Controller and method for controlling the temperature in a refrigerator

Abstract: A controller (1) for controlling the temperature in a food compartment of a refrigerator switches on the compressor (6) of the refrigerator when the actual temperature exceeds a cut-in temperature threshold (T on ) and switches off the compressor when the evaporator temperature falls below a cut-out temperature threshold (T off ). The controller (1) detects a thermal dispersion of the refrigerator for compensating the influence of the ambient temperature on the temperature inside the refrigerator. The cut-in temperature (T on ) and/or the cut-out temperature (T off ) are adjusted in accordance with a thermal dispersion ratio based on a ratio of a rate of fall of evaporator temperature when the compressor is on, to a rate of rise of temperature when said compressor is off.

Description: The present invention relates to a controller and a method for controlling the temperature in a food compartment of a refrigerator, according to the preamble of claims 1 and 8, respectively. A controller and a method of this kind are known from EP 0 686 818. From this document a controller for controlling the temperature in a compartment of a refrigerator is known, which addresses the wellknown problem that in conventional refrigerator appliances the air temperature within the food compartment shows a strong dependency on the ambient temperature of the refrigerator appliance. Reason for this phenomenon is, that the actual air temperatur resulting from the two point control of the evaporator temperatur in the food compartment does not only depend on evaporator temperatur thresholds T on and T off between which the evaporator temperatur changes periodically, but furthermore depends on the running time t on of the compressor and the pause time t off of the compressor. For a high ambient temperature, the running time t on of the compressor will be larger than for low ambient temperatures. The pause time t off will be higher for low ambient temperatures than for high ambient temperatures. Consequently, for high ambient temperatures the air temperature inside the food compartment tends to be lower than if the ambient temperature is low. In order to solve this problem, EP 0 686 818 suggests to measure a rate of rise in evaporator temperature when the compressor is switched off. This rate of rise in evaporator temperatur is dependent on the dispersion through the appliance insulation. The dispersion in turn depends on the temperature difference between the air temperature inside the food compartment and the ambient temperature of the refrigerator. In EP 0 686 818 the rate of rise in evaporator temperature when the compressor is switched off, is taken to adjust the lower temperature threshold T off in order to compensate the influence of the ambient temperature on the air temperature in the food compartment. The rate of rise in evaporator temperature is, however, not only dependent on the temperature dispersion through the appliance insulation. This rate of rise of evaporator temperature can be influenced by other factors, particularly the amount of food stored in the food compartment. If the food compartment is fully loaded, the rate of rise in evaporator temperature will be lower than when the food compartment is empty, given the same thermal dispersion, that is, given the same temperature difference between the air in the food compartment and the ambient temperature. For this reason the known solution of compensating the influence of different ambient temperatures is not always accurate. Accordingly, it is the object of the present invention to provide a controller and a method for controlling the temperature inside a food compartment of a refrigerator which allows to compensate the influence of different ambient temperatures on the air temperature inside the food compartment with increased accuracy. According to the present invention, this object is solved as defined in claims 1 and 8. According to the present invention, the ambient temperature is estimated on the basis of a ratio of the rate of rise of evaporator temperature when the compressor is off to the rate of fall of evaporator temperature when the compressor is on. The present invention is based on the recognition that factors like food load influence the rate of rise of evaporator temperature when the compressor is off in the same way as they influence the rate of fall of evaporator temperature when the compressor is on. Accordingly, an entity which reflects the ratio of the rate of rise of evaporator temperature when the compressor is off to the rate of fall of evaporator temperature when the compressor is on, can be taken as a basis for estimating the thermal dispersion of the food compartment with a reduced influence of factors like to food load. The thermal dispersion ratio of the rate of rise of evaporator temperature when the compressor is off to rate of fall of evaporator temperature when the compressor is on, can be evaluated in a number of different ways. These rates of rise and fall can be measured either over a constant time period or over a constant temperature change. If these rates of rise and fall are measured over a constant time period, the beginning of the time interval used for measuring a temperature change can be synchronized with that time instant when the compressor is switched off, to measure the rate of rise of evaporator temperature when the compressor is off, and can be synchronized with that time instant when the compressor is switched on, to measure the rate of fall of evaporator temperature when the compressor is on. If these rates of rise and fall are measured over a constant temperature change, it is advantageous to use the difference between a low temperature threshold, also called cut-out temperature, and a high temperature threshold, also called cut-in temperature, as the temperature change. The high temperature threshold defines a temperature value, which when exceeded by the evaporator temperature, results in that the compressor is switched on. Similarly, if the evaporator temperature falls below the low temperature threshold, the compressor is switched off. Since in this case the temperature change for measuring the rate of rise of evaporator temperature is the same as the temperature change for measuring the rate of fall of evaporator temperature, said ratio of the rate of rise of evaporator temperature when the compressor is off to the rate of fall of evaporator temperature when the compressor is on, can be evaluated from a ratio t on /t off of the compressor on-time period t on to the compressor off-time period t off . This evaluation of the thermal dispension ratio is to a large extent independent from factors such as food load. It provides an indication of the thermal dispersion of the food compartment which in turn indicates the ambient temperature of the refrigerator. An entity equivalent to the ratio t on /t off is the compressor duty cycle c = t on /(t on + t off ). Since the compressor duty cycle c can be expressed as a function of t on /t off alone, also the compressor duty cycle c provides an evaluation of the thermal dispersion ratio indicating the ambient temperature substantially without the influence of the actual food load in the refrigerator compartment. The thermal dispersion ratio is preferably calculated by the refrigerator control on a continuous basis. That is, preferably each time the compressor changes its operating state from off to on or from on to off, the rate of change of evaporator temperature is evaluated such that each time the operating state of the compressor has changed, an updated thermal dispersion ratio can be evaluated. According to the present invention, the low temperature threshold and/or the high temperature threshold are adjusted in accordance with the evaluation of the thermal dispersion ratio such that the influence of the ambient temperature on the actual average air temperature within the food compartment can be compensated. This compensation can be achieved by means of only adjusting the low temperature theshold based on the thermal dispersion ratio, or by means of only adjusting the high temperature threshold based on the thermal dispersion ratio, or by means of adjusting both the low temperature threshold and the high temperature threshold on the basis of the thermal dispersion ratio. The thermal dispersion ratio can be disturbed by factors such as frequent or long refrigerator door openings or the introduction of extremely warm or cold goods. If it is desired to also reduce the influence of these factors, it is advantageous to look for a stabilization of the thermal dispersion ratio before an adjustment of said at least one of the high and low temperature thresholds is made. Whether or not the thermal dispersion ratio has stabilized, can for instance be judged from a number of consecutive evaluations of this ratio and analyzing whether this predetermined number of consecutive evaluations differ from each other by less than a predetermined amount. If a predetermined number of consecutive evaluations of the thermal dispersion ratio does not differ by more than this predetermined amount, said adjustment of thresholds based on the thermal dispersion ratio is enabled, otherwise disabled. A controller implementing this method is advantageous in that it does not require an additional sensor for sensing the door position and needs no more than an input from the existing evaporator temperature sensor and an input from a user adjustable temperature dial for setting a target value of the air temperature within the food compartment. Preferably, the controller includes a lookup table which contains various thermal dispersion ratios and associated cut-in temperatures and cut-out temperatures for the compressor which have been chosen empirically during development testing of the refrigerator appliance, to suit the particular ambient conditions detected. It is convenient if these ambient temperatures are organized into temperature bands internationally recognized for refrigeration appliances. A controller according to the present invention is preferably implemented using an appropriately programmed microcontroller. In the following, preferred ambodiments of the present invention will be described with reference to the accompanying drawings. Identical or corresponding elements in the various drawings have been denoted with identical reference numerals. Fig. 1 shows a time chart schematically depicting the behaviour of the air temperature in the food compartment, the evaporator temperature and the compressor operating states over time; Fig. 2 shows a first embodiment of a controller for controlling a refrigerator according to the present invention; Fig. 3 shows a second embodiment of a controller according to the present invention; and Fig. 4 shows a third embodiment of a controller according to the present invention. Fig. 1 is a time chart with three curves A, B and C. All three curves use the same time axis t. Curve A shows the behaviour of the air temperature within a food compartment of a refrigerator over time. Curve B shows the behaviour of the temperature of the evaporator in the food compartment over time. Finally, curve C shows the operating states ON and OFF of the compressor in association with the temperature curves A and B. From Fig. 1 it can be taken that in the course of controlling the average air temperature corresponding to the temperature of the food in the refrigerator compartment, the compressor which circulates cooling fluid through the refrigeration circuit of the refrigerator, switches on when the evaporator temperature has risen to a high temperature value T on , called the cut-in temperature. The operation of the compressor results in that the evaporator temperature falls until a low temperature threshold T off is reached which causes the compressor to switch off again. In the off state of the compresssor, the evaporator temperature rises again until the high temperature threshold T on has been reached. This cycle repeats periodically. While the temperature difference T on - T off between the high temperature threshold and the low temperature threshold can be substantial, e.g. about 20°C, the resulting cyclic change of the air temperature in the food compartment is much lower, as indicated in curve A of Fig. 1. The average air temperature depends on the high temperature threshold T on and on the low temperature threshold T off and furthermore on the duration of each of the operating states of the compressor in each cycle. The present invention is based on the recognition that when the ambient temperature increases, the rate of fall of evaporator temperature when the compressor is on, decreases due to the thermal dispersion of the refrigerator insulation and other reasons. Moreover, the rate of rise of evaporator temperature when the compressor is off, will increase with an increase in the ambient temperature. On the other hand, an increased food load in the compartment will not only slow down the rate of fall of evaporator temperature when the compressor is on but also the rate of rise of evaporator temperature when the compressor is off. Accordingly, a ratio of the rate of rise of evaporator temperature when the compressor is off to the rate of fall'of evaporator temperature when the compressor is on, will be a sensitive indication of ambient temperature and will at the same time be insensitive against food load variations. Fig. 1 indicates one possibility how this thermal dispersion ratio can be evaluated. According to Fig. 1, a temperature difference ΔT off is measured over a predetermined time interval Δt which begins whenever the compressor changes its operating state from on to off. The ratio ΔT off /Δt is an evaluation of the rate of rise of evaporator temperature when the compressor is off. Similarly, a temperature difference ΔT on is measured over a predetermined time interval Δt which can be of the same length as that used for measuring ΔT off , the interval Δt for measuring ΔT on beginning when the compressor changes its operating state from off to on. The ratio ΔT on /Δt is an evaluation of the rate of fall of evaporator temperature when the compressor is off. The ratio of this rate of rise to this rate of fall represents the thermal dispersion ratio indicating the ambient temperature with low sensitivity towards food load variations. For evaluating this thermal dispersion, the time interval Δt can be kept constant when measuring the change of temperature ΔT off and ΔT on , or the change of temperature is kept constant and the time interval Δt is measured. According to an advantageous embodiment of the invention, ΔT on and ΔT off are chosen such that ΔT on = ΔT off = T on - T off = ΔT. From (ΔT/t off )/ΔT/t on ) = t on /t off it is evident that this ratio of the compressor on time to the compressor off time indicates an essentially food load independent thermal dispersion ratio of the refrigerator which is useful for estimating the ambient temperature of the refrigerator. Fig. 2 shows a first embodiment of a controller according to the present invention. In this figure, reference numeral 1 denotes the controller which can be implemented by means of software in a microcontroller. Reference numeral 7 denotes a temperature sensor, for instance a temperature dependent resistor. This temperature sensor is mounted in thermal contact with the evaporator in the food compartment and provides a temperature input to the controller 1. Reference numeral 8 denotes a temperature setting means which is user-adjustable, for presetting a target temperature inside the food compartment. The user-adjustable temperature setting means 8 can be a potentiometer which provides a target temperature input to controller 1. Reference numeral 5 denotes a power supply for the controller 1. Reference numeral 6 represents the compressor which is switched on and off by the controller 1. Reference numberal 4 denotes a comparator with hysteresis which compares the temperature sensed by the temperature sensor 7 with a high temperature threshold T on and with a low temperature threshold T off . As depicted in Fig. 7, the comparator 4 switches on the compressor as soon as the detected evaporator temperature has exceeded T on , and switches off the compressor as soon as the detected evaporator temperature falls below T off . Reference numeral 2 denotes a section for evaluating a thermal dispersion ratio based on a ratio of said rate of fall of evaporator temperature when said compressor is off to a rate of rise of temperature when said compressor is on. In order to evaluate this thermal dispersion ratio, section 2 receives the detected evaporator temperature and the operating state of the compressor 6 as inputs. For each of the operating states ON and OFF of the compressor, section 2 evaluates a rate of fall of evaporator temperature and a rate of rise of evaporator temperature, respectively, and evaluates a ratio of these rates each time the compressor 6 has changed its operating state. This ratio generated by section 2 which represents the thermal dispersion ratio of the refrigerator, is received by section 3. This section comprises a database which contains high temperature threshold values and low temperature threshold values for a variety of different values of the thermal dispersion ratio provided by section 2, and for a variety of different target temperature values set by a user. In this embodiment, section 3 furthermore contains memory locations for storing a predetermined number of most recent thermal dispersion ratios. Each time when section 2 outputs a new thermal dispersion ratio, the oldest of the stored previous thermal dispersion ratios is discarded and replaced by the newest. Section 3 furthermore contains means for judging whether the stored thermal dispersion ratios differ from each other by more than a predetermined amount. If this is the case, section 3 detects an unstable condition, e.g. resulting from frequent or long door openings or the introduction of extremely warm or cold goods. As long as the unstable condition prevails, section 3 will not update the high temperature threshold T on and the low temperature threshold T off output to comparator 4. As soon as the stored thermal dispersion ratios are found by section 3 to differ from each other by less than said predetermined amount, section 3 detects a stable condition. As long as the stable condition prevails, section 3 takes the most recent thermal dispersion ratio provided by section 2, furthermore takes the user-adjusted target temperature provided by setting means 8, reads from its database a low temperature threshold T off and a high temperature threshold T on and outputs these thresholds to comparator 4. The database of section 3 can be a table filled with threshold values which have been chosen empirically during development testing under different ambient temperature conditions. While in the embodiment of Fig. 2, section 3 provides both the low temperature threshold and the high temperature threshold to comparator 4 depending on the set target temperature and the evaluated thermal dispersion ratio, the same effect of compensating the influence of ambient temperature variations on the average air temperature in the food compartment can be obtained if only one of these temperature thresholds, preferably the low temperature threshold, is adjusted by section 3 while the other temperature threshold is kept fixed. Fig. 3 shows a second embodiment of the present invention. This embodiment differs from the embodiment of Fig. 2 in section 2 for evaluating a thermal dispersion ratio. All other elements of Fig. 3 are not different from the corresponding elements of Fig. 2, such that their description need not be repeated. According to the embodiment of Fig. 3, the section 2 for evaluating a thermal dispersion ratio based on a ratio of said rate of fall of evaporator temperature when said compressor is off to a rate of rise of temperature when said compressor is on, is constituted to measure the on time t on of the compressor and the off time t off of the compressor. To this end, section 2 receives information on the current operating status of the compressor 6 from comparator 4. Each time the compressor 6 has changed its operating state, section 2 calculates a ratio t on /t off for evaluating the thermal dispersion ratio of the refrigerator, and outputs this ratio to section 3. According to this embodiment, section 2 does not require an input from temperature sensor 7 on the evaporator because in this embodiment the evaluation of the thermal dispersion ratio is based on rates of fall and rise of evaporator temperature which in turn are based on the same temperature difference ΔT = T on - T off . During normal operation of the refrigerator, this temperature difference remains constant such that the ratio t on /t off is a reliable evaluation of the thermal dispersion ratio input into section 3. Fig. 4 shows a third embodiment of a controller according to the present invention. This embodiment differs from the embodiment of Fig. 3 only in section 2. All remaining elements are not different from the corresponding elements in Figs. 2 and 3 such that their description need not be repeated. According to this embodiment, section 2 evaluates a thermal dispersion ratio of the refrigerator based on the duty cycle of the compressor 6. To this end, section 2 receives an indication on the current operating state of compressor 6 from comparator 4. Section 2 measures the on time period t on of the compressor and the off time period toff of the compressor. Each time the operating state of the compressor has changed, section 2 calculates a new duty cycle from the most recently completed on period and the most recently completed off period of the compressor. The duty cycles calculated by section 2 are input into section 3 as an evaluation of the thermal dispersion ratio of the refrigerator. A controller according to the present invention is suitable for controlling the temperature in the freshfood compartment, also called fridge compartment of a refrigerator. It is similarly suitable for controlling the temperature in a freezer compartment for storing frozen food, of a refrigerator. A controller is particularly suitable for controlling the temperature in a refrigerator combining a freezer compartment and a fridge compartment. If such a refrigerator has a high compressor and a single refrigeration circuit with a series connection of the evaporator in the fridge compartment and the evaporator in the freezer compartment, and a temperature sensor only on the evaporator of the fridge compartment, the controller is able to maintain the temperature in the freezer compartment essentially stable in different ambient temperature conditions. The term freezer compartment refers to compartments having at least a one star rating. A controller according to the present invention can compensate the influence of ambient temperatures on the temperature inside the food compartment of a refrigerator essentially independent from the food load and does not require a separate ambient temperature sensor.

Claims: A controller for controlling the temperature in a food compartment of a refrigerator having an evaporator located in said food compartment and a compressor (6) for circulating cooling fluid through said evaporator, said controller (1) comprising: means (8) receiving a target temperature value from a target temperature setting means for said food compartment; means (7) receiving an actual temperature value of said evaporator in a refrigeration compartment of the refrigerator; means (4) switching on said compressor (6) when the actual temperature exceeds a high temperature threshold (T on ) and for switching off said compressor (6) when said actual temperature falls below a low temperature threshold (T off ), said temperature thresholds being determined depending on said food compartment target temperature; means (3) compensating a dependency of an average temperature in said food compartment from an ambient temperature of the refrigerator by adjusting said low temperature threshold (T off ) and/or said high temperature threshold (T on ), said compensation means (3) being adapted to measure a rate of rise of evaporator temperature when said compressor is off; characterized in that said compensation means (3) is adapted to evaluate a thermal dispersion ratio based on a ratio of said rate of rise of evaporator temperature when said compressor (6) is off, to a rate of fall of temperature when said compressor (6) is on, and to adjust said low temperature threshold (T off ) and/or said high temperature threshold (T on ) in accordance with said thermal dispersion ratio. The controller according to claim 1, characterized in that said compensation means (3) is adapted to determine said rate of fall and/or said rate of rise of evaporator temperature by measuring a change of evaporator temperature over a specified period of time (Δt) or by measuring a period of time during which the evaporator temperature changes by a specified amount (ΔT). The controller according to claim 1, characterized in that said compensation means (3) is adapted to measure a period t on when said compressor (6) is on and a period t off when said compressor is off, and to obtain said thermal dispersion ratio from a ratio t on /t off of said ON time period to said OFF time period. The controller according to claim 1, characterized in that said compensation means (3) is adapted to measure a period t on when said compressor (6) is on and a period t off when said compressor (6) is off, and to obtain said thermal dispersion ratio from a ratio of said Off time period t off to a sum t on +t off of said ON time period and said OFF time period. The controller according to any one of the preceding claims, characterized by look up table means (3) for storing low temperature threshold values and/or high temperature threshold values in association with various values of said thermal dispersion ratio and with food compartment target temperature values; said compensation means (3) being adapted to read said low temperature threshold and/or said high temperature threshold from a location in said lookup table indicated by said evaluated thermal dispersion ratio and said food compartment target temperature. The controller according to any of the preceding claims, characterized in that said compensation means (3) is adapted to store a predetermined number of consecutively evaluated thermal dispersion ratios; and to inhibit an adjustment of said low temperature threshold (T off ) and/or said high temperature threshold (T on ) if said predetermined number of thermal dispersion ratios differ from each other by more than a predetermined variance threshold. The controller according to any of the preceding claims, characterized in that at least said compensation means (3) is implemented in a microcontroller. A method of controlling the temperature in a food compartment of a refrigerator having an evaporator located in said food compartment and a compressor (6) for circulating cooling fluid through said evaporator, comprising the steps of receiving a target temperature value for said food compartment; receiving an actual temperature value of said evaporator in said food compartment of the refrigerator; switching on said compressor (6) when the actual temperature exceeds a high temperature threshold (T on ) and switching off said compressor (6) when said actual temperature falls below a low temperature threshold (T off ); at least one of said temperature thresholds being generated depending on said food compartment target temperature; measuring a rate of rise of evaporator temperature when said compressor (6) is off and compensating a dependency of the average temperature in said food compartment from an ambient temperature by adjusting said low temperature threshold and/or said high temperature threshold; characterized by evaluating a thermal dispersion ratio based on a ratio of said rate of rise of evaporator temperature when said compressor (6) is off, to a rate of fall of temperature when said compressor (6) is on, and adjusting said low temperature threshold (T off ) and/or said high temperature threshold (T on ) in accordance with said thermal dispersion ratio.

Inventors: BAILEY GRAHAM, HEGER HELMUT, HIRSCH MIKE, BAILEY, GRAHAM, HEGER, HELMUT, HIRSCH, MIKE
Applicants: RANCO INC, RANCO INCORPORATED OF DELAWARE
Application year: 1999
Main classification: F25D29
IPC codes: F25D29, G05D23