Cooling system with switched compressor control

Abstract

Described is, among other things, a cooling system. The cooling system comprises a compressor, a condenser, and an evaporator. The compressor, the condenser, and the evaporator are all connected in series to form a closed loop to allow circulation of a refrigerant. The cooling system further comprise a controller configured to control the operation of the compressor. The compressor can be controlled in at least a first mode of operation where the compressor is switched in cycles between an OFF state and an ON state, and a second mode of operation where the compressor is run continuously. The controller is configured to obtain cooling demand data, and to control the operation of the compressor based on the cooling demand data, and the controller is further configured to switch between the first mode of operation and the second mode of operation based on a variable threshold value.

Description

[0001] COOLING SYSTEM WITH SWITCHED COMPRESSOR CONTROL

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a domestic appliance such as a refrigerator and a cooling system therefor. In particular, the present disclosure relates to control of a compressor in a domestic appliance.

[0004] BACKGROUND

[0005] Cooling systems for domestic appliances such as refrigerators typically use compressor cooling systems to provide cooling. The compressor can be of fixed speed type where the compressor is switched ON and OFF to meet a set cooling target such as a set target temperature in a refrigerator compartment. The compressor can also be of a variable speed type, a variable speed compressor (VSC). In a variable speed compressor, the speed of the compressor is controlled to meet the required cooling demand.

[0006] There is a constant desire to improve the performance in domestic appliances having a cooling system comprising a compressor and to improve energy efficiency thereof. Hence, there is a need for an improved cooling system comprising a compressor.

[0007] SUMMARY

[0008] It is an object of the present invention to provide an improved domestic appliance having a cooling system, in particular a refrigerator. In particular it is an object to improve the performance of a cooling system for a domestic appliance.

[0009] This object and / or others are obtained by the domestic appliance and the cooling system as set out in the appended claims. As has been realized by the inventors, energy efficiency of a domestic appliance with a cooling system can be improved by controlling a variable speed compressor of the cooling system in two different modes. The two modes of operation can comprise a first mode (switched mode) of operation where the compressor is switched in cycles between an OFF state and an ON state, and a second mode (continuous mode) of operation where the compressor is run continuously. The control between the first and second mode of operation can be switched based on the cooling demand. For example, if the cooling demand corresponds to a compressor threshold speed below a minimum speed of the compressor (or another pre-determined compressor speed) the mode of operation is switched from continuous mode to switched mode and vice versa. Thus, to keep the temperature at the setpoint it is typically mandatory that the average speed of the compressor in a given period (say 1 hour) exactly matches the demand from the temperature regulator. If the demand of the regulator is below the minimum speed allowed for the VSC, the compressor needs to be in a switched mode. In the switched mode it is possible to make the compressor run for a fraction of the period at minimum speed and for the rest of the time keep it off such that the average speed in the period matches the request of the regulator. In case the cooling demand is above the minimum speed, of course the VSC could be made run for the whole period at the speed matching the cooling demand. Moreover, the threshold is computed in such way that it is guaranteed a minimum compressor OFF time.

[0010] As has been further realized by the inventors, when operating the variable speed compressor by switching between such two modes, there can be an instability in the control system when the cooling demand is such that the cooling demand is close to the compressor threshold speed. The control can then start to oscillate between the two modes of operation which is not desired. In other words, a problem can arise when the cooling demand is borderline and fluctuates around the minimum speed. What is observed is that the appliance may have one period switching and the next going continuously or vice versa, depending if the cooling demand is slightly below or above the minimum speed. Such an instable operation mode should be avoided to not cause instable operation of the domestic appliance. To solve this problem, the threshold used to switch between the two modes of operation can be a variable threshold. In particular the threshold can have a hysteretic behavior so that once a switch from one mode of operation to the other mode of operation has occurred, the threshold is moved to another value making it harder to re-enter the previous mode of operation.

[0011] In accordance with the present invention a cooling system is provided. The cooling system comprises a compressor, a condenser, and an evaporator. The compressor, the condenser, and the evaporator are all connected in series to form a closed loop to allow circulation of a refrigerant. The cooling system further comprise a controller configured to control the operation of the compressor. The compressor can be controlled in at least a first mode of operation where the compressor is switched in cycles between an OFF state and an ON state, and a second mode of operation where the compressor is run continuously. The controller is configured to obtain cooling demand data, and to control the operation of the compressor based on the cooling demand data, and the controller is further configured to switch between the first mode of operation and the second mode of operation based on a variable threshold value. Hereby it is possible to avoid operation where the compressor is operated around a threshold value where the operation is instable. This is because the threshold value can be set to another threshold value whereby the instable operation is avoided.

[0012] In accordance with one embodiment, the variable threshold value can be periodically updated. In particular the variable threshold value can be periodically updated in the first mode of operation and kept fixed to a predetermined value in the second mode of operations. Hereby a threshold value that avoids instable operation can be ensured.

[0013] In accordance with one embodiment, in the first mode of operation each cycle comprises one OFF state and one ON state and has a predetermined duration. Hereby a stable and energy efficient control of the compressor in the first state can be obtained. In accordance with one embodiment, the controller is configured to calculate the variable threshold value based on a hysteretic function such that when in the first mode of operation, the variable threshold is increased compared to when in the second mode of operation. Hereby it is achieved that the switching between the different modes of operation is made harder in that the threshold value is moved away from the current state of operation. For example, the threshold value can always be the minimum compressor speed when in the second mode of operation and a higher compressor speed (higher than the minimum compressor speed) when in the first mode of operation.

[0014] In accordance with one embodiment, the controller is configured to, when in the first mode of operation, continuously or periodically recalculate the duration of the of the OFF state and / or the ON state, respectively. Hereby a precise control of the cooling can be achieved and at the same time maintain an energy efficient control of the compressor.

[0015] In accordance with one embodiment, the controller is configured to, when in the first mode of operation, continuously or periodically recalculate the compressor speed in the ON state. Hereby a more accurate control of the cooling can be achieved.

[0016] In accordance with one embodiment, the controller is configured to, when in the first mode of operation, apply a minimum OFF state time. Hereby a more energy efficient control of the compressor can be achieved.

[0017] In accordance with one embodiment, the controller is configured to increase the speed of the compressor during the ON state when the application of the minimum OFF state time is determined to result in a reduced duration for the ON state. Hereby the cooling demand can be efficiently met. In accordance with one embodiment, the controller is configured to, when in the first mode of operation, apply a minimum ON state time. Hereby an energy efficient control of the compressor can be obtained.

[0018] In accordance with one embodiment, the controller is configured to reduce the speed of the compressor during the ON state when the application of the minimum ON state time is determined to result in an increased duration for the ON state. Hereby a more accurate control of the cooling can be obtained.

[0019] The invention also extends to a domestic appliance such as a refrigerator comprising a cooling system according to the above.

[0020] BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawings, in which:

[0022] - Fig. 1 illustrates a domestic appliance,

[0023] - Fig. 2 illustrates a cooling system for a refrigerator,

[0024] - Fig. 3 is a flow chart illustrating some steps performed when controlling a compressor of a refrigerator, and

[0025] - Fig. 4 illustrates a controller.

[0026] DETAILED DESCRIPTION

[0027] Aspects of the present invention will now be described more fully. Like numbers refer to like elements throughout. Well-known functions or constructions will not necessarily be described in detail for ease of understanding and / or clarity. It is further to be understood that the features described can be combined in any suitable manner to meet different implementational needs. Some elements can be omitted in some embodiments. Further, while the cooling system as described herein typically can be employed in a household refrigerator with multiple compartments, the cooling system can be used in any type of refrigerator and the like.

[0028] In Fig. 1, a typical household refrigerator 10 comprising a fresh food compartment 12 and a freezer compartment 16. The exemplary refrigerator 10 in Fig. 1 is of the type known as side by side. However, the cooling system as described herein can be employed in any type of refrigerator such as a French door type, top freezer type, bottom freezer type, or any other type of refrigerator. A door 14, shown in FIG. 1 as open, is mounted to the refrigerator body by hinges and serves to close the front of the fresh food compartment 12 as well as provide access to the interior of the fresh food compartment. A door 18, shown in FIG. 1 as open, also is mounted to the refrigerator body by hinges and serves to close the front of the freezer compartment 16 as well as provide access to the interior of the freezer compartment. The fresh food and freezer compartments can include a variety of shelves 20, closed drawers 22 and basket-like drawers 24 for storing articles of food and the like. Also, other configurations of the household refrigerator 10 can be envisaged depending on the type of refrigerator and to meet different needs. To cool the refrigerator a cooling system 30 is provided. An exemplary cooling system 30 will now be described in more detail with reference to the following Figures.

[0029] In Fig. 2, an exemplary cooling system 30 for a refrigerator apparatus is illustrated. The refrigerator apparatus can be any cooling device. In particular the refrigerator apparatus can be a refrigerator or a freezer or a combined refrigerator / freezer for example as described above in conjunction with Fig. 1. The cooling system 30 comprises a compressor 32, a condenser 34 and an evaporator 36. The cooling system 30 also comprises a capillary tube 38 (or a similar device such as an expansion valve). The cooling system 30 also comprises a controller 42. The cooling system 40 may typically also comprise other components such as a filter 44 and other conventional components not shown in Fig. 2. The compressor 32 drives a refrigerant in a cycle whereby the condenser 34 becomes hot and the evaporator 36 becomes cold. In order to reduce energy consumption, the compressor should be driven in an energy efficient manner to minimize energy consumption, but at the same time deliver the cooling capacity that meets the cooling demand of the refrigerator. The cooling demand can typically be a set temperature for a cooled compartment of the refrigerator. The controller 42 is configured to control the operation of the compressor 32. Here the compressor is a variable speed compressor 32 and the control can be performed as described below.

[0030] In Fig. 3, a flow chart illustrating some procedural steps that can be performed by the controller 42 when controlling the compressor 32. First, in a step 300, the procedure is started. In the starting position it is assumed that some initial data is made available to the controller 42. Such initial data can comprise, but is not limited to, a set temperature for one or many cooled compartments of the refrigerator, and some data of the compressor 42. Data of the compressor can for example be the maximum speed of the compressor and the minimum speed of the compressor.

[0031] Next, in a step 301, a mode switching threshold for switching between a first mode of operation and a second mode of operation is set. In the first mode of operation the compressor 32 is switched in cycles between an OFF state and an ON state. One cycle is typically one time period in an ON state followed by one time period in an OFF state, or vice versa. In the second mode of operation the compressor is run continuously. The threshold can initially be set to the minimum speed of the compressor.

[0032] Next, in a step 303 data indicative of a cooling demand is obtained. The cooling demand data can be a value indicating the current cooling demand in the refrigerator. The cooling demand data can be derived in a process where the different input parameters are processed. For example, a temperature regulator (not shown) can be employed to derive the cooling demand data. The temperature regulator (e.g., a PID, or MPC, or adaptive controller or other known control schemes) can be setup to determine a cooling capacity request. The cooling capacity request can for example be a value based on the error between the temperature feedback of one cooled compartment and a setpoint temperature (typically the freezer compartment in a refrigerator comprising a freezer), or by a linear combination of temperature errors in case of a multi-compartment refrigerator. The cooling capacity request can be used directly as cooling demand data by the controller or the cooling capacity request can be further pre-processed. For example, the cooling capacity request as calculated by the temperature regulator can be normalized to a value in the range [0 - 1] and set for a predetermined time period T. The normalized value can be normalized such that the value 0 corresponds to that the compressor should be OFF for the entire pre-determined time period T. The normalized value can further be normalized such that the value 1 corresponds to that the compressor should be run at maximum speed for the entire pre-determined time period T. If it is determined that the compressor is to be run in an OFF state or a maximum speed state for the entire pre-determined time period T, the procedure executes this step in a step 304 and can then return to step 301.

[0033] If the cooling demand data corresponds to any other state of operation of the compressor 32 other than that the compressor should be OFF for an entire pre-determined time period T or run at full speed (maximum speed) for the entire pre-determined time period T, the following steps can be performed.

[0034] In a step 305, if the cooling demand data obtained by the controller is such that the controller switch mode of operation from the second mode of operation to the first mode of operation, the mode switch threshold is increased. For example, if the mode switch threshold currently is the compressor minimum speed, the mode switch threshold is set to a compressor speed higher value than the compressor minimum speed. In other words, if the compressor is run with a compressor continuously running (second mode of operation) and the cooling data indicates that the mode is to be switched to a switched mode of operation (first mode of operation). The mode is switched and the mode switch threshold is increased. Thus, the compressor speed at which the controller 42 will switch back to the second mode of operation will be increased.

[0035] In a step 307, if the cooling demand data obtained by the controller is such that the controller switch mode of operation from the first mode of operation to the second mode of operation, the mode switch threshold is reduced. This assumes that the mode switch threshold when in the first mode of operation is higher than the compressor minimum speed (the mode switch threshold cannot be lower than the compressor minimum speed). Typically, the mode switch threshold can now be set to the compressor minimum speed.

[0036] Thus, the controller 42 is configured to obtain cooling demand data, and to control the operation of the compressor based on the cooling demand data, and the controller is further configured to switch between the first mode of operation and the second mode of operation based on a variable threshold value given by the mode switch threshold. Because the threshold value is variable, the switching between the first and second mode can have a hysteretic behavior. This in turn can reduce problems arising from instable control that can occur if the threshold value is fix and the control starts to oscillate between the first and second mode of operation because the cooling demand is close to the threshold value.

[0037] The hysteretic behavior of the variable threshold can be implemented in various manners. In accordance with one embodiment the variable threshold is always set to the minimum speed of the variable speed compressor when the compressor is run in a continuous mode of operation. When the compressor is run in the switched mode the variable threshold can be increased so that the variable threshold is a speed higher than the minimum speed of the variable speed compressor. Further, the variable threshold applied in the continuous mode of operation (second mode of operation), can be periodically adjusted. For example, if the control of the compressor switches between the first mode of operation and the second mode of operation frequently (above some threshold), the threshold can be increased to a higher speed of the compressor. Next, in a step 309, the compressor 42 is run in the selected mode for a set time period. The duration of the set time period can advantageously be fixed for consecutive set time periods and can typically correspond to the length of the pre-determined time period T set out above.

[0038] If the compressor in step 309 is run in the first mode of operation, where the compressor is switched between an ON state and an OFF state (or vice versa), advantageously only one ON state and only one OFF state is set for each set time period. The duration of the ON state and the speed of the compressor can be calculated beforehand. However, during the ON state, the speed can be adjusted to respond to updates of the cooling demand obtained by the controller during the ON state. For example, a new value for the speed can be re-calculated periodically within the set time period such as once every second. Also, additional requirements for the time in ON state and the time in OFF state can be applied. For example, the controller can enforce a minimum time in ON state and a minimum time in OFF state for a set time period. The compressor speed during the ON time can be used to deliver adequate cooling capacity during the set time period while ensuring that the minimum time in ON and OFF state respectively is complied with. Similarly, if the compressor is run the second state the compressor speed can be adjusted during the time period during which the step 309 is executed. Further, In the first mode of operation (ON / OFF switching) also the threshold speed can be continuously or periodically such as updated second by second according to the cooling request. However, the threshold speed can be kept fixed to a predetermined threshold value in the second mode of operations.

[0039] When the set time in step 309 has elapsed, the procedure can return to step 301, and the procedure restarts and the variable threshold value can again be updated. Hence, the control allows for a periodic update of the variable threshold value that controls when switching between the two modes of operation shall occur. Further, the controller 42 can be implemented using suitable hardware and or software. An exemplary controller is depicted in Fig. 4. The hardware can comprise one or many processors 401 that can be arranged to execute software stored in a readable storage media 402. The processor(s) can be implemented by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared or distributed. Moreover, a processor or may include, without limitation, digital signal processor (DSP) hardware, ASIC hardware, read only memory (ROM), random access memory (RAM), and / or other storage media. The processor 42 is adapted to send and receive signals from other entities such as the compressor 32, a cooling demand signal such as an error signal indicative of the difference between a set temperature and an actual temperature in a cooled compartment of the refrigerator. The controller can also in some embodiments obtain data from other entities such as a cooled compartment temperature sensor, and different external sensors or other units such as a user interface using an interface 403. The controller 42 can in particular be configured to implement the control procedures as described herein.

[0040] By controlling the compressor of a cooling system as set out herein provides a more efficient cooling system that can be used for example in a refrigerator.

Claims

CLAIMS1. A cooling system (30) comprising:- a variable speed compressor (32),- a condenser (34),- an evaporator (36),- wherein the compressor, the condenser, and the evaporator are all connected in series to form a closed loop to allow circulation of a refrigerant, and- a controller (42) wherein- the controller is configured to control the operation of the compressor in at least:- a first mode of operation where the compressor is switched in cycles between an OFF state and an ON state, and- a second mode of operation where the compressor is run continuously, and wherein- the controller is configured to obtain cooling demand data, and to control the operation of the compressor based on the cooling demand data, and wherein the controller is further configured to switch between the first mode of operation and the second mode of operation based on a variable threshold value.

2. The cooling system (30) according to claim 1, wherein the variable threshold value is periodically updated in the first mode of operation, while kept fixed to a predetermined value in the second mode of operation.

3. The cooling system (30) according to claim 1 or 2 wherein when in the first mode of operation each cycle comprising of one OFF state and one ON state has a predetermined duration.

4. The cooling system (30) according to any one of claims 1 - 3, wherein the controller is configured to calculate the variable threshold value based on a hysteretic function such that when in the first mode of operation, the variable threshold is increased compared to when in the second mode of operation.

5. The cooling system (30) according to any one of claims 1 - 4, wherein the controller is configured to, when in the first mode of operation, continuously or periodically recalculate the duration of the of the OFF state and / or the ON state, respectively.

6. The cooling system (30) according to any one of claims 1 - 5, wherein the controller is configured to, when in the first mode of operation, continuously or periodically recalculate the compressor speed in the ON state.

7. The cooling system (30) according to any one of claims 1 - 6, wherein the controller is configured to, when in the first mode of operation, apply a minimum OFF state time.

8. The cooling system (30) according to claim 7, wherein the controller is configured to increase the speed of the compressor during the ON state when the application of the minimum OFF state time is determined to result in a reduced duration for the ON state.

9. The cooling system (30) according to any one of claims 1 - 8, wherein the controller is configured to, when in the first mode of operation, apply a minimum ON state time.

10. The cooling system (30) according to claim 9, wherein the controller is configured to reduce the speed of the compressor during the ON state when the application of the minimum ON state time is determined to result in an increased duration for the ON state.

11. A household appliance (10) comprising the cooling system according to any one of claims 1 - 10.

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