input the
current defrost time DT and compares it to an optimum defrost
time ODT. The timer module 12 defines the optimum defrost time
ODT, which is correlated with an optimum operation of the
refrigerator wherein the energy efficiency of the refrigerator
operation is optimized. As described above, it can be
understood that the optimum defrost time is related to the size
of the defrost heater. In the preferred embodiment, if the
timer defrost module is associated with a refrigerator having a
600 watt defrost heater, the optimum time is 13 minutes, while
for a refrigerator having a 400 watt defrost heater, the
optimum defrost time is 17 minutes.
Several examples are illustrative of the logic used in
developing the fuzzy control 20b.
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1. Generally, if a continuous compressor run time
CCT which exceeds the demand defrost time is a result of
excessive frost build up on the evaporator coil, the
resulting defrost heater on time will be quite long. This
signals to the fuzzy control 20b that the refrigerator has
not adapted properly to the existing environment and
should reset the target compressor run time 'rCT to the
nominal value from ROM.
2. If a continuous compressor run time CCT which
exceeds the demand defrost time is a result of a moderate
frost build up on the evaporator coil, a long door
opening, or a heavy food load addition to the
refrigerator, the resulting defrost heater on time may be
in the nominal range approximately near the target defrost
time. In this case, the control may proceed to the next
target compressor run time TCT or build cycle with caution
such that the next be target compressor run time will be
reduced but not reset to nominal.
3. If a continuous compressor run time CCT which
exceeds the demand defrost time occurs but is not a result
of a large frost load but is rather the result of a long
door opening or a heavy food load addition to the w
refrigerator, the resulting defrost heater on time may be
quite short. This would signal that frost was not a
problem and therefore the fuzzy control 20b would not
reset or reduce the target compressor run time TCT. The
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_ ,n",~
control 20b would allow the next target compressor run
time TCT to equal the last.
The fuzzy control 20b executes three fuzzy logic stages:
fuzzification, rule application and defuzzification, according
to the fuzzy theory. In the fuzzification stage, the system
input, the current defrost time DT, is manipulated and mapped
to linguistic values or through fuzzy inputs, a set of
predetermined membership functions. FIG. 8 shows the
membership functions for the input value of current defrost
time DT. In FIG. 8, the ordinate represents the degree of
membership and the abscissa represents the defrost time DT,
in 0.5 minute increments. The trapezoidally shaped membership
functions for RESETS (reset small), NB (negative big) and
RESETB (reset big) and the triangularly shaped functions NS
(negative small), Z (zero) and PS (positive small) map the
range of defrost times to the degree of membership in the fuzzy
functions based on an experts knowledge of defrost functioning.
In this fashion, the defrost operation may be controlled in an
optimum fashion in accordance with the expert knowledge as
represented in the fuzzy system. In the rule application
stage, logic rules are applied to the set of linguistic values
or fuzzy inputs values resultant from mapping the current ,_
defrost time DT to the input membership functions. From this
application of the logic rules, a set of linguistic output
values or conclusions are derived. FIG. 9 illustrates the
fuzzy logic rule base applied to the input membership values
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for determining conclusions of fuzzy outputs in the present
invention. By use of these fuzzy logic rules, an inference may
be made regarding the fuzzy inputs. The construction of-the
input membership functions, the fuzzy logic rule base
represents an experts knowledge of defrost operation based on
the length of the current and previous defrost times.
The rule application stage includes two separate
operations: rule evaluation and rule aggregation. In the rule
evaluation operation, the degree to which each rule is fired is
controlled using a min-max Inference method. In this manner,
the degree of membership of the conclusions or fuzzy output
values resultant from the rules fired is equal to the minimum
degree of membership for the fuzzy input values. In the rule
aggregation operation, the set of output values, representing
degrees of membership in the output membership functions, are
aggregated. Specifically, for each output membership function,
the rule fired with the maximum degree of membership or maximum
rule strength controls the degree of membership.
In the defuzzification stage, the aggregated fuzzy output
values are applied to an output membership function,
illustrated in FIG. 10, for determining the fuzzy controller
20b output for controlling the amount of change in the target
compressor run time TCT. Specifically, a center of gravity
method is used in the defuzzification stage.
A sample case is described herein below to demonstrate the
control system implemented in the fuzzy logic 20b.
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Case 3:
Initial Conditions:
Target compressor run time TCT: 18 hours
Cumulative compressor run time CT: 16 hours
Min. cumulative comp. run time MINCT: 10 hours
Cumulative compressor run time CCT: > 1 hour
Demand defrost time DDT: 1 hour
Defrost time DT: 14.2 min.
Fuzzification: (See Fig. 8)
Defrost time DT: 0.2 Z, 0.8 PS
Rule Application: (See Fig. 9)
Rule Evaluation: Set of linguistic output values or
conclusion: 0.2 NS and 0.8 NB.
Rule Aggregation: Linguistic output values: 0.2 NS and 0.8 NB
Defuzzif ication: (See Fig. 10)
Since the linguistic output values are 0.2 NS and 0.8 NB,
change in target compressor run time TCT is -9 hours (utilizing
the center of gravity method).
The logic behind the fuzzy control 20b can simplified and
be reduced to a look-up table, as shown in Fig. il. FIG.
11 shows a look-up table for a refrigerator having a 13 min
defrost target (600 watt defrost heater) or a 17 min defrost
target (400 watt defrost heater). It can be understood,
therefore, that the implementation of the fuzzy control 20b may
be achieved by encoding the look-up table of FIG. 11 into the w
controller 20.
In this fashion therefore, a novel adaptive defrost system
for determining the frost build period or the target compressor
run time for a refrigerator is provided. More specifically, a
fuzzy control system utilizing the inputs of the defrost length
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and the previous defrost length is provided for determining the
optimum frost built period. Further, a control system
responsive to excessive continuous run times for the compressor
is provide wherein the frost build period is modified in
response to the defrost time.
Although the present invention has been described with
reference to a specific embodiment, those of skill in the Art
will recognize that changes may be made thereto without
departing from the scope and spirit of the invention as set
forth in the appended claims.
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