Information
-
Patent Grant
-
6606870
-
Patent Number
6,606,870
-
Date Filed
Friday, January 5, 200123 years ago
-
Date Issued
Tuesday, August 19, 200321 years ago
-
Inventors
-
Original Assignees
-
Examiners
Agents
- Houser, Esq.; H. Neil
- Armstrong Teasdale LLP
-
CPC
-
US Classifications
Field of Search
US
- 062 151
- 062 152
- 062 153
- 062 154
- 062 155
- 062 156
- 062 140
- 062 234
-
International Classifications
-
Abstract
A defrost control system for a self-defrosting refrigerator is configured to monitor a compressor load, determine whether at least a first defrost cycle is required based on the compressor load, execute at least one defrost cycle when required; and regulate the defrost cycle to conserve energy. A controller is operatively coupled to a compressor, a defrost heater, and a refrigeration compartment temperature sensor. The controller makes defrost decisions based on temperature conditions in the refrigeration compartment in light of other events, such as refrigerator door openings, completed defrost cycles, and power up events. Defrost cycles are automatically adjusted as operating conditions change, thereby avoiding unnecessary energy consumption that would otherwise occur in a fixed defrost cycle.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to refrigerators and, more particularly, a method and apparatus for controlling refrigeration defrost cycles.
Known frost free refrigerators include a refrigeration defrost system to limit frost buildup on evaporator coils. An electromechanical timer is used to energize a heater after a pre-determined run time of the refrigerator compressor to melt frost buildup on the evaporator coils. To prevent overheating of the freezer compartment during defrost operations when the heater is energized, in at least one type of defrost system the compartment is pre-chilled. After defrost, the compressor is typically run for a predetermined time to lower the evaporator temperature and prevent food spoilage in the refrigerator and/or fresh food compartments of a refrigeration appliance.
Such timer-based defrost systems, however are not as energy efficient as desired. For instance, they tend to operate regardless of whether ice or frost is initially present, and they often pre-chill the freezer compartment regardless of initial compartment temperature. In addition, the defrost heater is typically energized without temperature regulation, and the compressor typically runs after a defrost cycle regardless of the compartment temperature. Such open loop defrost control systems, and the accompanying inefficiencies are undesirable in light of increasing energy efficiency requirements.
While efforts have been made to provide defrost on demand systems employing limited feedback, such as door openings and compressor and evaporator conditions, for improved energy efficiency of defrost cycles, an adaptive defrost on-demand system is desired to alter defrost operation to conserve energy in light of refrigerator operating conditions.
BRIEF SUMMARY OF THE INVENTION
In an exemplary embodiment of the invention, a defrost control system for a self-defrosting refrigerator is configured to monitor compressor load, determine whether at least a first defrost cycle is required based on the compressor load, execute at least one defrost cycle when required; and regulate the defrost cycle to conserve energy.
More specifically a controller is provided for a refrigerator including a compressor, a defrost heater, at least one refrigeration compartment and a temperature sensor thermally coupled to the refrigeration compartment. The controller is operatively coupled to the compressor, the defrost heater, and the temperature sensor, and makes defrost decisions based on temperature conditions in the refrigeration compartment in light of other events, such as refrigerator door openings, completed defrost cycles, and power up events. Defrost cycles are automatically adjusted as operating conditions change, thereby avoiding unnecessary energy consumption that would otherwise occur in a fixed defrost cycle.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a perspective view of a refrigerator;
FIG. 2
is a block diagram of a refrigerator controller in accordance with one embodiment of the present invention;
FIGS. 3A-3C
is a block diagram of the main control board shown in
FIG. 2
;
FIG. 4
is a block diagram of the main control board shown in
FIG. 2
;
FIG. 5
is a defrost state diagram executable by a state machine of the controller shown in
FIG. 2
;
FIG. 6
is a sealed system/defrost system block diagram;
FIG. 7
is a defrost algorithm flow chart;
FIG. 8
is a state diagram for sensor based on-demand defrost; and
FIG. 9
is a state diagram for implicit defrost control.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1
illustrates a side-by-side refrigerator
100
in which the present invention may be practiced. It is recognized, however, that the benefits of the present invention apply to other types of refrigerators, freezers, and refrigeration appliances wherein frost free operation is desirable. Consequently, the description set forth herein is for illustrative purposes only and is not intended to limit the invention in any aspect.
Refrigerator
100
includes a fresh food storage compartment
102
and a freezer storage compartment
104
. Freezer compartment
104
and fresh food compartment
102
are arranged side-by-side.
Refrigerator
100
includes an outer case
106
and inner liners
108
and
110
. A space between case
106
and liners
108
and
110
, and between liners
108
and
110
, is filled with foamed-in-place insulation. Outer case
106
normally is formed by folding a sheet of a suitable material, such as pre-painted steel, into an inverted U-shape to form top and side walls of case. A bottom wall of case
106
normally is formed separately and attached to the case side walls and to a bottom frame that provides support for refrigerator
100
. Inner liners
108
and
110
are molded from a suitable plastic material to form freezer compartment
104
and fresh food compartment
102
, respectively. Alternatively, liners
108
,
110
may be formed by bending and welding a sheet of a suitable metal, such as steel. The illustrative embodiment includes two separate liners
108
,
110
as it is a relatively large capacity unit and separate liners add strength and are easier to maintain within manufacturing tolerances. In smaller refrigerators, a single liner is formed and a mullion spans between opposite sides of the liner to divide it into a freezer compartment and a fresh food compartment.
A breaker strip
112
extends between a case front flange and outer front edges of liners
108
,
110
. Breaker strip
112
is formed from a suitable resilient material, such as an extruded acrylo-butadiene-styrene based material (commonly referred to as ABS).
The insulation in the space between liners
108
,
110
is covered by another strip of suitable resilient material, which also commonly is referred to as a mullion
114
. Mullion
114
also preferably is formed of an extruded ABS material. Breaker strip
112
and mullion
114
form a front face, and extend completely around inner peripheral edges of case
106
and vertically between liners
108
,
110
. Mullion
114
, insulation between compartments
102
,
104
, and a spaced wall of liners
108
,
110
separating compartments
102
,
104
, sometimes are collectively referred to herein as a center mullion wall
116
.
Shelves
118
and slide-out drawers
120
normally are provided in fresh food compartment
102
to support items being stored therein. A bottom drawer or pan
122
partly forms a quick chill and thaw system (not shown) and selectively controlled, together with other refrigerator features, by a microprocessor (not shown in
FIG. 1
) according to user preference via manipulation of a control interface
124
mounted in an upper region of fresh food storage compartment
102
and coupled to the microprocessor. A shelf
126
and wire baskets
128
are also provided in freezer compartment
104
. In addition, an ice maker
130
may be provided in freezer compartment
104
.
A freezer door
132
and a fresh food door
134
close access openings to fresh food and freezer compartments
102
,
104
, respectively. Each door
132
,
134
is mounted by a top hinge
136
and a bottom hinge (not shown) to rotate about its outer vertical edge between an open position, as shown in
FIG. 1
, and a closed position (not shown) closing the associated storage compartment. Freezer door
132
includes a plurality of storage shelves
138
and a sealing gasket
140
, and fresh food door
134
also includes a plurality of storage shelves
142
and a sealing gasket
144
.
In accordance with known refrigerators, refrigerator
100
also includes a machinery compartment (not shown) that at least partially contains components for executing a known vapor compression cycle for cooling air. The components include a compressor (not shown in FIG.
1
), a condenser (not shown in FIG.
1
), an expansion device (not shown in FIG.
1
), and an evaporator (not shown in
FIG. 1
) connected in series and charged with a refrigerant. The evaporator is a type of heat exchanger which transfers heat from air passing over the evaporator to a refrigerant flowing through the evaporator, thereby causing the refrigerant to vaporize. The cooled air is used to refrigerate one or more refrigerator or freezer compartments via fans (not shown in FIG.
1
). Collectively, the vapor compression cycle components in a refrigeration circuit, associated fans, and associated compartments are referred to herein as a sealed system. The construction of the sealed system is well known and therefore not described in detail herein, and the sealed system is operable to force cold air through the refrigerator subject to the following control scheme.
FIG. 2
illustrates a controller
160
in accordance with one embodiment of the present invention. Controller
160
can be used, for example, in refrigerators, freezers and combinations thereof, such as, for example side-by-side refrigerator
100
(shown in FIG.
1
).
Controller
160
includes a diagnostic port
162
and a human machine interface (HMI) board
164
coupled to a main control board
166
by an asynchronous interprocessor communications bus
168
. An analog to digital converter (“A/D converter”)
170
is coupled to main control board
166
. A/D converter
170
converts analog signals from a plurality of sensors including one or more fresh food compartment temperature sensors
172
, a quick chill/thaw feature pan (i.e., pan
122
shown in
FIG. 1
) temperature sensors
174
(shown in FIG.
8
), freezer temperature sensors
176
, external temperature sensors (not shown in FIG.
2
), and evaporator temperature sensors
178
into digital signals for processing by main control board
166
.
In an alternative embodiment (not shown), A/D converter
170
digitizes other input functions (not shown), such as a power supply current and voltage, brownout detection, compressor cycle adjustment, analog time and delay inputs (both use based and sensor based) where the analog input is coupled to an auxiliary device (e.g., clock or finger pressure activated switch), analog pressure sensing of the compressor sealed system for diagnostics and power/energy optimization. Further input functions include external communication via IR detectors or sound detectors, HMI display dimming based on ambient light, adjustment of the refrigerator to react to food loading and changing the air flow/pressure accordingly to ensure food load cooling or heating as desired, and altitude adjustment to ensure even food load cooling and enhance pull-down rate of various altitudes by changing fan speed and varying air flow.
Digital input and relay outputs correspond to, but are not limited to, a condenser fan speed
180
, an evaporator fan speed
182
, a crusher solenoid
184
, an auger motor
186
, personality inputs
188
, a water dispenser valve
190
, encoders
192
for set points, a compressor control
194
, a defrost heater
196
, a door detector
198
, a mullion damper
200
, feature pan air handler dampers
202
,
204
, and a quick chill/thaw feature pan heater
206
. Main control board
166
also is coupled to a pulse width modulator
208
for controlling the operating speed of a condenser fan
210
, a fresh food compartment fan
212
, an evaporator fan
214
, and a quick chill system feature pan fan
216
.
FIGS. 3 and 4
are more detailed block diagrams of main control board
166
. As shown in
FIGS. 3 and 4
, main control board
166
includes a processor
230
. Processor
230
performs temperature adjustments/dispenser communication, AC device control, signal conditioning, microprocessor hardware watchdog, and EEPROM read/write functions. In addition, processor
230
executes many control algorithms including sealed system control, evaporator fan control, defrost control, feature pan control, fresh food fan control, stepper motor damper control, water valve control, auger motor control, cube/crush solenoid control, timer control, and self-test operations.
Processor
230
is coupled to a power supply
232
which receives an AC power signal from a line conditioning unit
234
. Line conditioning unit
234
filters a line voltage which is, for example, a 90-265 Volts AC, 50/60 Hz signal Processor
230
also is coupled to an EEPROM
236
and a clock circuit
238
.
A door switch input sensor
240
is coupled to fresh food and freezer door switches
242
, and senses a door switch state. A signal is supplied from door switch input sensor
240
to processor
230
, in digital form, indicative of the door switch state. Fresh food thermistors
244
, a freezer thermistor
246
, at least one evaporator thermistor
248
, a feature pan thermistor
250
, and an ambient thermistor
252
are coupled to processor
230
via a sensor signal conditioner
254
. Conditioner
254
receives a multiplex control signal from processor
230
and provides analog signals to processor
230
representative of the respective sensed temperatures. Processor
230
also is coupled to a dispenser board
256
and a temperature adjustment board
258
via a serial communications link
260
. Conditioner
254
also calibrates the above-described thermistors
244
,
246
,
248
,
250
, and
252
.
Processor
230
provides control outputs to a DC fan motor control
262
, a DC stepper motor control
264
, a DC motor control
266
, and a relay watchdog
268
. Watchdog
268
is coupled to an AC device controller
270
that provides power to AC loads, such as to water valve
190
, cube/crush solenoid
184
, a compressor
272
, auger motor
186
, a feature pan heater
206
, and defrost heater
196
. DC fan motor control
266
is coupled to evaporator fan
214
, condenser fan
210
, fresh food fan
212
, and feature pan fan
216
. DC stepper motor control
266
is coupled to mullion damper
200
, and DC motor control
266
is coupled to one of more sealed system dampers.
Processor logic uses the following inputs to make control decisions:
Freezer Door State—Light Switch Detection Using Optoisolators,
Fresh Food Door State—Light Switch Detection Using Optoisolators,
Freezer Compartment Temperature—Thermistor,
Evaporator Temperature—Thermistor,
Upper Compartment Temperature in FF—Thermistor,
Lower Compartment Temperature in FF—Thermistor,
Zone (Feature Pan) Compartment Temperature—Thermistor,
Compressor On Time,
Time to Complete a Defrost,
User Desired Set Points via Electronic Keyboard and Display or Encoders,
User Dispenser Keys,
Cup Switch on Dispenser, and
Data Communications Inputs.
The electronic controls activate the following loads to control the refrigerator:
Multi-speed or variable speed (via PWM) fresh food fan,
Multi-speed (via PWM) evaporator fan,
Multi-speed (via PWM) condenser fan,
Single-speed zone (Special Pan) fan,
Compressor Relay,
Defrost Relay,
Auger motor Relay,
Water valve Relay,
Crusher solenoid Relay,
Drip pan heater Relay,
Zonal (Special Pan) heater Relay,
Mullion Damper Stepper Motor IC,
Two DC Zonal (Special Pan) Damper H-Bridges, and
Data Communications Outputs.
The foregoing functions of the above-described electronic control system are performed under the control of firmware implemented as small independent state machines
FIG. 5
is a defrost state diagram
300
illustrating a state algorithm executable by a state machine of controller
160
(shown in FIGS.
2
-
4
). As will be seen, controller
160
adaptively determines an optimal defrost state based upon effectiveness of defrost cycles as they occur, while accounting for power losses that may interrupt a defrost operation.
By monitoring evaporator temperature over time, it is determined whether defrost cycles are deemed “normal” or “abnormal.” More specifically, when it is time to defrost, i.e. after an applicable defrost interval (explained below) has expired, the refrigerator sealed system is shut off, defrost heater
196
is turned on (at state
2
), and a defrost timer is started. As the evaporator coils defrost, the temperature of the evaporator increases. When evaporator temperature reaches a termination temperature (60° F. in an exemplary embodiment) defrost heater
196
is shut off and the elapsed time defrost heater was on (Δt
de
) is recorded in system memory. Also, if the termination temperature is not reached within a predetermined maximum time, defrost heater
196
is shut off and the elapsed time the defrost heater was on is recorded in system memory.
The elapsed defrost time Δt
de
is then compared with a predetermined defrost reference time Δt
dr
representative of, for example, an empirically determined or calculated elapsed defrost heater time to remove a selected amount of frost buildup on the evaporator coils that is typically encountered in the applicable refrigerator platform under predetermined usage conditions. If elapsed defrost time Δt
de
is greater than reference time Δt
dr
, thereby indicating excessive frost buildup, a first or “abnormal” defrost interval, or time until the next defrost cycle, is employed If elapsed defrost time Δt
de
is less than reference time Δt
dr
, a second or “normal” defrost interval, or time until the next defrost cycle is employed. The normal and abnormal defrost intervals, as defined below, are selectively employed, using Δt
dr
as a baseline, for more efficient defrost operation as refrigerator usage conditions change, thereby affecting frost buildup on the evaporator coils.
More specifically, the following control scheme automatically cycles between the first or abnormal defrost interval and the second or normal defrost interval on demand. When usage conditions are heavy and refrigerator doors
132
,
134
(shown in
FIG. 1
) are opened frequently, thereby introducing more humidity into the refrigeration compartment, the system tends to execute the first or abnormal defrost interval repeatedly. When usage conditions are light and the doors opened infrequently, thereby introducing less humidity into the refrigeration compartments, the system tends to execute the second or normal defrost interval repeatedly. In intermediate usage conditions the system alternates between one or more defrost cycles at the first or abnormal defrost interval and one or more defrost cycles at the second or normal defrost interval.
Upon powerup, controller
160
reads freezer thermistor
246
(shown in
FIG. 3
) over a predetermined period of time and averages temperature data from freezer thermistor
146
to reduce noise in the data. If the freezer temperature is determined to be substantially at or below a set temperature, thereby indicating a brief power loss, a defrost interval is read from EEPROM memory
236
(shown in
FIG. 3
) of controller
160
, and defrost continues from the point of power failure without resetting defrost parameters. Periodically, controller
160
saves a current time till defrost value in system memory in the event of power loss. Controller
160
therefore recovers from brief power loses and associated defrost cycles due to resetting of the system from momentary power failures are therefore avoided.
If freezer temperature data indicates that freezer compartment
104
(shown in
FIG. 1
) is warm, i.e., at a temperature outside a normal operating range of freezer compartment, humid air is likely to be contained in freezer compartment
104
, either because of a sustained power outage or opened doors during a power outage. Because of the humid air, a defrost timer is initially set to the first or abnormal defrost interval. In one embodiment the first or abnormal defrost interval is set to, for example, eight hours of compressor run time. For each second of compressor run time, the first defrost interval is decremented by a predetermined amount, such as one second, and the first defrost interval is generally unaffected by any other event, such as opening and closing of fresh food and freezer compartment doors
134
,
132
. In alternative embodiments, a first or abnormal defrost interval of greater or lesser than eight hours is employed, and decrement values of greater or lesser than one second are employed for optimal performance of a particular compressor system in a particular refrigerator platform.
When the first defrost interval has expired, controller
160
runs compressor
272
(see
FIG. 3
) for a designated pre-chill period or until a designated pre-chill temperature is reached (at state
1
). Defrost heater
196
(shown in
FIGS. 2-4
) is energized (at state
2
) to defrost the evaporator coils. Defrost heater
196
is turned on to defrost the evaporator coils either until a predetermined evaporator temperature has been reached or until a predetermined maximum defrost time has expired, and then a dwell state is entered (at state
3
) wherein operation is suspended for a predetermined time period.
Upon completion of an “abnormal” defrost cycle after the first or abnormal defrost interval has expired, controller
160
(at state
0
) sets the time till defrost to the second or normal pre-selected defrost interval that is different from the first or abnormal time to defrost. Therefore, using the second defrost interval, a “normal” defrost cycle is executed. For example, in one embodiment, the second defrost interval is set to about 60 hours of compressor run time. In alternative embodiments, a second defrost interval of greater or lesser than 60 hours is employed to accommodate different refrigerator platforms, e.g., top-mount versus side-by-side refrigerators or refrigerators of varying cabinet size.
In one embodiment, the second defrost interval, unlike the first defrost interval, is decremented (at state
5
) upon the occurrence of any one of several decrement events. For example, the second defrost interval is decremented (at state
5
) by, for example, one second for each second of compressor run time. In addition, the second defrost interval is decremented by a predetermined amount, e.g., 143 seconds, for every second freezer door
132
(shown in
FIG. 1
) is open as determined by a freezer door switch or sensor
242
(shown in FIG.
3
). Finally, the second defrost interval is decremented by a predetermined amount, such as 143 seconds in an exemplary embodiment, for every second fresh food door
134
(shown in
FIG. 1
) is open. In an alternative embodiment, greater or lesser decrement amounts are employed in place of the above-described one second decrement for each second of compressor run time and 143 second decrement per second of door opening. In a further alternative embodiment, the decrement values per unit time of opening of doors
132
,
134
are unequal for respective door open events. In further alternative embodiments, greater or fewer than three decrement events are employed to accommodate refrigerators and refrigerator appliances having greater or fewer numbers of doors and to accommodate various compressor systems and speeds.
When the second or normal defrost interval has expired, controller
160
runs compressor
272
for a designated pre-chill period or until a designated pre-chill temperature is reached (at state
1
). Defrost heater
196
is energized (at state
2
) to defrost the evaporator coils. Defrost heater
196
is turned on to defrost the evaporator coils either until a predetermined evaporator temperature has been reached or until a predetermined maximum defrost time has expired. Defrost heater
196
is then shut off and the elapsed time defrost heater
196
was on (Δt
de
) is recorded in system memory. A dwell state is then entered (at state
3
) wherein operation is suspended for a predetermined time period.
The elapsed defrost time Δt
de
is then compared with a predetermined defrost reference time Δt
dr
. If elapsed defrost time Δt
de
time is greater than reference time Δt
dr
, thereby indicating excessive frost buildup, the first or abnormal defrost interval is employed for the next defrost cycle. If elapsed defrost time Δt
de
is less than reference time Δt
dr
, the second or normal defrost interval is employed for the next defrost cycle. The applicable defrost interval is applied and a defrost cycle is executed when the defrost interval expires. The elapsed defrost time Δt
de
of the cycle is recorded and compared to reference time Δt
dr
to determine the applicable defrost interval for the next cycle, and the process continues. Normal and abnormal defrost intervals are therefore selectively employed on demand in response to changing refrigerator conditions.
Because the defrost function introduces heat to the system and the sealed system provides cold air, it is desirable that the sealed system and defrost system do not negatively interact. Therefore, a defrost system/sealed system interaction algorithm
310
is defined as follows, and as illustrated in
FIGS. 6 and 7
.
Defrost algorithm
300
, as described above, determines when it is time to begin the defrost process, and in one embodiment further includes a defrost cycle hold-off or delay. In an exemplary embodiment, refrigerator compartment doors
132
,
134
(shown in
FIG. 1
) are to be closed for at a least a predetermined time period, such as two hours, before freezer compartment pre-chill is initiated prior to actual defrost. If the predetermined door closed time, e.g., two hours, is not satisfied, the hold-off will wait until the door closed condition is satisfied, up to a predetermined maximum time, such as, for example, sixteen hours after the originally desired pre-chill entry time determined by defrost algorithm
300
. When either the door closed condition is satisfied or when the predetermined maximum time has expired, pre-chill operation is entered Hold-off timing values, including but not limited to the above-described values, may be stored in ROM, EEPROM
236
(shown in FIG.
3
), or other programmable memory in order to accommodate the needs of different styles of refrigerator units.
When defrost algorithm
300
requests pre-chill from sealed system
312
, sealed system
312
initiates pre-chill. When pre-chill is complete, defrost begins. Sealed system
312
then waits until the freezer temperature is above an upper set point and then turns on.
More particularly, instead of checking the freezer for a lower set point to be achieved, sealed system
312
runs for a fixed pre-chill time. e.g., two hours, to keep the average temperature in the freezer from warming up too much during the defrost cycle. Upon completion of the two hour pre-chill, sealed system
312
shuts down and defrost algorithm
300
takes over. Defrost algorithm
300
runs defrost heater
196
(shown in
FIGS. 2-4
) until a termination temperature or a time out occurs. Defrost algorithm
300
then goes into a dwell period (five minutes in an exemplary embodiment) that holds the sealed system and defrost heater
196
off.
Following the dwell period, compressor
272
(shown in
FIG. 3
) and condenser fan
210
(shown in FIGS.
2
-
4
), in one embodiment, are started for a period of time during which controller
160
keeps evaporator fan
214
(shown in
FIGS. 2-4
) and fresh food fan
212
(shown in
FIGS. 2-4
) off and mullion damper
200
(shown in
FIGS. 2-4
) closed. Once the period ends, or when evaporator temperature achieves a threshold temperature via operation of compressor
272
and condenser fan
210
, mullion damper
200
is opened, and evaporator fan
214
and fresh food fan
212
are started in their high speed. Control is then returned to sealed system
312
for normal cooling operation.
In an alternative implementation of an on-demand defrost system, two temperature sensors (thermistor
248
shown in FIG.
3
and another like thermistor) capable of measuring a temperature differential across the evaporator are utilized in conjunction with a current sensor on the compressor motor, freezer compartment sensor
246
, and a state machine algorithm, such as algorithm
320
illustrated in FIG.
8
. State algorithm
320
may be used in a stand-alone defrost system or in combination with aspects of state algorithm
300
(shown in FIG.
5
), such as, for example, to determine initiation of either the normal or abnormal defrost cycles. A defrost decision can then be made by comparing the relative loads of the evaporator and compressor
272
.
A relationship exists between the evaporator and the compressor load such that compressor
272
experiences a largest load when the refrigerant is wholly in a liquid state and must be converted to a gas state. In this instance, liquid refrigerant in the evaporator closest to compressor
272
vaporizes before liquid refrigerant that is farther away from compressor
272
, producing a large temperature differential between a first sensor, such as thermistor
248
located on one end of the evaporator close to compressor
272
and a second sensor located on a second end of the evaporator away from compressor
272
. Further, when most of the refrigerant is converted, the temperature differential between the ends of the evaporator will reduce because the entire evaporator approaches a substantially uniform temperature (i.e., the vapor temperature of the refrigerant) as the refrigerant is converted.
Therefore, at each refrigerant cycle, when compressor startup is demanded
322
, power to compressor
272
is delayed
324
by a fixed predetermined period. Following fixed time delay
324
, a temperature differential across the evaporator (ΔT) is measured
326
, compressor load current which is proportional to the condenser load is measured
328
, and a defrost decision may be made.
If the compressor current indicates a light compressor load and the temperature differential across the evaporator is large, a fault condition is established
330
and an error flag is set.
If the compressor current indicates a light compressor load and the temperature differential across the evaporator is small, most of the refrigerant is vaporized, the system is operating normally, and a normal refrigerant cycle continues to execute
332
.
If the compressor current indicates a heavy compressor load and the temperature differential across the evaporator is large, most of the refrigerant is liquified, the system is operating normally, and a normal refrigerant cycle continues to execute
334
.
If, however, the compressor current measurement indicates a large compressor load, but the differential temperature measurement across the evaporator is small, it is likely that that frost or ice is causing a uniform temperature gradient across the surface of the evaporator. A need for a defrost cycle is therefore indicated. Before initiating a defrost, a temperature of freezer compartment
104
(shown in
FIG. 1
) is determined
336
. If freezer temperature is at or above a predetermined point, a pre-chill cycle is executed
338
as described above, and defrost heater
196
(shown in
FIGS. 2-4
) is turned on
340
after the pre-chill cycle completes.
If freezer compartment temperature is below a predetermined point, a pre-chill cycle is not executed, therefore saving energy the pre-chill cycle would have otherwise used, and defrost heater
196
is turned on
340
.
In one embodiment, defrost heater
196
is controlled with PID (Proportional, Integral, Derivative) control or other suitable closed loop control to create and execute an optimal heat profile that defrosts the evaporator coils without unnecessarily warming freezer compartment
104
, thereby producing further energy savings.
Upon completion of a defrost heater cycle, freezer compartment temperature is again measured to
342
to determine whether a cooling cycle is required for optimal food preservation. If freezer temperature is at or above a predetermined point, sealed system
312
is turned on to lower the temperature of freezer compartment
104
, thereby chilling
344
freezer compartment
104
. A normal refrigeration cycle is thereafter maintained
346
. If, however, freezer temperature is below a predetermined point, a normal refrigeration cycle is maintained
346
without chilling
344
of freezer compartment
102
.
In an alternative embodiment, instead of using two temperature sensors to measure the differential temperature across the evaporator, a known thermal time constant of the evaporator is used with a single sensor, such as thermistor
248
on the evaporator. Data acquired from the single sensor, i.e., rate of change data, is combined with the known characteristics of the evaporator coil to determine the temperature differential.
Referring to
FIG. 9
, another defrost system state machine or state algorithm
360
is realized using switches or sensors
242
(shown in
FIG. 30
) on refrigerator doors
132
,
134
(shown in
FIG. 1
) to determine when the doors are opened, and temperature sensors
244
,
246
(shown in
FIG. 3
) in the cooling cavities or compartments
102
,
104
. State algorithm
360
may be used as a stand-alone defrost system or in combination with aspects of state algorithm
300
(shown in FIG.
5
), such as, for example, to determine initiation of either the normal or abnormal defrost cycles.
In one embodiment, the normal refrigeration cycle measures refrigeration compartment temperature, and more specifically, freezer compartment
104
temperature to determine operation of sealed system
312
. When refrigeration compartment temperature rises above a set point, compressor
272
(shown in
FIG. 30
) is turned on
362
to initiate cooling, and a timer is set
364
to measure elapsed compressor on time. This cooling cycle continues until the refrigeration compartment temperature falls below a lower threshold set point and compressor is shut down. As the compressor is shut down, the timer is stopped and the elapsed compressor run time () is recorded
366
in controller memory.
Two implicit measurements determine whether defrost is required, namely the amount of time that compressor
272
takes to cool the refrigeration compartment and the cumulative amount of time a door
132
,
134
has been open since the last defrost cycle. Since frost buildup is a result of humidity entering refrigeration compartments when the doors are open there is no need to expend energy executing defrost cycles if the door has not been opened or has only been opened for a short period of time.
A primary indicator for defrost is the length of time (ΔT) that compressor
272
runs to cool the compartment. If the system measures ΔT during the first cooling cycle after a defrost cycle, it can be determined if the time to cool the compartment is increasing thereafter. Because ΔT is a function of compressor load, a threshold time differential ΔT
t
is established during the first cooling cycle that can be used to determine when defrost is required thereafter. In an alternative embodiment, a fixed, pre-programmed ΔT
t
value is employed in lieu of establishing a baseline ΔT
t
during the first cooling cycle.
Thus, when sealed system
312
is shut down and a measured compressor run time ΔT
m
is recorded
366
for that cooling cycle, ΔT
m
is compared to the threshold ΔT
t
. If ΔT
m
is less than or substantially equal to ΔT
t
, defrost is not needed and a normal cooling cycle continues to execute
368
.
If ΔT
m
is greater than the threshold ΔT
t
, a need for defrost is indicated. Before initiating a defrost, a temperature of freezer compartment
104
(shown in
FIG. 1
) is determined
370
. If freezer temperature is at or above a predetermined point, a pre-chill cycle is executed
372
as described above, and defrost heater
196
(shown in
FIGS. 2-4
) is turned on
374
after the pre-chill cycle completes.
Upon completion of a defrost heater cycle, freezer compartment temperature is again measured
376
to determine whether a cooling cycle is required for optimal food preservation. If freezer temperature is at or above a predetermined point, sealed system
312
is turned on to lower the temperature of freezer compartment
104
and chill
378
the freezer compartment. A normal refrigeration cycle is thereafter maintained
380
. If, however, freezer temperature is below a predetermined point, a normal refrigeration cycle is maintained
346
without chilling
378
the freezer compartment.
A fail safe maximum door open time to trigger defrost is also included in the event that there have been several door openings, but no increase in cooling time has been measured.
In addition, since door open and cooling times are implicit indicators of a need for defrost, a maximum time between defrost cycles is also maintained as a fail safe mechanism.
Yet another implementation of an on-demand defrost system can be realized using a combination of the embodiments described above. In this embodiment, compressor on time, i.e., (ΔT) is used to determine compressor load instead of using a current sensor on the compressor.
Still yet another implementation of an on-demand defrost system can be realized using any of the hardware scenarios described above but without using a state machine for making defrost decisions. Rather, Fuzzy Logic is used to make defrost decisions. Using Fuzzy inputs of compressor load (CL), evaporator temperature differential (ETD) and compartment temperature (CT) and Fuzzy outputs of defrost required (DR) and pre-chill required (PCD) a rule set can be constructed as follows:
IF CL is Large and ETD is Small THEN DR is Large
IF DR is Large and CT is Large THEN PCD is Large
Since these are Fuzzy variables, they represent continuous overlapping values. This multivariate system produces a weighting factor (DR) that is de-fuzzied using a fuzzy impulse response to determine whether a defrost is required. The PCD variable grows as the time to defrost approaches and pre-chill begins as required. Additional rules may also be used in alternative embodiments in order to optimize defrost operation across multiple refrigerator platforms.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
Claims
- 1. A method for controlling a self-defrosting refrigerator including a compressor, a defrost heater and a controller operatively coupled to the compressor and the defrost heater, said method comprising the steps of:monitoring a compressor load; determining whether at least a first defrost cycle is required based on the compressor load; executing at least one defrost cycle when required; and determining whether a normal defrost interval is required or an abnormal defrost interval is required for a subsequent defrost cycle, each of said normal and abnormal defrost interval having a predetermined value, said normal defrost interval value greater than said abnormal defrost interval value.
- 2. A method in accordance with claim 1, the refrigerator including an evaporator, said method further comprising the step of monitoring an evaporator load.
- 3. A method in accordance with claim 2, said step of determining whether at least a first defrost cycle is required comprises the step of comparing the evaporator load and the compressor load.
- 4. A method in accordance with claim 3 wherein said step of monitoring a compressor load comprises the step of sensing a compressor current.
- 5. A method in accordance with claim 4 wherein said step of monitoring the evaporator load comprises the step of monitoring a temperature differential across the evaporator.
- 6. A method in accordance with claim 1 wherein said step of monitoring a compressor load comprises the step of monitoring a compressor run time.
- 7. A method in accordance with claim 6 wherein said step of determining whether at least one defrost is required comprises the step of comparing the compressor run time to a predetermined compressor run time.
- 8. A method in accordance with claim 7, said step of monitoring a compressor run time further comprises the step of decrementing the predetermined run time by a predetermined amount for each second of compressor run time.
- 9. A method in accordance with claim 8, said step of monitoring a compressor run time further comprising the step of decrementing the predetermined run time by a predetermined amount for each second that the door is open.
- 10. A method in accordance with claim 1, the controller including a memory, said step of determining whether a normal defrost cycle is required or an abnormal defrost is required comprising the steps of:monitoring an elapsed defrost time to complete a defrost cycle; storing the elapsed time in controller memory; and comparing the elapsed time to a predetermined reference time.
- 11. A method in accordance with claim 10 wherein said step of executing at least one defrost cycle comprises the steps of:executing a first defrost cycle when the elapsed time is less than the reference time; and executing a second defrost cycle when the elapsed time is greater than the reference time, said second defrost cycle different than said first defrost cycle.
- 12. A method in accordance with claim 1, the refrigerator including at least one refrigeration compartment, said step of regulating the defrost cycle comprising the steps of:determining a temperature of the refrigeration compartment, and executing a pre-chill cycle only when the determined temperature is above a predetermined temperature.
- 13. A method in accordance with claim 1 wherein said step of regulating the defrost cycle comprises the steps of:monitoring an evaporator temperature during defrost; and terminating the defrost when the evaporator reaches a predetermined temperature.
- 14. A method in accordance with claim 1, the refrigerator including a refrigeration compartment, the controller including a memory, the memory containing a time till defrost value and a refrigeration compartment temperature setpoint, said step of regulating the defrost comprising the steps of:reading the time till defrost and the refrigeration compartment temperature setting upon powerup; determining the temperature of the refrigeration compartment; and resuming the time till defrost if the determined temperature is substantially at the refrigeration compartment temperature setting.
- 15. A method in accordance with claim 1, the refrigerator including a refrigeration compartment, the controller including a memory, the memory containing a refrigeration compartment temperature setpoint, said step of regulating the defrost cycle comprising the steps of:determining the temperature of the refrigeration after the defrost is completed; comparing the determined temperature to the compartment temperature setpoint; and executing a cooling cycle only when the determined temperature exceeds the compartment temperature setpoint.
- 16. A method in accordance with claim 1 wherein said step of determining whether at least a first defrost cycle is required comprises the step of determining a need for a defrost cycle using fuzzy inputs.
- 17. A defrost control system for a frost-free refrigerator including a compressor, a defrost heater, at least one refrigeration compartment and a temperature sensor thermally coupled to the refrigeration compartment, said control system comprising:a controller operatively coupled to the compressor, the defrost heater, and the temperature sensor, said controller configured to: monitor a compressor load; determine whether at least a first defrost cycle is required based on the compressor load; execute at least one defrost cycle when required; and determine, for a subsequent defrost cycle, whether a normal defrost cycle corresponding to a first predetermined defrost interval or whether an abnormal defrost cycle corresponding to a second predetermined defrost interval is required for the subsequent defrost cycle.
- 18. A defrost control system accordance with claim 17, the refrigerator including an evaporator, said controller further configured to monitor an evaporator load.
- 19. A defrost control system in accordance with claim 18, said controller further configured to compare the evaporator load and the compressor load.
- 20. A defrost control system in accordance with claim 19 said controller further configured to monitor a compressor load by sensing a compressor current.
- 21. A defrost control system in accordance with claim 20, said controller further configured to monitor a temperature differential across the evaporator.
- 22. A defrost control system in accordance with claim 17, said controller further configured to monitor a compressor run time.
- 23. A defrost control system in accordance with claim 22, said controller further configured to compare the compressor run time to a predetermined compressor run time.
- 24. A defrost control system in accordance with claim 23, said controller further configured to decrement the predetermined run time by a predetermined amount for each second of compressor run time.
- 25. A defrost control system in accordance with claim 24, said controller further configured to decrement the predetermined run time by a predetermined amount for each second that the door is open.
- 26. A defrost control system in accordance with claim 17, said controller comprising a memory, said controller further configured to:monitor an elapsed defrost time to complete a defrost cycle; store the elapsed time in said controller memory; and compare the elapsed time to a predetermined reference time.
- 27. A defrost control system in accordance with claim 26, said controller further configured to:execute a first defrost cycle when the elapsed time is less than the reference time; and execute at least a second defrost cycle when the elapsed time is greater than the reference time, said second defrost cycle different than said first defrost cycle.
- 28. A defrost control system in accordance with claim 17, said controller further configured todetermine a temperature of the refrigeration compartment, and execute a pre-chill cycle only when the determined temperature is above a predetermined temperature.
- 29. A defrost control system in accordance with claim 17, said controller further configured to:monitor an evaporator temperature during defrost; and terminate the defrost when the evaporator reaches a predetermined temperature.
- 30. A defrost control system in accordance with claim 17, said controller comprising a memory, said memory containing a time till defrost value and a refrigeration compartment temperature setpoint, said controller further configured to:read the time till defrost and the refrigeration compartment temperature setting upon powerup; determine the temperature of the refrigeration compartment; and resume the time till defrost if the determined temperature is substantially at the determined temperature.
- 31. A defrost control system in accordance with claim 17, said controller comprising a memory, said memory containing a refrigeration compartment temperature setpoint, said controller further configured to:determine the temperature of the refrigeration after the defrost is completed; compare the determined temperature to the compartment temperature setpoint; and execute cooling cycle only when the determined temperature exceeds the compartment temperature setpoint.
- 32. A defrost control system in accordance with claim 17 said controller further configured to determine a need for a defrost cycle using fuzzy inputs.
US Referenced Citations (52)