Method, system and device for identifying oscillations in a regulation

Abstract

The invention relates to a method, a system and a device for detecting a cause of an oscillation in a control circuit. The method comprises the following steps: determining a target value (w) and an actual value (x) of a control valve with respect to time. The setpoint value (w) and the actual value (x) are then checked whether there is a permanent oscillation. If it is determined that there is oscillation, the amplitude and the cycle time are determined. If only the actual value (x) or only the target value (w) oscillates, the cycle time is compared with a characteristic cycle time of the control valve. If the actual value (x) and the target value (w) oscillate, the amplitudes are compared with each other and / or the cycle time is compared with the characteristic cycle time of the control valve. Oscillation causes are identified from the comparison. Oscillation causes can be delineated in the regulating loop of the regulating valve by using the method, so that the equipment fault checking expenditure is reduced. Production accidents can thus be reduced to the minimum and costs can be avoided.

Description

Technical Field

[0001] The invention relates to a method for identifying the cause of oscillations in a control circuit of a control valve of a regulated process system.

[0002] Control valves are used in technical plants to control process parameters such as flow rate, pressure, differential pressure, temperature, pH value, etc. For this purpose, the control valve is connected to the process and the controller assigned to the process in such a way that a cascade control is present (see Figure 1 ). The controller assigned to the process is used to bring the process parameter or the associated process actual value closer to the process setpoint value. To this end, the controller assigned to the process calculates an output variable in the cascade structure as a setpoint value for the control valve spool, for example a position setpoint value. The control circuit of the control valve forms the inner control circuit of the cascade (see Figure 2 ) and generates, based on the control valve setpoint value, an actual value, for example, for the valve core position, which in turn is an input parameter for the actual process.

[0003] One of the main requirements for control loops is to ensure stable overall system behavior. For process control, this means that the process setpoint value is set and, apart from tolerable deviations, it should correspond to the corresponding process parameters. Undesirable unstable system behavior can manifest itself, for example, as permanent signal oscillations within the control loop. These can occur, for example, because the control valve control signal itself has oscillations that occur due to changes in one or more process variables or because the control valve introduces oscillations that are caused, for example, by unstable position control or different frictions.

[0004] In the case of unstable cascade control loops in process plants, such permanent oscillations must be eliminated, since otherwise, for example, out-of-specification products could be produced, and production batches would have to be rejected or corrected. To do this, maintenance personnel must find the cause of the error on site in the plant. The cause may lie in one of the components of the cascade, i.e. the process controller, the valve controller, the physical properties of the valve or its drive, or in the process itself. Background Art

[0005] Various evaluation methods and devices for detecting control loop oscillations are known, but they are often not suitable for cascade structures.

[0006] Thus, for example, a fault diagnosis device and a corresponding method for monitoring the operation of a control loop with a networked structure are known from the publication US 2016 / 0239015 A1. The fault diagnosis device receives a data string of a setpoint value and an actual value of the control loop. A random characteristic related to the distribution of the actual value and the setpoint value is determined, so that the control quality can be evaluated from their quotient.

[0007] Publication US 2007 / 0150079 A1 describes a self-testing process control loop for a process system, wherein signal data from normal operation of components of the process control loop are measured for fault diagnosis, from which control loop parameters are determined that allow conclusions to be drawn about the state of the process control loop.

[0008] But in both cases it is not possible to determine the possible causes of this unstable behavior, poor regulation quality or oscillation. Summary of the invention

[0009] Task

[0010] The object of the present invention is to provide a method and a device which allow the causes of oscillations to be determined or at least localized in a control circuit of a control valve in a process system which is itself regulated.

[0011] Solution

[0012] The use of the singular should not exclude the plural and vice versa, unless otherwise indicated.

[0013] Some method steps will be described in detail below. In a preferred variant of the present invention, the steps are performed in the specified order. However, the steps do not necessarily have to be performed in the specified order, and the method to be described may also have other steps not mentioned.

[0014] In order to achieve this object, a method is proposed for identifying the cause of at least one oscillation in a process plant, the process plant comprising at least one inner control circuit and at least one outer control loop of a control valve, the method comprising the following steps:

[0015] - The setpoint value and the actual value of the internal control circuit of the control valve are determined over time. This control circuit regulates the position of the valve core, ie the setpoint value and the actual value influence exactly this position.

[0016] The setpoint value determined over time and the actual value determined over time are then each checked for oscillations, ie in particular for permanent oscillations.

[0017] If at least one oscillation is determined, its amplitude and / or period time is determined. As an alternative to the period time, the frequency can of course also be determined, since these two parameters can be easily converted into one another.

[0018] If only the actual value or only the setpoint value is found to have oscillations, the period time and / or amplitude of the oscillations are compared with predetermined characteristic parameters of the control valve. For this purpose, a switching hysteresis may be provided to avoid excessively rapid and / or frequent changes between states.

[0019] If oscillations are detected for both the actual value and the setpoint value, a phase shift of the two oscillations relative to one another is determined and / or the respective amplitudes are compared and / or at least one of the cycle times is compared with a predetermined characteristic parameter of the control valve.

[0020] - identifying a possible cause of the at least one oscillation from the performed comparison and / or the phase shift.

[0021] The identified possible oscillation causes are preferably output, for example as a diagnosis and / or a message. The output can be made directly on the control valve or, for example, via an interface on the process plant control system or other monitoring system. The identified possible cause of the at least one oscillation can also be stored on a data carrier. This makes it possible to use the knowledge gained at a later stage, for example as part of a planned maintenance of the system or the control valve.

[0022] The method makes it possible to determine or at least localize the cause of oscillations in the control valve control circuit, so that the expenditure on the part of maintenance personnel in the plant, which may require on-site inspection of errors, can be kept low, since the cause of the problem can be targeted and located at the right point. If possible, the method identifies whether the instability or oscillation is caused by the process control or the valve control. Production accidents can be minimized in this way and unnecessary costs can be avoided.

[0023] The oscillations to be identified by the method preferably exceed some oscillation processes in the signal. The oscillations to be identified are preferably substantially stable or repetitive and in particular do not decay during the period of occurrence of the oscillations. The number of oscillation processes of the oscillations to be identified is preferably greater than 20, preferably greater than 30 and particularly preferably greater than 40. In particular, the oscillations to be identified are formed as instabilities of the signal of the control loop or loops. The oscillations to be identified are preferably permanent oscillations. It is conceivable that the oscillations to be identified consist of a plurality of oscillations, wherein in particular a plurality of causes are possible for these oscillations.

[0024] Preferably, the identified possible cause of at least one oscillation is assigned to the inner control loop or the outer control loop. This advantageously makes it possible to easily diagnose and / or eliminate the cause.

[0025] In addition to the setpoint values ​​and actual values ​​of the control circuit of the control valve, preferably no other parameters are measured and / or considered for identifying the cause of at least one oscillation, which are only available at the control valve. The parameters available at the control valve are generally the drive pressure and the air inlet pressure (in the case of a control valve with a pneumatic drive), the structure-borne noise at the valve, the control signal (air pressure or current) and their respective time stamps. This allows the method to be run without problems, for example, on a position controller of a control valve, without requiring additional ports or without the need for appropriate configuration of any other device. Furthermore, no additional sensors or the like are required.

[0026] Incorrect or unreliable diagnoses can be avoided by determining whether the setpoint values ​​and actual values ​​of the control circuit of the control valve have noise and only performing further steps if the noise is below a predetermined threshold value. This prevents, for example, the determination of an erroneous oscillation characteristic due to a noisy signal. Noise detection is therefore preferably performed immediately before the check for the presence of permanent oscillations.

[0027] Simplified conditions and lower maintenance costs can be achieved if the method additionally provides that at least one identified possible cause of the oscillation is automatically eliminated by adjusting parameters in the valve position control. p If it is too large, it can be automatically reduced, with the concern being whether the permanent oscillations are damped. The amplification parameter k for the integral part of the position control can be i If the permanent oscillations decay, the problem is solved and maintenance, for example by a technician, is no longer necessary.

[0028] The method can be carried out with particularly low demands on computing power and memory if the check of whether the setpoint and / or actual value has oscillations includes local extreme values ​​of the setpoint and / or actual value determined over time within a predetermined time interval and / or is evaluated. The determination of the local extreme values ​​ultimately requires only low computing power. For example, the cycle time can be inferred from the spacing of successive local maxima or minima, while the amplitude can be obtained from the difference between two successive local maxima and minima. Whether there is a permanent oscillation can be detected, for example, by whether the detected amplitude is constant or at least does not decrease within a predetermined error from one predetermined time interval to the next.

[0029] Preferably, if only the setpoint value is determined to have oscillations, a diagnosis and / or maintenance of the control valve is requested. Since the actual value does not follow the oscillations of the setpoint value, the cause is a defect in the valve or its drive or maintenance is required. If the control valve has a position controller with an on-board diagnostic function, it can be used for this purpose and, for example, automatically triggered. The results of the diagnosis and / or maintenance, if any, can be used to improve the identification of the cause of the at least one oscillation. If such a diagnostic state is present, a comparison of the identified cause of the oscillation with the valve diagnostic state can be performed here.

[0030] In a preferred improvement of the method, a characteristic cycle time or frequency of the control valve is used as a predetermined characteristic parameter. If it is determined that only the actual value has oscillations and / or the actual value oscillation amplitude is greater than the theoretical value oscillation amplitude and / or the cycle time or frequency of at least one oscillation corresponds to the characteristic cycle time or frequency of the control valve, then the setting of the control valve regulation that is unfavorable to the operation of the equipment is identified as a possible cause of at least one oscillation. As an alternative to the cycle time, the oscillation frequency can obviously also be considered. The characteristic cycle time is preferably determined here from the valve core running time when fully open or fully closed, that is, from the time required for the control valve to be fully opened and fully closed. The individual performance of each valve is generally determined or ascertained when the valve is set up or put into use. The characteristic cycle time therefore depicts the oscillation at the maximum possible speed at which the valve core can move. This is generally significantly higher than the speed of the oscillation caused, for example, by an erroneous process regulation.

[0031] If the control of the control valve has a proportional part (P part) and / or an integral part (I part), a more accurate diagnosis can be obtained in the following way: It should be taken into account that the control of the control valve may also have other parts, such as a differential part. A so-called PID controller is usually used, which has all the mentioned control components. All possible combinations of the control parts are considered here. If, as already described, an unfavorable setting of the control valve control is identified as a possible cause of at least one oscillation, the parameters of the proportional part of the control valve control that are unfavorable to the operation of the control valve are identified as possible causes of oscillations that can be refined when the oscillation cycle time is less than a predetermined characteristic cycle time and / or the frequency is greater than a predetermined characteristic frequency and / or the phase shift between the oscillation of the actual value and the theoretical value is below a predetermined threshold. The parameters of the integral part of the control valve control that are unfavorable to the operation of the control valve are identified as possible causes of oscillations that can be refined when the oscillation cycle time is greater than a predetermined characteristic cycle time and / or the oscillation frequency is less than a predetermined characteristic frequency and / or the phase shift between the oscillation of the actual value and the theoretical value is above a predetermined threshold. Larger or smaller means here and below that, taking into account error factors, one of the multiple parameters to be compared is in each case multiplied by a predetermined factor, which ensures that the respective comparison with a sufficient distance is fulfilled.

[0032] In a preferred development of the method, a characteristic cycle time or frequency of the control valve is used as the predetermined characteristic parameter as described above. If it is determined that only the setpoint value oscillates and / or the setpoint value oscillation amplitude is greater than the actual value oscillation amplitude and / or the cycle time or frequency of the setpoint value oscillation is greater than the characteristic cycle time of the control valve or the setpoint value oscillation frequency is less than the control valve characteristic frequency, a control problem of the process plant is identified as a possible cause of the oscillation.

[0033] The hysteresis width of the regulating valve and in particular the drive, the regulation of the regulating valve and / or the valve core can also be used as a predetermined characteristic parameter. The hysteresis is very significant in the case of pneumatic drives. Preferably, when it is determined that only the setpoint value oscillates, the oscillation amplitude is compared with the predetermined characteristic parameter, wherein diagnosis and / or maintenance of the regulating valve is requested only when the oscillation amplitude exceeds the width of the combined hysteresis. It can thus be taken into account that the actual value may not follow the oscillation of the setpoint value because its amplitude is too small and the valve drive cannot perform a corresponding movement. In this case, there is no indication that the valve is disturbed and / or defective, so diagnosis and / or maintenance of it is not necessary.

[0034] If an acceleration sensor and / or a structure-borne sound sensor is provided, a further possible cause of the oscillations can be identified. These sensors detect or measure tremors and vibrations within the system. In the manner described, the signals of the acceleration sensor and / or the structure-borne sound sensor are then checked for oscillations. If oscillations are detected, their period time and / or frequency are determined. If oscillations are detected in the actual value and in the signal of the acceleration sensor and / or the structure-borne sound sensor, but not in the target value, the period time and / or frequency of the oscillations are compared with one another. If the period time and / or frequency of the oscillations in the actual value and in the signal of the acceleration sensor and / or the structure-borne sound sensor are identical within a predetermined error, a vibration within the system is identified as a possible cause of the at least one oscillation.

[0035] If a process actual value is provided, a further possible cause of oscillations can be identified. In the method, the process actual value is subsequently checked for oscillations, wherein if an oscillation is determined, its period time and / or frequency are determined. If an oscillation is determined for the actual value and the process actual value, and no oscillation is determined for the setpoint value, the period time and / or frequency of the oscillations are compared with one another. If the period time and / or frequency of the oscillations in the actual value and the process actual value are identical within a predetermined error, a pulsating fluid flow in the device is identified as a possible cause of the at least one oscillation.

[0036] If a process setpoint value is additionally provided, an expected oscillation which does not require maintenance measures can be identified. It is then checked whether there is an oscillation. If an oscillation is determined, its period time and / or frequency are determined. If an oscillation is determined for the actual value, the setpoint value and the process setpoint value, the period time and / or frequency of the oscillation are compared. If the period time and / or frequency of the oscillation in the actual value, the setpoint value and the process setpoint value are identical within a predetermined error, the at least one oscillation is identified as expected.

[0037] Furthermore, the object is achieved by a position controller for regulating a valve, wherein the position controller comprises a computing unit and a device suitable and set up for carrying out the steps of the method as described above. The device comprises, for example, a position sensor for measuring the actual position of the valve core. The computing unit can be, for example, a control electronics, a digital signal processor (DSP), a microcontroller, a computer or a plurality thereof networked together with corresponding programming. The programming can be realized, for example, within the scope of a fixed circuit arrangement of the control electronics, DSP and / or microcontroller or by means of a field programmable gate array (FPGA).

[0038] This object is also achieved by a diagnostic box for use with a control valve, wherein the diagnostic box, like the position controller just described, comprises a computing unit with means suitable and designed to carry out the steps of the method described above. Such a diagnostic box can be used directly by a maintenance technician when troubleshooting a process plant, perhaps even on a control valve that only has a position controller that does not adequately meet the above requirements. Such a diagnostic box can also be retrofitted.

[0039] This object is also achieved by a control valve having a position controller as described above.

[0040] Furthermore, the object is achieved by a computer program which contains instructions which cause the above-mentioned position controller and / or the above-mentioned diagnostic box to execute the method steps according to the method described further above.

[0041] This task is also accomplished by a computer readable medium having stored thereon the computer program just described.

[0042] Further details and features are derived from the following description of preferred embodiments in conjunction with the accompanying drawings. In this case, the respective features can be implemented individually or in combination with one another. The possible ways of accomplishing the task are not limited to the described embodiments. Thus, for example, an interval description always includes all unmentioned intermediate values ​​and all conceivable sub-intervals. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The embodiments are schematically shown in the drawings. The same reference numerals in these drawings represent the same or functionally identical components or components corresponding to each other with respect to their functions, in particular:

[0044] Figure 1 A schematic diagram of a cascade consisting of a process control loop and a control valve is shown;

[0045] Figure 2 Shows Figure 1 Schematic diagram of the regulating circuit of the regulating valve;

[0046] Figure 3AThe time diagram of the actual value and the setpoint value of the outer control loop for a simulation example is shown, in which no permanent oscillation occurs;

[0047] Figure 3B shows a time diagram of actual and setpoint values ​​of an inner control loop for a simulation example, in which no permanent oscillation occurs;

[0048] Figure 4A The time diagram of actual and setpoint values ​​for an outer control loop is shown for a simulation example, in which the valve control results in permanent oscillations;

[0049] Figure 4B The time diagram of actual and setpoint values ​​of an inner control loop is shown for a simulation example, in which the valve control results in permanent oscillations;

[0050] Figure 5A Shows Figure 4A An enlarged portion of the curve graph;

[0051] Figure 5B Shows Figure 4B An enlarged portion of the curve graph;

[0052] Fig. 6A A time diagram of actual and setpoint values ​​for an outer control loop is shown for a simulation example, in which the process control causes permanent oscillations;

[0053] Figure 6B shows a time diagram of actual and setpoint values ​​of an inner control loop for a simulation example in which the process control leads to permanent oscillations;

[0054] Fig. 7A Shows Fig. 6A An enlarged portion of the curve graph;

[0055] Figure 7B Shows Figure 6B An enlarged portion of the curve graph;

[0056] Figure 8 A process diagram is shown for distinguishing one possible situation when both the actual value and the setpoint value of the valve position control oscillate;

[0057] Fig. 9 An overview of the entire method is shown.

[0058] Glossary

[0059] equipment

[0060] A plant is a fully planned combination of technical components. The components may include machines, instruments, devices, storage devices, pipelines or conveyor sections and / or control or regulating elements. They may be interconnected, wired or logically connected in terms of function, control technology and / or safety technology. Plants are operated for a wide variety of purposes in many different fields, including, for example, process plants or process technology plants, which in many cases belong to the chemical industry. The term plant also includes refineries, district heating systems, geothermal or solar thermal plants, food production plants, fresh water supply plants or wastewater discharge plants, biogas plants, etc.

[0061] Low pass filter cutoff frequency

[0062] The limit frequency of a low-pass filter is generally defined as the frequency at which the output signal A is attenuated by 3 dB compared to the sinusoidal input signal E. This corresponds to A=0.707*E.

[0063] Regulation loop

[0064] The control loop (or control loop) has a regulator, a control section and a feedback loop. The control section acts on the controlled parameter. A theoretical value w is set for this. The actual value x of the controlled parameter is measured. The regulator determines the control parameter y from the actual value x and the theoretical value w according to the desired dynamics of the control loop, which acts on the controlled parameter through the control section in order to make the actual value x close to the theoretical value w.

[0065] Regulation with integral part

[0066] Integral controllers (also abbreviated as I controllers) are used in order to completely isolate the control deviation at every operating point. The control difference or control deviation e is here the difference between the setpoint value w and the actual value x: e=wx. As long as the control deviation is not equal to zero, the value of the control parameter y changes. The control is only stable when the controlled variable, i.e. the setpoint value w and the actual value x, are the same, but at the latest the control parameter reaches its limit value determined by the system (e.g. the maximum voltage). The mathematical expression of the integral characteristic is that the value of the control parameter y is proportional to the time integral of the control difference e:

[0067] y=k i ∫edt

[0068] Here, the amplification parameter k i It is usually defined as the inverse of the integration time. Usually, a dead zone is provided before the integral part of the regulator, for example in the form of a characteristic curve.

[0069] Regulation with proportional part

[0070] In the case of a proportional controller (also abbreviated as P controller), the control variable y is always proportional to the control difference e (difference between setpoint value and actual value) obtained. As a result, the P controller reacts to the control difference without delay and generates the control variable y when such a deviation exists. The amplitude of the control variable depends on the control difference e and the proportionality factor k, which is also called the amplification factor. p value:

[0071] y=k p *e

[0072] The effect of the disturbance variable on the compensation controller is such that it generates a correspondingly inverse control variable. This control variable can only be generated by the P controller when there is a control deviation. Therefore, the remaining disturbances cannot be completely eliminated by the P controller, and a residual control deviation always remains. Large k p Because of the stronger regulatory intervention, the smaller the regulation deviation is. However, too large k p The value increases the tendency of the regulation loop to form oscillations.

[0073] Control valve

[0074] Control valves, also called process valves or regulating valves, are used to throttle or regulate fluid flows. To this end, the valve core is moved in the flow opening of the valve seat by means of a drive. This allows the flow opening to be opened or closed, whereby the flow volume is varied until the flow opening is fully opened or fully closed. Usually, pneumatic or electric drives are used for this purpose.

[0075] Position regulator

[0076] A position controller is a component of a regulating or control valve that actuates a drive of a valve element in order to open or close the valve. A position controller contains a control circuit in order to adjust the position of the valve element as a function of set conditions, for example a signal from a control console. The valve element drive (in many cases an electric or fluid drive, the latter of which can be hydraulically actuated or operated with compressed air) is part of the control circuit and is therefore clearly assigned to the position controller, even if the drive is located outside the actual position controller.

[0077] Reference numerals

[0078] 100 external regulation loop

[0079] 110 process regulator

[0080] 120 Internal regulation loop

[0081] 130 Process

[0082] 210 Valve Positioner

[0083] 230 Control valve

[0084] 240 Control valve drive device

[0085] 900 Situation Distinction

[0086] 910 Maintenance Requirement (Non-Specific)

[0087] 920 No disposal required

[0088] 930 Control Valve Maintenance Requirements

[0089] 940 Process Adjustment Maintenance Requirement

[0090] p Actual value of process regulation

[0091] r Theoretical value of process regulation

[0092] w Theoretical value of valve position adjustment

[0093] x Actual value of valve position control

[0094] y Control parameters for valve position control

[0095] The amplitude of the permanent oscillation of W w

[0096] Amplitude of the perpetual oscillation of X

[0097] T x x Oscillation period

[0098] T c Characteristic cycle time associated with the control valve

[0099] v max Maximum moving speed of valve core

[0100] References

[0101] Patent literature cited:

[0102] US2007 / 0150079 A1

[0103] US2016 / 0239015 A1 DETAILED DESCRIPTION

[0104] Figure 1The wiring diagram of the process control loop forming the outer control loop 100 of the cascade is shown, which has a process controller 110, to which the setpoint value r for the process parameter to be regulated is set. The process controller 110 generates a setpoint value w for the control valve position as a control parameter, which is regulated in the inner control loop 120 of the cascade. The actual value x of the control valve position affects the process 130 due to the cascade regulation. The process parameter to be regulated is thus changed, which is measured and provided to the process controller 110 as an actual value p. The elements associated with the outer control loop 100 of the cascade are highlighted by a dashed frame.

[0105] The internal control loop 120 formed by the position control device of the control valve 230 is Figure 2 2 is shown by a dashed box. The setpoint value w for regulating the valve position predetermined by the process controller 110 serves as an input parameter for the valve position controller 210. The valve position controller 210 generates a control variable y from it, which influences the drive 240 of the regulating valve 230, generally the current intensity or the air pressure, depending on the type of the current valve drive 240. Accordingly, the valve core position of the regulating valve 230 changes, which is used by the valve position controller 210 as the actual value x.

[0106] The position regulator of the control valve in such a process control device usually only provides the theoretical value w and the actual value x of the valve core position. In addition, there may be a diagnostic function available, which can analyze the current state of the control valve and store the results of this diagnosis in the protocol, such as possible causes of errors. There may also be certain characteristic parameters of the control valve, such as the valve operating time, which indicates how long it usually takes to open and close the valve. Other data, such as the theoretical value r and the actual value p of the process control, are generally not available to the position regulator of the control valve. In order to determine the cause of instability or permanent oscillation in the cascade control despite this, the following steps can be specified:

[0107] - Evaluate the signal noise and determine the signal noise of the setpoint value w and the actual value x of the valve slide position. If the checked signal has excessive noise, it should not be analyzed further, because in particular the oscillation behavior cannot be determined without error. Instead, a message is preferably issued and / or stored indicating that the method cannot be performed due to excessive signal noise.

[0108] An oscillation analysis is performed to detect whether oscillations of the actual value x and the setpoint value w of the valve position are present and to determine characteristic oscillation parameters, ie preferably amplitude and period time and possibly also a phase shift of the oscillations with respect to one another.

[0109] - The identified system characteristics are differentiated into:

[0110] Case 1: No permanent oscillation is identified and no action is required.

[0111] Case 2: Permanent oscillation of the actual value x only. The cause may be found in the valve control. The characteristic oscillation parameters are compared with the characteristic parameters of the control valve.

[0112] Case 3: Only permanent oscillation of the setpoint value w. The cause of the oscillation can be found in the process control. Since there is no oscillation of the actual value, the valve does not follow the oscillation. Therefore, there may be a blockage on the valve. Therefore, it is possible to call up the diagnostic function or request a check of the valve.

[0113] Case 4: Both the setpoint value w and the actual value x have permanent oscillations, in which case a more precise check is required, so that characteristic oscillation parameters of the setpoint value and actual value are compared with one another and possibly with characteristic parameters of the control valve.

[0114] - generating a diagnostic message for the detected situation, which may be output and / or stored.

[0115] The oscillation analysis can be performed, for example, with the aid of Fourier analysis. However, the following approach is preferred, which is sufficient with much smaller requirements for computing power and memory space. By evaluating the number and / or interval of local extreme values ​​of the timing of the theoretical value w and the actual value x within a predetermined time interval Δt, for example, a flag, i.e. a Boolean parameter, is set, which indicates whether the current signal has permanent oscillations. Whether the permanent oscillation continues to exist within the next time interval Δt is identified by whether the amplitudes of the signals respectively identified as oscillations remain approximately equal. This also requires checking. In the case of a permanent oscillation, the amplitude and cycle time of the identified oscillation are determined. If permanent oscillations are identified in both the theoretical value w and the actual value x, the phase shift between the two oscillations may also be determined.

[0116] When distinguishing between the individual cases, the most complex case exists if both the setpoint value w and the actual value x of the valve position have permanent oscillations. In this case, considering the amplitudes of the two oscillations can provide an indication of their possible causes.

[0117] In this case, it can be used that both the control valve control loop and the process control have overall low-pass filter characteristics, ie the oscillation amplitudes of the actual values ​​of the respective control loop are attenuated at higher frequencies compared to the setpoint value amplitudes.

[0118] In this way, two cases can be distinguished:

[0119] If the control valve couples permanent oscillations into the overall control loop, the actual value x oscillation amplitude is attenuated above the limit frequency by the strong low-pass filter characteristics of the process control, and is therefore fed back to the setpoint value w with a smaller amplitude.

[0120] If the process controller causes permanent oscillations and couples them into the valve position control, the resulting oscillation amplitude of the setpoint value w can be damped by the low-pass filter performance of the valve position control, and in this case there is a smaller oscillation amplitude of the actual value x. However, it is also possible that the frequency of the permanent oscillation in the process control loop is sufficiently below the limit frequency of the low-pass filter performance of the valve position control loop, and the actual value x can follow the setpoint value w. In this case, both have the same amplitude.

[0121] exist FIG. 3A to FIG. 7B , a simulation of the behavior of actual and setpoint values ​​for a process control loop and a control valve position control loop for different situations is shown by way of example. This shows how the overall oscillation situation can be inferred from the behavior of the actual value x and the setpoint value w of the valve position.

[0122] Figure 3A and Figure 3B The desired situation is shown, when neither the process control loop nor the control valve position control loop has permanent oscillations. In this example, the setpoint value r of the process variable changes from 0% to 25%. Figure 3A The change process is shown by the dashed line. The jump response of the actual value p of the process variable is presented by the solid line. It can be seen that the actual value approaches the theoretical value and the previously existing oscillations are significantly damped when approaching the theoretical value. There is no permanent oscillation.

[0123] exist Figure 3B The corresponding course of the setpoint value w (dashed line) and the actual value x (solid line) of the valve position can be seen in the figure. In this example, the valve position at which the process variable reaches 25% of the desired value is 50%, i.e. the flow opening is half open. The entire cascade consisting of the inner and outer control loops is stable.

[0124] Figure 4A and Figure 4B The same basic situation is shown, i.e. the setpoint value r of the process variable jumps from 0% to 25%. However, in this case, the cascade consisting of the outer control loop and the inner control loop is unstable, because the valve position controller inflow causes permanent oscillations due to the excessive amplification of its proportional component (P component). In addition, the diagram corresponds to Figure 3A and Figure 3B .

[0125] Figure 5A and Figure 5B Shows Figure 4A and Figure 4B A zoomed-in section of the 300-340 s region. In this region, any possible oscillations have decayed, so only permanent oscillations can be seen. Figure 5A and Figure 5B In the comparison, it can be seen that the control valve, that is, the inner control loop, oscillates, but the process control, that is, the outer control loop, does not oscillate. Figure 5BIt can also be seen that the actual value x oscillates with a greater amplitude than the setpoint value w. Thus, according to the above embodiment, the oscillations are caused by the position control of the control valve or the valve core. This process, i.e. the outer control loop, is hardly affected by the oscillations in this example, as can be seen from Figure 5A As seen in.

[0126] exist Fig. 6A In the example above, the theoretical value r of the process variable also jumps from 0% to 25%. Fig. 6A and Figure 6B The diagram corresponds to Figure 3A and Figure 3B As can be seen in Fig. 6A and Figure 6B As can be seen in Figure 1, the cascade consisting of the inner and outer control loops is unstable in this case. Since the setpoint value w and the actual value x follow the setpoint conditions of the process controller with almost the same amplitude, the cause of the permanent oscillation may lie in the process controller. It may, for example, have an excessively strong amplification of its control proportional part.

[0127] Fig. 7A and Figure 7B It also shows Fig. 6A and Figure 6B Magnified detail of the 300-340 second region of the valve position. At this higher resolution it can be clearly seen that in this case the oscillation amplitude of the setpoint value w of the valve position is greater than the oscillation amplitude of the actual value x. This also indicates that the cause of the permanent oscillation is in the process control loop.

[0128] If you will Figure 5B and Figure 7B A comparison will show that, as an alternative to the differentiation criterion, the period time of the oscillation in question can also be taken into account. Figure 5B The oscillations shown, which are caused by the relatively fast regulation of the control valve, have a cycle time of approximately 3 seconds in this example, whereas Figure 7B The oscillation shown, which is caused by a relatively slow process control, exhibits a cycle time of approximately 15.5 seconds. Knowing the respective characteristic behavior of the control loop, it is easy to set limits by which the described cases can be clearly distinguished.

[0129] Figure 8 The process diagram shows how different possible situations can be distinguished from one another, in which both the actual value x and the setpoint value w of the control valve position control have permanent oscillations (also referred to as situation 4 above and below).

[0130] The magnitudes of the parameters x, w are in this case denoted by capital letters X, W. They have the same units as the parameters x and w, ie in typical applications they are stated in %.

[0131] A possible check whether it is the expected permanent oscillation (case 4d) is based on Figure 8before the process.

[0132] First in Figure 8 Check whether the amplitude X of the actual value x of the valve position is greater than the amplitude W of the theoretical value w.

[0133] Preferably, all such comparisons are carried out with the aid of factors f (f1, f2, f3, f4) which define suitable error thresholds. This ensures that the comparisons are carried out reliably and always provide unambiguous results. As an alternative or in addition thereto, switching hysteresis can be provided in order to avoid too rapid and / or frequent changes between states. These can also be taken into account for setting the size of the error factors, if necessary.

[0134] If the amplitude X is now greater than f1*W, it can be assumed that the cause of the permanent oscillation lies in the position control of the control valve. In order to be able to indicate the cause of the oscillation even more precisely, the oscillation period time T of the valve position actual value x is now further checked. x Is it significantly greater than the characteristic cycle time T assigned to the control valve? c (T c Depends on X and v max , which will be explained below). If this is not the case, i.e. the control valve vibrates approximately at the maximum possible speed, it is assumed that the oscillations are caused by incorrect parameters in the proportional part of the position control. Usually, the amplification parameter k p is chosen too large (case 4a). This incorrect distribution is due to the fact that the proportional part of the regulation always acts directly without delay.

[0135] If the period time T of the oscillation of the actual valve position value x is x In fact, it is significantly longer than the characteristic cycle time T of the control valve c , it is assumed that the cause of the oscillation is an incorrect parameter in the integral part of the position control (case 4b). This is based on the fact that the integral controller adds the control deviations together, i.e. the memory parameter is increased or decreased. This behavior takes effect later than a proportional controller. In addition, the gain factor of the integral part of the control is usually smaller than the gain factor of the proportional part, which also leads to a slower behavior. If case 4b is detected, the cause of the error is usually a too small dead zone of the integral part of the position control or a too large gain parameter k i .

[0136] Alternatively, the phase shift of the oscillations of the setpoint value and the actual value can also be taken into account here, since the different behaviors of proportional control and integral control are used to produce correspondingly smaller or larger phase shifts between oscillations due to the immediate (proportional) or delayed (integral) control response. The phase shift is preferably determined when determining the oscillation characteristics (i.e. the cycle time and / or the frequency and the amplitude). The time stamps of the local maxima and / or minima of the actual value and the setpoint value can be compared with each other for this purpose. If they are different, the difference can be averaged, for example, to form the phase shift.

[0137] If it is determined in the initial query that the amplitude X of the oscillation of the actual value x is not greater than the amplitude W of the oscillation of the setpoint value w, then a check is made as to whether the opposite is true. If so, in this case the period time T of the oscillation of the valve position actual value x is also checked. x Is it significantly greater than the characteristic cycle time T assigned to the control valve? c Or, here it is obvious that T x The corresponding value T of the theoretical value oscillation is adopted w The oscillation cycle times should be equal in any case. If this condition is met, the valve is therefore oscillating much more slowly than at its maximum possible speed, in which case it is assumed that the oscillation is caused by a superordinate process control (case 4c).

[0138] If it is first determined that X is not greater than W and therefore W is not greater than X or it is determined that the oscillation is rapid despite W being greater than X, the cause of the oscillation cannot be explained with current means (case 4e).

[0139] Characteristic cycle time T of the control valve c The consideration can be determined as follows: Starting from the initialization of the control valve when it is put into use, the time for the valve exhaust and ventilation process is known. From this, the valve core movement speed can be determined for both directions. This value is averaged and used as the reaction speed v max The characteristic period time can now be estimated, for example based on a triangular oscillation or a sinusoidal oscillation. In the case of a triangular oscillation, for an amplitude X, the period T is obtained. c =4*X / v max In the case of sinusoidal oscillation x(t) = X*sin(ωt), v max =X*ω and ω=2π / T c , so T c =2π*X / v max .

[0140] Fig. 9An overview of one embodiment of the entire method is shown. For example, a user initiation can serve as a starting point, for example by operating a switch or by requesting a diagnosis, etc. As measured data, only the actual value x and the setpoint value w of the control valve position control are required. It is therefore particularly advantageous to carry out the method on the position controller of the control valve, provided that it has the required computing power, for example with the aid of a digital signal processor. However, for optional additionally distinguishable subdivisions, further data, for example from sensors and / or process control, are required.

[0141] The measurement signals of the actual value x and the setpoint value w are preferably first checked for noise, using known noise identification methods, depending on whether this is to be expected for the noise in the current process equipment. If the signal has too much noise, the further method cannot be meaningfully carried out, because the oscillation identification will provide the most likely erroneous or unreliable results, so the method ends with a corresponding message 910. This message indicates that there is strong noise and that the equipment may need maintenance, but does not specifically circle them, because the cause of the noise is not immediately identified.

[0142] If the signal is not noisy, an oscillation analysis is performed in each case, i.e., as described above, it is determined whether there are permanent oscillations in x and / or w, and if so, characteristic oscillation parameters, i.e., period time and amplitude, are determined. The oscillation analysis results serve as the basis for case differentiation 900. Depending on whether the setpoint value w and / or the actual value x oscillates, the situation is assigned to one of four cases:

[0143] Case 1: No permanent oscillation. In this case, the method ends with the message 920 that everything is OK.

[0144] Case 2: Only the actual value x has permanent oscillations. Here we distinguish further subdivisions which may require different responses. For this purpose, we set the period time T of the oscillation x The characteristic cycle time T of the control valve c In comparison, as in Figure 8 As indicated for Case 4 and described earlier.

[0145] Case 2a: T x No significant greater than T c The cause of the permanent oscillation may be an incorrect setting of the proportional part of the valve position control. The method ends with a message 930 that the control valve requires maintenance. This message can of course also contain more precise information in the specific case, for example the amplification parameter k must be checked because it may be too large. p .

[0146] Case 2b: T x Significantly greater than T cThe cause of the permanent oscillation may be an incorrect setting of the integral part of the valve position control. The method ends with a message 930 that the control valve requires maintenance. In this case, the message also omits additional information.

[0147] Case 2c: Vibrations in the system are superimposed on the valve drive or position measurement signal. However, this case can only be detected with optional additional sensor devices (such as acceleration sensors or structure-borne sound sensors). It occurs when the sensor signal has an oscillation indication, the period time or frequency of which corresponds to the period time or frequency of the oscillation of the actual value x of the valve control. This is preferably checked before distinguishing between cases 2a and 2b. If this occurs, the method ends with message 940 that the system requires maintenance and the cause of the oscillation is to be found outside the control valve.

[0148] Case 2d: For example, a pulsating fluid flow transmits oscillations to the valve core, which in turn forwards the oscillations to the position measurement signal via the valve drive. However, in order to identify it, the process actual value p must also be provided and checked for oscillations, which cannot be achieved in every embodiment. This situation occurs when the process actual value p and the valve position actual value x oscillate with almost the same cycle time or frequency, but the valve position theoretical value w does not oscillate. This is also preferably checked between the distinction between cases 2a and 2b. If this situation occurs, the method ends with a message 940 that the equipment needs maintenance and the cause of the oscillation is to be found outside the control valve.

[0149] Case 2e: This covers all other situations in which only the actual value of the control valve position has permanent oscillations, but they cannot be assigned to one of the other cases 2a-2d. For example, when in addition to the actual value x, the process target value p and the acceleration sensor signal also have oscillations with similar cycle times. This situation does not necessarily have to be implemented in every embodiment. In a particularly preferred design without additional sensor devices and without knowledge of the process actual value p, only a distinction is made between subdivided cases 2a, 2b, which in any case provide clear results when a reasonable error selection is made. If this situation occurs, the method ends with a non-specific message 910 that maintenance is required.

[0150] Case 3: Permanent oscillations only in the setpoint value w. Here, a distinction is made between further subdivided cases which may require different responses.

[0151] Case 3a: If the position controller has a control valve diagnostic function, this is implemented, since the permanent oscillation of the actual value x not following the setpoint value w can be traced to a defect in the control valve (e.g. no compressed air supply to the compression device, mechanical clamp, defective I / P converter, etc.). The method ends with a message 930 that the control valve requires maintenance.

[0152] Case 3b: If the setpoint value signal w has an amplitude that is too small, the position control device of the valve generates a too small control signal, which is not enough for the valve drive to overcome the control valve hysteresis, which is preferably composed of the hysteresis of the general pneumatic drive device, the I / P converter, the control device of the control valve and / or the valve core, and the mechanical friction at the valve. Therefore, the valve core does not move. This is preferably checked first, that is, before considering case 3a. In this case, the method ends with a message 940 that the device needs maintenance and the cause of the oscillation is to be found outside the control valve. The message can also contain information that the strength of the setpoint value signal w on the control valve must be checked.

[0153] Case 3c: This covers all other situations in which only the setpoint value w of the control valve position has permanent oscillations, but which cannot be assigned to cases 3a or 3b, for example when the oscillation amplitude of w is large enough, but a check of the control valve (e.g. by means of a partial stroke test) confirms its functional capability. In this case, the method ends with a non-specific message 910 that maintenance is required.

[0154] Case 4: Both the actual value x and the setpoint value w of the control valve position have permanent oscillations. In this case, a more precise investigation is required, which can be done as follows: Figure 8 As shown and described further above.

[0155] Case 4a: In this case there is an amplification parameter k for the P part of the control valve position control p The method ends with a corresponding message 930 that the regulating valve requires maintenance.

[0156] Case 4b: In this case, there is an incorrect setting of the I component of the control valve position control. The method ends with a corresponding message 930 that the control valve requires maintenance.

[0157] Case 4c: In this case it is then assumed that the process controller is the cause of the permanent oscillation. The method ends with a corresponding message 940 that the system requires maintenance and the cause of the oscillation is to be sought outside the control valve.

[0158] Case 4d: In some operating scenarios, an oscillating process setpoint value may be required. However, in order to detect this situation, the process setpoint value r must be provided and checked for oscillations, which cannot be achieved in all embodiments. This situation occurs when the process setpoint value r, the valve position setpoint value w and the valve position actual value x oscillate with almost the same cycle time or frequency. This situation is checked in Figure 8 Not shown in and subject to the other checks shown there as a prerequisite. If this occurs, the method ends with a message 920 that everything is normal.

[0159] Case 4e: This covers all other situations in which only the actual value and the setpoint value of the control valve position have permanent oscillations, but which cannot be assigned to one of the other cases 4a to 4d. In this case, the method ends with a non-specific message 910 that maintenance is required.

[0160] In some of the cases described, it can also be provided that assistance is automatically provided within the scope of the method. This applies in particular to cases 2a, 2b, 4a and 4b. If an amplification parameter k of the proportional part of the control valve position control is identified p If it is too large, it can be automatically reduced, observing whether the permanent oscillation decays. i Corresponding cases can be envisaged by reducing and / or increasing the dead zone of the integral part of the position control. In case 4c, the position control parameters can also be adjusted to achieve process stabilization. For example, the amplification parameter k of the proportional part is adjusted p Can be reduced, or the regulating differential part (if any) can be amplified. Since the cause of the oscillation is located outside the regulating valve, it is also preferred in this case to provide for maintenance of the device.

[0161] The method allows to indicate whether the permanent oscillations that occur are caused by control valves or other devices or components of the process system. Regardless of whether the source of the oscillations is more precisely identified, this is already extremely important for eliminating or compensating for it. It is conceivable that the user can determine the source of the oscillations according to Fig. 9 or with Fig. 9 Similarly, only the type of situation determined is informed, or additional diagnostic steps and / or additional devices or sensors are also activated and / or added depending on the type of situation in order to further narrow down the source of the permanent oscillation if necessary.

Claims

1. A method for identifying the cause of at least one oscillation in a process plant, the process plant comprising at least one inner regulating circuit (120) and at least one outer regulating circuit of a regulating valve (230), the method comprising the following steps: 1.1 determining a setpoint value (w) and an actual value (x) of the inner control loop (120) of the control valve (230) with respect to time; 1.2 The theoretical value (w) determined with respect to time and the actual value (x) determined with respect to time are then checked for oscillations; 1.3 If at least one oscillation is determined, determining the amplitude and / or period time of the oscillation; 1.4.1 if it is determined that only the actual value (x) or only the setpoint value (w) has oscillations, the period time and / or amplitude of the oscillations are compared with predetermined characteristic parameters of the regulating valve (230); 1.4.2 if it is determined that both the actual value (x) and the setpoint value (w) have oscillations, then determining the phase shift of the two oscillations relative to each other and / or comparing the respective amplitudes and / or comparing at least one of the cycle times with a predetermined characteristic parameter of the regulating valve (230); and 1.5 Identifying a possible cause of the at least one oscillation from the performed comparison and / or phase shift.

2. The method according to claim 1, characterized in that: The identified possible cause of the at least one oscillation is assigned to the inner control loop (120) or to the outer control loop.

3. The method according to claim 1 or 2, characterized in that: In order to identify the cause of the at least one oscillation, in addition to the setpoint value (w) and the actual value (x) of the control circuit (120) of the control valve (230), other parameters which are only available at the control valve are measured and / or taken into account.

4. The method according to any one of the preceding claims, characterized in that 4.1 determining whether the theoretical value (w) and the actual value (x) of the regulating loop (120) of the regulating valve (230) have noise; and 4.2 Perform further steps only if the noise is below a predetermined threshold.

5. The method according to any one of the preceding claims, characterized in that An additional step is provided in which the identified possible cause of the at least one oscillation is automatically eliminated by adjusting a parameter in the valve position control.

6. The method according to any one of the preceding claims, characterized in that In order to check whether the setpoint value (w) and / or the actual value (x) has oscillations, local extreme values ​​of the setpoint value (w) and / or the actual value (x) determined over time are selected and / or evaluated within a predetermined time interval.

7. The method according to any one of the preceding claims, characterized in that If it is determined that only the setpoint value (w) has oscillations, a diagnosis and / or maintenance of the regulating valve (230) is required.

8. The method according to any one of the preceding claims, characterized in that 8.1 The characteristic cycle time or frequency of the regulating valve (230) is used as a predetermined characteristic parameter; and 8.2 If it is determined that only the actual value (x) oscillates and / or the oscillation amplitude of the actual value (x) is greater than the oscillation amplitude of the theoretical value (w) and / or the period time or frequency of at least one oscillation corresponds to a characteristic period time or frequency of the regulating valve (230), then the setting conditions of the regulation (210) of the regulating valve (230) that are unfavorable for the operation of the device are identified as possible causes of the at least one oscillation.

9. The method according to claim 8, 9.1 Among them, The regulation (210) of the regulating valve (230) has a proportional part and / or an integral part, It is characterized in that 9.2 if the period time of the oscillation is less than a predetermined characteristic period time and / or if the frequency of the oscillation is greater than a predetermined characteristic frequency and / or if the phase shift between the oscillation of the actual value and the setpoint value is below a predetermined threshold value, a parameter of the proportional part of the regulation (210) of the regulating valve (230) which is unfavorable for the operation of the regulating valve is identified as a possible cause of the at least one oscillation; and 9.3 If the period time of the oscillation is greater than the predetermined characteristic period time while taking into account error factors and / or the frequency of the oscillation is less than the predetermined characteristic frequency while taking into account error factors and / or the phase shift between the oscillation of the actual value and the theoretical value is higher than the predetermined threshold value, then a parameter of the integral part of the regulation (210) of the regulating valve (230) that is unfavorable to the operation of the regulating valve is identified as a possible cause of at least one of the oscillations.

10. The method according to any one of the preceding claims, characterized in that 10.1 The characteristic cycle time or frequency of the regulating valve (230) is used as a predetermined characteristic parameter; and 10.2 If it is determined that only the theoretical value (w) oscillates and / or the oscillation amplitude of the theoretical value (w) is greater than the oscillation amplitude of the actual value (x) and / or the cycle time of the oscillation is greater than the characteristic cycle time of the control valve (230) or the frequency of the oscillation is less than the characteristic frequency of the control valve (230), then a problem with the regulating device (110) of the process equipment is identified as a possible cause of the oscillation.

11. The method according to any one of the preceding claims, characterized in that 11.1 The width of the hysteresis of the regulating valve (230), in particular the drive device, the regulating device and / or the regulating valve core, is used as a predetermined characteristic parameter; and 11.2 If it is determined that only the theoretical value (w) has oscillations, the amplitude of the oscillations is compared with the predetermined characteristic parameter; 11.3 In which case diagnosis and / or maintenance of the regulating valve (230) is required only when the amplitude of the oscillation exceeds the width of the combined hysteresis of the regulating valve.

12. The method according to claim 1, characterized in that 12.1 Provide acceleration sensors and / or structure-borne sound sensors; 12.1.1 Thereafter, the signals of the acceleration sensor and / or structure-borne sound sensor are checked for oscillations; 12.1.2 Among them, If oscillation is determined, determining the period time and / or frequency of the oscillation; and 12.2 if oscillations are detected in the actual value (x) and in the signal of the acceleration sensor and / or structure-borne sound sensor, but not in the setpoint value (w), comparing the period times and / or frequencies of the oscillations with one another; 12.3 In which, if the period time and / or frequency of the oscillations in the actual value (x) and the signal of the acceleration sensor and / or structure-borne sound sensor are identical within a predetermined error, a vibration within the device is identified as a possible cause of the at least one oscillation.

13. The method according to claim 1, characterized in that 13.1 Provide the actual value of the process (p); 13.1.1 wherein it is then checked whether the process actual value (p) has oscillations; 13.1.2 Among them, If oscillation is determined, determining the period time and / or frequency of the oscillation; 13.2 if oscillations are detected in both the actual value (x) and the process actual value (p), but no oscillations are detected in the setpoint value (w), comparing the period times and / or frequencies of the oscillations with one another; 13.3 wherein, if the periodic time and / or frequency of the oscillations in the actual value (x) and the process actual value (p) are identical within a predetermined error, a pulsating fluid flow in the device is identified as a possible cause of the at least one oscillation.

14. The method according to claim 1, characterized in that 14.1 Provide the process theoretical value (r); 14.1.1 wherein it is then checked whether the process target value (r) has oscillations; 14.1.2 Among them, If oscillation is determined, determining the period time and / or frequency of the oscillation; and 14.2 if oscillations are detected in the actual value (x), the setpoint value (w) and the process setpoint value (r), the period times and / or the frequencies of the oscillations are compared with one another; 14.3 wherein the at least one oscillation is identified as expected if the period times and / or frequencies of the oscillations in the actual value (x), the setpoint value (w) and the process setpoint value (r) are identical within a predetermined error.

15. A position regulator (210) for a regulating valve (230), wherein: The position controller (210) comprises a computing unit and means which are suitable and designed to carry out the steps of the method according to any of the preceding method claims 1 to 14.

16. A diagnostic kit for use with a regulating valve (230), wherein: The diagnostic box comprises a computing unit having means suitable and configured for carrying out the steps according to any of the preceding method claims 1 to 14 .

17. A regulating valve (230), comprising the position regulator (210) according to claim 15.

18. A computer program comprising instructions causing a position controller (210) according to claim 15 and / or a diagnostic box according to claim 16 to execute the method steps according to any one of the preceding method claims 1 to 14.

19. A computer readable medium having stored thereon the computer program according to claim 18.

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