DK201470338A1 - A method for detecting instability in a refrigeration system - Google Patents

Abstract

A method for detecting instability in a refrigeration system (1) is disclosed. A control parameter of the refrigeration system (1) is continuously monitored, thereby obtaining a monitored signal (10). Online analysis is performed on the monitored signal (10) in order to determine whether or not the monitored signal (10) is oscillating. In the case that it is determined that the monitored signal (10) is oscillating, a time constant and/or an amplitude is/are derived from the monitored signal (10), and it is indicated that the monitored signal (10) is oscillating, and that a possible instability is occurring in the refrigeration system (1).

Description

A METHOD FOR DETECTING INSTABILITY IN A REFRIGERATION SYSTEM FIELD OF THE INVENTION

The present invention relates to a method for detecting instability in a refrigeration system comprising a compressor, a condenser, an expansion valve and an evaporator arranged in a refrigerant path. The invention further relates to a controller for performing the method, and to a refrigeration system comprising such a controller.

BACKGROUND OF THE INVENTION

Instabilities in a refrigeration system may originate from a mismatch between the dynamics of the refrigeration system and control of one or more components of the refrigeration system, for instance control of the expansion valve. Such a mismatch may, for example, occur when a setpoint of the refrigeration system, such as a temperature setpoint, is changed, or if a controller of the refrigeration system is improperly tuned.

Instabilities in a refrigeration system may lead to poor, or even unacceptable, controller performance, leading to an increase in energy consumption of the refrigeration system, wear of physical components, such as the expansion valve, due to frequent on-off cycles, risk of system damage, risk of liquid refrigerant in the suction line, etc. It is therefore desirable to avoid such instabilities.

When instabilities occur in a refrigeration system, one or more signals obtainable from the refrigeration system may be oscillating, in the sense that the signal is constantly changing periodically between maximum and minimum values. Accordingly, instability in a refrigeration system can sometimes be detected by detecting oscillations in an appropriate signal. US 4,848,099 discloses an adaptive refrigerant control algorithm. A superheat signal is monitored and provided to an input circuit of a control device. A differentiator within the control means provides a differentiation function to monitor the rate of change of the superheat signal with respect to time. The output signal from the differentiator is compared to a threshold slope, and an appropriate transfer function needed to induce proper opening and closing of the expansion valve Is determined on the basis of the comparison, in order to stabilize the superheat.

DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a method for detecting instability in a refrigeration system, which allows for easy and reliable detection of instabilities.

It is a further object of embodiments of the invention to provide a method for detecting instability in a refrigeration system, which can be performed without the need for heavy computer power.

It is an even further object of embodiments of the invention to provide a method for detecting Instability in a refrigeration system, which allows the system to be stabilized fast.

According to a first aspect, the invention provides a method for detecting instability in a refrigeration system, the refrigeration system comprising a compressor, a condenser, an expansion valve and an evaporator arranged in a refrigerant path, and a controller arranged to control an opening degree of the expansion valve, the method comprising the steps of: continuously monitoring a control parameter of the refrigeration system, thereby obtaining a monitored signal, - performing online analysis on the monitored signal in order to determine whether or not the monitored signal is oscillating, - in the case that it is determined that the monitored signal is oscillating: deriving a time constant and / or an amplitude from the monitored signal, indicating that the monitored signal is oscillating, and that a possible instability is occurring in the refrigeration system.

In the present context, the term 'refrigeration system' should be interpreted to mean any system in which a flow of fluid medium, such as refrigerant, circulates and is alternately compressed and expanded, thereby providing either refrigeration or heating of a volume. Thus, the refrigeration system may be a cooling system, a freezing system, an air conditioning system, a beat pump, etc.

The refrigeration system comprises a compressor, a condenser, an expansion valve and an evaporator arranged in a refrigerant path. Thus, refrigerant flow in the refrigerant path is alternately compressed by the compressor and expanded by the expansion valve. Heat exchange takes place in the condenser and the evaporator in such a manner that heat is rejected from the refrigerant in the condenser, and heat is consumed by the refrigerant in the evaporator. The expansion vaive is arranged upstream with respect to the evaporator, and thereby the flow of refrigerant towards the evaporator can be controlled by controlling an opening degree of the expansion vaive.

The refrigeration system further comprises a controller arranged to control an opening degree of the expansion vaive, and thereby the flow of refrigerant towards the evaporator. Accordingly, the controller is responsible for ensuring that the supply of refrigerant to the evaporator matches various ambient conditions as well as dynamics of the refrigeration system.

According to the method of the invention, a control parameter of the refrigeration system is continuously monitored, thereby obtaining a monitored signal. The monitored control parameter may be a processing signal used for controlling one or more components of the refrigeration system. For instance, the monitored control parameter may be a superheat value of refrigerant leaving the evaporator. The superheat value is often used to control the opening degree of the expansion vaive. Alternatively, the monitored control parameter may be a liquid level, such as a liquid level of a tank or an accumulator arranged in the refrigeration system. As another alternative, the monitored control parameter may be a refrigerant pressure obtained at a suitable position along the refrigerant path.

In the present context, the term 'continuous monitoring' should be interpreted to mean that values of the control parameter are obtained during operation of the refrigeration system, either in a constant manner or by repetitive sampling, e.g. every 10 seconds. Thus, the continuous monitoring of the control parameter ensures that a current value of the control parameter is available at all times during the operation of the refrigeration system.

Online analysis is performed on the monitored signal in order to determine whether or not the monitored signal is oscillating. In the present context the term 'online analysis' should be interpreted to mean that the analysis of the monitored signal is performed on the fly and as the monitored signal is obtained and during operation of the refrigeration system. Thus, the result of the performed analysis is also readily available during operation of the refrigeration system, and it is possible to immediately adjust the operation of the refrigeration system in order to take the result of the analysis into account, and possibly improve the performance of the refrigeration system. This will be described in further detail below.

In the case that the analysis reveals that the monitored signal is oscillating, a time constant and / or an amplitude ls / are derived from the monitored signal, and it is indicated that the monitored signal is oscillating, and that a possible instability is therefore occurring in the refrigeration system.

As mentioned above, a signal is oscillating if it comprises a component which is constantly changing periodically between maximum and minimum values. Thus, an oscillating signal exhibits a time constant, such as a typical duration of an oscillation period, and an amplitude, such as a typical difference between a mean value of the signal and extreme values of the signal. The time constant as well as the amplitude provides information regarding the nature of the oscillations of the monitored signal, and one or both of them may therefore be used to characterize the oscillations, and possibly to identify how to adjust the operation of the refrigeration system in order to remove the oscillations from the monitored signal This will be described in further detail below.

When it is indicated that the monitored signal is oscillating, this information may be communicated to an operator who may then determine whether or not to adjust the operation of the refrigeration system, and possibly perform the required adjustments. Alternatively or additionally, the information may be communicated directly to a controller, which then automatically adjusts one or more parameters in order to adjust operation of the refrigeration system.

The indication that the monitored signal is oscillating may, for example, include a flag being set, an alarm or a warning being generated for an operator and / or initiation of auto-tuning of one or more controllers of the refrigeration system.

It is an advantage of the method of the invention that the analysis step is performed online, because thereby the result of the analysis is readily available during operation of the refrigeration system, and it is therefore possible to adjust operation of the refrigeration system on the fly ', instead of having to wait for off-line analysis of the signal, which would delay the adjustment of the operation of the refrigeration system, thereby resulting in poor performance of the refrigeration system for a longer period of time.

Furthermore, the method of the invention provides an easy and reliable manner of detecting instability in a refrigeration system, and allows fast stabilization of the refrigeration system. Thus, the method of the invention does not require heavy or complex calculations. Furthermore, the method does not require heavy processing power, and a simple controller can be used instead of a device, such as a general purpose computer, which provides heavy processing power. This ensures that the refrigeration system is cost effective. Furthermore, a genera! purpose computer the processing power is divided among several tasks, and thereby the timing of the performed calculations may be unreliable. This is avoided when a simple, dedicated device, such as a controller, is used.

The method may further comprise the step of generating a warning to an operator of the refrigeration system in the event that it is determined that the monitored signal is oscillating.

According to this embodiment, the indication that the monitored signal is oscillating results in a warning, or an alarm, being generated, and possibly communicated to an operator. In response to the warning the operator may determine whether or not adjustments to the operation of the refrigeration system are required and, if this is determined, how the operation of the refrigeration system should be adjusted in order to remove the oscillations, thereby stabilizing the refrigeration system. The warning may, for example, include information regarding the nature of the oscillations and / or recommendations or suggestions for adjustments to be made to the operation of the refrigeration system. Such information may assist the operator in determining whether or not to perform adjustments, and which adjustments to perform.

The method may further comprise the step of selectively adjusting one or more parameters of the controller in accordance with the monitored signal, in the event that it is determined that the monitored signal is oscillating. As described above, the controller is arranged to control the opening degree of the expansion valve. Therefore, adjusting one or more parameters of the controller affects the control of the opening degree of the expansion valve, and thereby the supply of refrigerant to the evaporator. The adjustment may be performed automatically by the system in response to the detection of oscillations, in this case it may be selected whether or not to perform the adjustment on the basis of the derived time constant and / or amplitude of the oscillations. Alternatively, the adjustments may be performed manually by an operator. In this case, the operator determines whether or not to perform the adjustments.

The controller may be of a proportional integral (PI) or proportional integral (PI) type, and the step of selectively adjusting one or more parameters of the controller may comprise adjusting a time constant and / or a gain of the controller. Adjusting the time constant and / or the gain of the controller results in tuning the controller. In addition, the control of the expansion valve can be made to match the dynamics of the refrigeration system more closely, thereby stabilizing the refrigeration system.

For instance, the step of selectively adjusting one or more parameters of the controller may further comprise the steps of: comparing the time constant derived from the monitored signal to a current time constant of the controller, in the case that the derived time constant is larger than the time constant of the controller, adjusting the time constant of the controller, and in the case that the derived time constant is smaller than the time constant of the controller, adjusting the gain of the controller.

If the time is constantly derived from the monitored signal, i.e. the time constant of the oscillations is greater than the current time constant of the controller, this is an indication that the controller reacts faster than the dynamics of the refrigeration system. This may lead to instability of the refrigeration system, and is therefore undesirable. By adjusting the time constant of the controller in this case, the control of the expansion valve is made to follow the dynamics of the refrigeration system more closely. According to one embodiment, the time constant of the controller may be increased. For instance, the time constant of the controller may be adjusted to a value which is 1.5 times the previous time constant.

On the other hand, if the derived time constant is smaller than the time constant of the controller, then the time constant of the controller is probably well matched to the dynamics of the refrigeration system. However, since oscillations have been detected in the monitored signal, there is most likely an Instability in the refrigeration system. In order to remove this instability, the gain of the controller is adjusted. According to one embodiment, the gain of the controller may be decreased. For instance, the gain may be adjusted to a value which is 0.5 times the previous gain.

The step of selectively adjusting one or more parameters of the controller may be performed automatically. This may be referred to as 'auto-tuning'. According to this embodiment, the adjustment of the one or more parameters of the controller may also be performed without informing an operator of the refrigeration system. Thus, in the event that an instability of the refrigeration system is detected, the system simply auto-tunes the controller, thereby stabilizing the refrigeration system, and the operator simply experiences that the refrigeration system remains stable.

Alternatively, the step of selectively adjusting one or more parameters of the controller may be performed manually by an operator. In this case, a warning may be generated for the operator, as described above, in order to inform the operator that oscillations are present in the monitored signal, and that an instability may therefore be occurring in the refrigeration system. The warning may, for example, contain information regarding the nature of the detected oscillations, the time constant and / or amplitude derived from the monitored signal and / or one or more suggestions for how to adjust the one or more parameters of the controller In order to stabilize the refrigeration system, it is then up to the operator to decide whether or not to perform adjustments, which adjustments to perform, and when to perform the adjustments.

The step of performing online analysis on the monitored signal may comprise the steps of: detrending the monitored signal, thereby obtaining a detrended monitored signal, identifying local maxima and local minima of the detrended monitored signal, and determining the magnitude of said local maxima and local minima, comparing the magnitudes of the identified local maxima and local minima of the detrended monitored signal to a predefined band, and determining that the monitored signal is oscillating if a predefined number of successive iocai maxima and local minima are outside the predefined band.

According to this embodiment, a trend of the monitored signal is initially removed, Le. the monitored signal is detrended, in order to be able to identify the true oscillating behavior of the monitored signal. The detrended monitored signal represents the variations of the monitored signal with respect to a general increasing or decreasing trend of the monitored signal

Once a detrended monitored signal has been obtained, local maxima and local minima of the detrended monitored signal are identified. In the present context the term local maximum 'should be interpreted to mean a point in the detrended monitored signal, where the time derivative of the signal is zero, and where the signal values of points preceding and of points succeeding said point are lower than the signal value of said point. Similarly, in the present context the term 'local minimum' should be interpreted to mean a point in the detrended monitored signal, where the time derivative of the signal is zero, and where the signal values of points preceding and of points succeeding said point are higher than the signal value of said point. Accordingly, this step results in the identification of a number of positive (local maximum) and negative (local minima) peaks in the detrended signal.

Next the magnitudes of the identified iocai maxima and local minima of the detrended monitored signal are compared to a predefined band. In the present context the term 'band' should be interpreted to mean a range of signal values. The predefined band is a range of signal values within which the signal values of the detrended monitored signal should preferably be maintained. The magnitudes of the identified local maxima and local minima represent a typical amplitude of variations in the monitored signal, if such an amplitude exceeds a certain value, it may be an indication that the monitored signal is oscillating, and that an instability is occurring in the refrigeration system.

Thus, if the comparison of the magnitudes of the identified local maxima and local minima of the detrended monitored signal and the predefined band reveals that a predefined number of successive local maxima and local minima are outside the predefined band, then it is determined that the monitored signal Is oscillating. Thus, it may be acceptable that the magnitudes of a single or a few of the identified local maxima and local minima are outside the predefined band, but it is not acceptable that the magnitudes of the local maxima and local minima continue to be outside the predefined band. For instance, it may be acceptable that the variations of the monitored signal behave like a damped oscillation, where the amplitude is continuously decreasing.

According to one embodiment, the process above may be performed in the following manner. Once the monitored signal has been detrended, a number of successive peaks in the form of local maxima and local minima are identified in the signal, and the magnitudes of the identified peaks are compared to the predefined band. When it is detected that the magnitude of one of the identified peaks is outside the predefined band, this peak is labeled as the first peak. Then it is investigated whether or not the magnitudes of the two following peaks are also outside the predefined band. The two following peaks are labeled the second and the third peak, respectively. If this is the case, it is determined that the monitored signal is oscillating. On the other hand, if the magnitude of at least one of the second and the third peak is within the predefined band, then it is determined that the enlarged magnitude of the first peak is of an acceptable kind, and that the monitored signal is not osciilating.

According to the embodiment described above, three successive peaks are used to determine whether or not the monitored signal is oscillating. In the case that the first peak is a local maximum, the process requires two local maxima (the first peak and the third peak, respectively) and one local minimum (the second peak). Similarly, in the case that the first peak is a local minimum, the process requires two local minima (the first peak and the third peak, respectively) and one local maximum (the second peak). Thus, in this case the predefined number of successive local maxima and local minima is three. It should be noted that another predefined number of successive local maxima and local minima could be selected, such as two, four, five, six, or even a higher number.

The step of performing online analysis on the monitored signal may be performed continuously on a running window of obtained data. According to this embodiment, the online analysis is always performed on the latest data obtained, while the oldest data is discarded as new data is obtained. In addition, it is ensured that oscillations in the monitored signal can be detected fast. Furthermore, in the event that the analysis includes detrending the monitored signal as described above, the running window of obtained data ensures that sufficient data is available for performing appropriate detrending. According to this embodiment, the method may be performed in the following manner. When the refrigeration system is started, a period of time corresponding to the running window, is initially allowed to lapse, while data is obtained, before analysis of the monitored signal is initiated. Once the period of time has elapsed, the analysis is initiated on the data obtained during the period, while new data is continuously obtained and the oldest data is continuously discarded. For instance, the running window may be a window of approximately 30 minutes duration, but a running window of a longer or shorter duration could aiso be applied.

According to one embodiment, the monitored control parameter may be a liquid level in an accumulator arranged in the refrigerant path. The accumulator may, for example, be arranged in the refrigerant path between the expansion valve and the evaporator. In this case, two-phase refrigerant leaving the expansion valve enters the accumulator, where liquid refrigerant is separated from gaseous refrigerant, and only the liquid refrigerant is supplied from the accumulator to the evaporator.

The liquid level in the accumulator is an indicator of the balance between the amount of refrigerant supplied from the expansion valve and the amount of refrigerant passing through the evaporator. If more refrigerant passes through the evaporator than the amount of refrigerant being supplied from the expansion valve, then the liquid level in the accumulator decreases. On the other hand, if the expansion vaive supplies more refrigerant than the amount of refrigerant passing through the evaporator, then the liquid level in the accumulator increases. If the amount of refrigerant supplied from the expansion valve substantially matches the amount of refrigerant passing through the evaporator, then the liquid level in the accumulator is substantially constant, the refrigerant supply substantially matches the cooling load of the refrigeration system, and the refrigeration system is stack up.

If the liquid level in the accumulator oscillates, this is an indication that the control of the refrigeration system is not capable of keeping the refrigeration system stable, i.e. an instability is occurring in the refrigeration system. Accordingly, the liquid level in an accumulator is a suitable parameter for detecting an instability in the refrigeration system.

According to an alternative embodiment, the monitored control parameter may be a superheat value of refrigerant leaving the evaporator. The superheat value is the difference between the temperature of refrigerant leaving the evaporator and the dew point of refrigerant leaving the evaporator, it is normally desired to operate a refrigeration system in such a manner that a superheat value which is small but positive is obtained , because it is thereby obtained that the potential cooling capacity of the evaporator is utilized to the greatest possible extend, without risking that liquid refrigerant reaches the compressor, if the superheat value oscillates, this is an indication that the control of the refrigeration system is not succeeding in obtaining a stable superheat value, but is continuously 'hunting' the optimal superheat value without reaching it, oscillating about the optima! value. Accordingly, an instability is occurring in the refrigeration system. The instability may, in this case, be removed by adjusting the control of the expansion valve to such an extent that it matches the dynamics of the refrigeration system to a greater extent. Thus, the superheat value is a suitable parameter for defecting an instability of the refrigeration system.

According to a second aspect the invention provides a controller for controlling an opening degree of an expansion valve of a refrigeration system, the controller being arranged to perform the method according to the first aspect of the invention.

It should be noted that a person skilled in the art would readily recognize that any feature described in combination with the first aspect of the invention could also be combined with the second aspect of the Invention, and vice versa. Thus, the remarks set forth above are equally applicable here.

In the present context the term 'controller' should be Interpreted to mean a dedicated processing device used for performing control of the opening degree of the expansion valve. The controller could also be used for other processing tasks and / or controlling tasks of the refrigeration system, but it would normally not be a general purpose computer or a general purpose processor. The controller could, for example, be in the form of a Programmable Logic Controller (PLC), i.e. a standard component which is programmable to fulfill a desired purpose.

The controller may form part of a refrigeration system.

LETTER DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which

FIG. 1 is a diagrammatic view of a refrigeration system for applying a method according to a first embodiment of the invention,

FIG. 2 is a diagrammatic view of a refrigeration system for applying a method according to a second embodiment of the invention,

FIG. 3 is a graph illustrating part of a method according to an embodiment of the invention. FIG. 4 is a graph illustrating measurements obtained from a stable refrigeration system, and FIG. 5 is a graph illustrating measurements from an unstable refrigeration system.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a refrigeration system 1 for applying a method according to a first embodiment of the invention. The refrigeration system 1 comprises a compressor 2, a condenser 3, an expansion valve 4, a liquid separator 5 and an evaporator 6 arranged in a refrigerant path 7. Thus, refrigerant flow in the refrigerant path 7 is aberrantly compressed by the compressor 2 and expanded by the expansion valve 4, and heat exchange takes place in the condenser 3 and the evaporator 6, in such a manner that heat is rejected from the refrigerant in the condenser 3 and heat is absorbed by the refrigerant in the evaporator 6.

The expanded refrigerant leaving the expansion valve 4 is in the form of a mixture of liquid and gaseous refrigerant. This refrigerant is supplied to the liquid separator 5, where the liquid refrigerant is separated from the gaseous refrigerant. Liquid refrigerant Is supplied from the liquid separator 5 to the evaporator 6. The liquid refrigerant passing through the evaporator 6 is evaporated, while exchanging heat with an air flow across the evaporator 6, thereby providing cooling to a closed volume.

The refrigerant leaving the evaporator 6 is led back to the liquid separator 5, where any liquid refrigerant remaining is separated from the gaseous refrigerant, and the gaseous refrigerant is supplied to the compressor 2.

The liquid separator 5 is provided with a level sensor 8, which measures the liquid level. In the liquid separator 5. The level sensor 8 provides a monitored signal, which is analyzed in accordance with the method of the invention. As described above, the liquid level in the liquid separator 5 reflects whether or not the amount of refrigerant supplied by the expansion valve 4 matches the amount of refrigerant passing through the evaporator 6. If the liquid level oscillates, this is an indication that the control of the refrigeration system 1, notably the control of the opening degree of the expansion valve 4, Is not matching the dynamics of the refrigeration system 1, and that an instability of the refrigeration system i is therefore occurring. Thus, an instability of the refrigeration system 1 can be detected by determining that the liquid level in the liquid separator 5 is oscillating, and therefore the liquid level in the liquid separator 5 is a suitable parameter for detecting an instability of the refrigeration system 1.

FIG. 2 is a diagrammatic view of a refrigeration system 1 for applying a method according to a second embodiment of the invention. The refrigeration system 1 of FIG. 2 is similar to the refrigeration system 1 of FIG. 1 in that it comprises a compressor 2, a condenser 3, an expansion valve 4 and an evaporator 6 arranged in a refrigerant path 7. However, the refrigeration system 1 of FIG. 2 is not provided with a liquid separator. Instead, the refrigerant leaving the expansion valve 4 is supplied directly to the evaporator 6, and the refrigerant leaving the evaporator 4 is supplied directly to the compressor 2. A superheat value of refrigerant leaving the evaporator 6 is obtained at point 9 in the refrigerant path 7 The superheat value is the difference between the temperature of the refrigerant leaving the evaporator 6 and the dew point of the refrigerant leaving the evaporator 6. The superheat value may, for example, be obtained from simultaneous measurements of the temperature and the pressure of the refrigerant leaving the evaporator 6.

The obtained superheat value is provided as a monitored signal, which is analyzed in accordance with the method of the invention. Variations in the opening degree of the expansion valve 4 will normally result in variations in the superheat value, but the exact response in the superheat value following a given variation in the opening degree is difficult to predict, and will depend on other factors as well. If the superheat value oscillates, this is an indication that the control of the refrigeration system 1, notably the control of the expansion valve 4, is not successful in obtaining a stable superheat value, but is continuously 'hunting' the optimal superheat value without reaching it, oscillating about the optimal value. Accordingly, an instability is occurring in the refrigeration system 1. Thus, the superheat value is a suitable parameter for detecting an instability of the refrigeration system 1.

FIG. 3 is a graph illustrating part of a method according to an embodiment of the invention. The upper part of the graph of FIG. 3 shows an original monitored signal 10, when measured directly. The monitored signal 10 may, e.g., be a liquid level in an accumulator or a liquid separator, as described above with reference to FIG. 1. Alternatively, monitored signal 10 may be a superheat value of refrigerant leaving an evaporator, as described above with reference to FIG. 2. Alternatively, the monitored signal 10 may relate to any other suitable parameter obtained from a refrigeration system, and which is capable of indicating an instability of the refrigeration system due to oscillations occurring in the monitored signal 10.

The monitored signal 10 appears to be about a generally increasing trend. The trend has been identified and is indicated as straight line 11.

The lower part of the graph of FIG. 3 shows a detrended monitored signal 12, which is obtained by subtracting the trend signal 11 from the original monitored signal 10. It can be seen that the detrended monitored signal 12 varies between maxima and minima about zero. Even though the graph of FIG. 3 shows that the detrended monitored signal 12 varies about zero, it should be noted that the detrended monitored signal 12 could alternatively vary about a non-zero off-set value.

The detrended monitored signal 12 is then analyzed in order to determine whether or not the monitored signal 10 is oscillating, thereby indicating that an instability is occurring in the refrigeration system.

Fig. 4 is a graph illustrating measurements obtained from a stable refrigeration system. The graph shown in FIG. 4 could, e.g., be the detrended signal of FIG. Third

The detrended signal varies between maxima and minima about zero. The maxima and minima are identified. Notably, the first two maxima (Pi and P3) and the first minimum (P2) are identified, and their magnitudes are determined. The magnitudes of the identified maxima and minimum are compared to a predefined band, marked by the dotted lines. It can be seen that the magnitude of the first maximum, P-, and of the first minimum, P2, are outside the predefined band. However, the magnitude of the second maximum, P3, is within the predefined band. This indicates that, although the magnitudes of the variations of the detrended signal have exceeded the limits defined by the predefined band, the variations behave as a damped oscillation. It can therefore be concluded that the refrigeration system is stable, or will stabilize itself, and there is no reason for adjusting the control of the refrigeration system.

The analysis described above may, for example, be performed whenever it is detected that a maximum or a minimum has a magnitude which is outside the predefined band. Then that peak and the foliowing two peaks are analyzed as described above in order to determine whether it is acceptable that the first peak is outside the predefined band, or it can be concluded that the monitored signal is oscillating, and that an instability is occurring in the refrigeration system.

FIG. 5 Is a graph illustrating measurements from an unstable refrigeration system. The graph of FIG. 5 may also be a detrended signal, and the analysis performed on the signal is identical to the analysis described above with reference to FIG. 4. However, in the situation illustrated In FIG. 5, the third peak, P3, also has a magnitude which is outside the predefined band. This indicates that the signal is in fact oscillating, i.e. the variations of the signal are not damped. This in turn indicates that instability is occurring in the refrigeration system, and that adjustments to the control of the refrigeration system are therefore required in order to stabilize the refrigeration system.

Once it has been determined that the monitored signal is oscillating, a time constant and an amplitude of the signal are derived. The time constant is the period of the oscillations, and the amplitude is the magnitude of the peaks.

Claims (12)

1. A method for detecting instability in a refrigeration system (1). the refrigeration system (1) comprising a compressor (2), a condenser (3), an expansion valve (4) and an evaporator (6) arranged in a refrigerant path (7). and a controller arranged to control an opening degree of the expansion valve (4), the method comprising the steps of: - continuously monitoring a control parameter of the refrigeration system (1). thereby obtaining a monitored signal (10), performing online analysis on the monitored signal (10) in order to determine whether or not the monitored signal (10) is oscillating, in the case that it is determined that the monitored signal (10) is osciiiating: - deriving a time constant and/or an amplitude from the monitored signal (10), and indicating that the monitored signal (10) is osciiiating, and that a possible instability is occurring in the refrigeration system (1).

2. A method according to claim 1, further comprising the step of generating a warning to an operator of the refrigeration system (1) in the case that it is determined that the monitored signal (10) is osciiiating.

3. A method according to claim 1 or 2, further comprising the step of selectively adjusting one or more parameters of the controller In accordance with the monitored signal (10), In the case that it is determined that the monitored signal (10) Is oscillating.

4. A method according to claim 3, wherein the controller is of a proportional integral (PI) or proportional integral (PI) like type, and wherein the step of selectively adjusting one or more parameters of the controller comprises adjusting a time constant and/or a gain of the controller.

5. A method according to claim 4, wherein the step of selectively adjusting one or more parameters of the controller further comprises the steps of: comparing the time constant derived from the monitored signal (10) to a current time constant of the controller, in the case that the derived time constant Is larger than the time constant of the controller, adjusting the time constant of the controller, and in the case that the derived time constant is smaller than the time constant of the controller, adjusting the gain of the controller.

6. A method according to any of claims 3-5, wherein the step of selectively adjusting one or more parameters of the controller is performed automatically.

7. A method according to any of the preceding claims, wherein the step of performing online analysis on the monitored signal (10) comprises the steps of: - detrending the monitored signal (10), thereby obtaining a detrended monitored signal (12), identifying local maxima and local minima of the detrended monitored signal (12), and determining the magnitude of said local maxima and local minima, comparing the magnitudes of the identified local maxima and local minima of the detrended monitored signal (12) to a predefined band, and determining that the monitored signal (10) is oscillating if a predefined number of successive local maxima and iocai minima are outside the predefined band.

8. A method according to any of the preceding claims, wherein the step of performing online analysis on the monitored signal (10) is performed continuously on a running window of obtained data,

9. A method according to any of the preceding claims, wherein the monitored control parameter is a liquid level in an accumulator (5) arranged in the refrigerant path (7).

10. A method according to any of claims 1-8, wherein the monitored control parameter is a superheat value of refrigerant leaving the evaporator (6).

11. A controller for controlling an opening degree of an expansion valve (4) of a refrigeration system (1), the controller being arranged to perform the method according to any of claims 1-10.

12. A controller according to claim 11, wherein the controller forms part of a refrigeration system (1).