VALVE CONTROL IN AN HVAC SYSTEM WITH SENSORS

Abstract

A method of controlling opening of a valve in an HVAC system is provided. The method includes controlling the opening of a six-way-valve according to a default control mode in dependence of a default sensor signal, the six-way-valve fluidicly coupling an inlet side and an outlet side of the heat exchanger alternatively with a first fluidic circuit in a first default control mode or a second fluidic circuit in a second default control mode; selecting a selected default control mode from the first default control mode and the alternative second default control mode; determining, while controlling the valve in the selected default control mode, whether the sensor signal is faulty; and switching, in case the signal is faulty, to controlling the opening according to a first fallback control mode or an alternative second fallback control mode, where the opening is controlled independently of the faulty sensor signal.

Claims

1. A method of controlling opening of a valve (10) in an HVAC system (100) to regulate the flow of a fluid through a heat exchanger (2) of the HVAC system (100), the method including: controlling the opening of a six-way-valve according to a default control mode in dependence of a default sensor signal, the default sensor signal being indicative of at least one parameter of the HVAC system, wherein the six-way-valve fluidicly couples an inlet side and an outlet side of the heat exchanger alternatively with a first fluidic circuit in a first default control mode or a second fluidic circuit in a second default control mode; selecting the default control mode as selected default control mode from the first default control mode and the alternative second default control mode; determining, while controlling the opening of the six-way-valve according to the selected default control mode, whether the default sensor signal is faulty; and switching, in case of the default sensor signal being faulty, to controlling the opening of the six-way-valve according to a first fallback control mode or an alternative second fallback control mode in dependence of the selected default control mode, with the opening of the six-way-valve being controlled independently of the faulty sensor signal in the first fallback control mode or the alternative second fallback control mode.

2. The method according to claim 1, wherein the method further includes determining, while controlling the opening of the six-way-valve according to a fallback control mode, whether the previously faulty default sensor signal is non-faulty, and, in case of the default sensor signal being non-faulty, switching back to controlling the opening of the six-way-valve according to a default control mode.

3. The method according claim 1, wherein the default control mode is a power control mode, with an amount of energy per time, E, that is exchanged by the heat exchanger (2) being controlled to a set point energy per time value in the power control mode.

4. The method according to claim 1, wherein a fallback control mode is a pressure-independent control mode, with the flow being controlled to a setpoint flow value in the pressure-independent control mode.

5. The method according to claim 1, wherein a fallback control mode is an opening control mode, with the valve (10) being controlled to a setpoint degree of valve opening in the opening control mode.

6. The method according to claim 1, wherein the default sensor signal includes at least one of: a flow signal *, the flow signal * being indicative of the flow ; and a temperature difference signal T*, the temperature difference signal T* being indicative of a temperature difference T=T.sub.inT.sub.out between a supply temperature T.sub.in of the fluid entering the heat exchanger (2) and a return temperature T.sub.out of the fluid exiting the heat exchanger (2).

7. The method according to claim 1, wherein the six-way-valve fluidicly couples the inlet side and the outlet side of the heat exchanger alternatively with a heating circuit, the first default control mode being a heating mode and the first fallback control mode being a heating mode, or a cooling circuit, the second default control mode being a cooling mode and the second fallback control mode being a cooling mode.

8. A control device (1) for controlling opening of a valve (10) in an HVAC system (100) to regulate the flow of a fluid through a heat exchanger (2) of the HVAC system (100) and adjust the amount of energy per time, E, that is exchanged by the heat exchanger (2), the control device (1) being configured to operatively couple to at least one sensor (13, 21, 22, 141) for receiving a default sensor signal, the control device further including a processor (14), the processor (14) being configured: to control the opening of a six-way-valve according to a default control mode in dependence of the default sensor signal, the sensor signal being indicative of at least one parameter of the HVAC system, wherein the six-way-valve fluidicly couples an inlet side and an outlet side of the heat exchanger alternatively with a first fluidic circuit in a first default control mode or a second fluidic circuit in a second default control mode; to select the default control mode as selected default control mode from the first default control mode and the alternative second default control mode; to determine, while controlling the opening of the six-way-valve according to the selected default control mode, whether the default sensor signal is faulty; to switch, in case of the default sensor signal being faulty, to controlling the opening of the six-way-valve according to a first fallback control mode or an alternative second fallback control mode in dependence of the selected default control mode, with the opening of the six-way-valve being controlled independently of the faulty sensor signal in the first fallback control mode or the alternative second fallback control mode.

9. The control device according to claim 8, wherein the processor (14) is further configured to determine, while controlling the opening of the six-way-valve according to a fallback control mode, whether the previously faulty default sensor signal is non-faulty, and, in case of the default sensor signal being non-faulty, switching back to controlling the opening of the six-way-valve according to a default control mode.

10. The control device according to claim 8, wherein the at least one sensor includes at least one of a flow sensor (13), the flow sensor (13) being configured to provide a flow signal *, the flow signal * being indicative of the flow , and a temperature difference sensor (21, 22, 141), the temperature difference sensor (21, 22, 141) being configured to provide a temperature difference signal T*, the temperature difference signal T* being indicative of a temperature difference T=T.sub.inT.sub.out between a supply temperature T.sub.in of the fluid entering the heat exchanger (2) and a return temperature T.sub.out of the fluid exiting the heat exchanger (2).

11. The control device according to claim 8, wherein the processor (14) is further configured to control the opening of the six-way-valve using as the default control mode a power control mode, with an amount of energy per time, E, that is exchanged by the heat exchanger (2) being controlled to a set point energy per time value in the power control mode.

12. The control device according to claim 8, wherein the processor (14) is further configured to control the opening of the six-way-valve using as a fallback control mode a pressure-independent control mode, with the flow being controlled to a setpoint flow value in the pressure-independent control mode.

13. The control device according to claim 8, wherein the processor (14) is further configured to control the opening of the six-way-valve using as a fallback control mode an opening control mode, with the valve (10) being controlled to a setpoint degree of valve opening in the opening control mode.

14. The control device according to claim 8, wherein the six-way valve fluidicly couples the inlet side and the outlet side of the heat exchanger alternatively with a heating circuit, wherein the processor (14) is configured to control the opening of the six-way-valve using as the first default control mode and as the first fallback control mode a heating mode, or with a cooling circuit, whereby the processor (14) is configured to control the opening of the six-way-valve using as the second default control mode and the second fallback control mode a cooling mode.

15. A computer program product including a non-transient computer readable medium having stored therein computer program code configured to direct a processor of a control device (1) for controlling opening of a valve (10) in an HVAC system (100) to regulate the flow of a fluid through a heat exchanger (2) of the HVAC system (100) and adjust the amount of energy per time, E, that is exchanged by the heat exchanger (2) by: controlling the opening of a six-way-valve according to a default control mode in dependence of a default sensor signal, the default sensor signal being indicative of at least one parameter of the HVAC system, wherein the six-way-valve fluidicly couples an inlet side and an outlet side of the heat exchanger alternatively with a first fluidic circuit in a first default control mode or a second fluidic circuit in a second default control mode; selecting the default control mode as selected default control mode from the first default control mode and the alternative second default control mode; determining, while controlling the opening of the six-way-valve according to the selected default control mode, whether the default sensor signal is faulty; and switching, in case of the default sensor signal being faulty, to controlling the opening of the six-way-valve according to a first fallback control mode or an alternative second fallback control mode in dependence of the selected default control mode, with the opening of the six-way-valve being controlled independently of the faulty sensor signal in the first fallback control mode or the alternative second fallback control mode.

16. The computer program product according to claim 15, wherein the computer program code is further configured to direct the processor of the control device (1) to determine, while controlling the opening of the six-way-valve according to a fallback control mode, whether the previously faulty default sensor signal is non-faulty, and, in case of the default sensor signal being non-faulty, switching back to controlling the opening of the six-way-valve according to a default control mode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The present invention will be explained in more detail, by way of example, with reference to the drawings in which:

[0056] FIG. 1: shows a block diagram illustrating schematically an HVAC system with a fluid circuit comprising a pump, a valve, and a heat exchanger, and a control device for controlling the opening of the valve to regulate the amount of power exchanged by the heat exchanger.

[0057] FIG. 2: shows the major functional units that are hosted on a processor of the control device in operation together with main functional links between those units.

[0058] FIG. 3: shows a flow diagram illustrating steps for sensor testing and control mode switching when controlling the opening of the valve in a power control mode.

[0059] FIG. 4: shows a flow diagram illustrating steps for sensor testing and control mode switching when controlling the opening of the valve in a pressure-independent control mode.

[0060] FIG. 5: shows a flow diagram illustrating steps for sensor testing and control mode switching when controlling the opening of the valve in an opening control mode.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0061] In FIG. 1, reference numeral 100 refers to an HVAC system with a fluid circuit 101 comprising a pump 3, a valve 10, a heat exchanger 2, for example a heat exchanger for heating or cooling a room, with the components of the fluid circuit being interconnected by way of pipes. The valve 10 is provided with an actuator 11, for example an electrical motor, for opening and closing the valve 10 and thus controlling the flow through the fluid circuit 101, using different positions (or sizes of orifice) of the valve 10, corresponding to different degrees of opening of the valve 10. Further, the pump 3 may vary the flow through the fluid circuit 101.

[0062] As illustrated schematically, the HVAC system 100 further comprises an optional building control system 4 connected to the valve 10 or actuator 11, respectively. One skilled in the art will understand that the depiction of the HVAC system 100 is very simplified and that the HVAC system 100 may include a plurality of fluid circuits 101, having in each case one or more pumps 3, valves 10, heat exchangers 2, and further heat exchangers, such as chillers. The HVAC system 100 may further include a heater for heating the liquid.

[0063] In the present description, the fluid circulating in the fluid circuit 101 is considered to be a liquid heat transportation medium, such as water. In alternative embodiments, the fluid may be a gas, such as air, or any special purpose medium as used in the internal fluidics of an HVAC system. The thermal heat exchanger may alternatively be, for example, a cooler or a thermal heat exchanger internal to the HVAC system.

[0064] As further illustrated schematically in FIG. 1, the heat exchanger 2 is provided with two temperature sensors 21, 22 that are arranged at the inlet of the heat exchanger 2, for measuring the supply temperature T.sub.in of the fluid entering the heat exchanger 2, and at the exit of the heat exchanger 2, for measuring the return temperature T.sub.out of the fluid exiting the heat exchanger 2. The temperature sensors provide signals T*.sub.in, T*.sub.out which are indicative of and reflect the supply temperature T.sub.in and the return temperature T.sub.out in an analogue or digital representation. As will be discussed further below, the temperature sensors 21, 21 are part of a temperature difference sensor (or temperature difference sensor system).

[0065] The fluid circuit 101 further comprises a flow sensor 13 for measuring the flow , i.e.

[0066] the rate of fluid flow, through the valve 10 or fluid circuit 101, respectively. Depending on the embodiment, the flow sensor 13 is arranged in or at the valve 10, or in or at a pipe section 12 connected to the valve 10. For example, the flow sensor 13 is an ultra-sonic flow sensor, an inductive-magnetic flow sensor, or a heat transport sensor. The flow sensor 13 provides a signal * which is indicative of and reflects the flow in an analogue or digital representation. In an alternative embodiment where the fluid is a gas, such as air, a flow sensor may typically be implemented by a set of two pressure sensors and a flow-restricting aperture that is fluidically arranged between the pressure sensors.

[0067] In FIG. 1, reference numeral 1 refers to a control device for controlling the valve 10 or the actuator 11, respectively, to adjust the opening (or position or size of orifice) of the valve 10. Accordingly, the control device 1 regulates the flow , i.e. the rate of fluid flow, through the valve 10 and, thus, through the heat exchanger 2. Consequently, the control device 1 regulates the amount of thermal energy per time that is exchanged by the heat exchanger 2 with its environment. Depending on the embodiment, the control device 1 is arranged at the valve 10, for example as an integral part of the valve 10 or attached to the valve 10, or the control device 1 is arranged at a pipe section 12 connected to the valve 10.

[0068] The control device 1 comprises a processor 14, for example an operational microprocessor or microcontroller with program and data memory or another programmable circuit. The control device 1 comprises computer program code configured to direct the processor 14 or another programmable circuit of the control device 1 to perform various functions, as will be explained later in more detail in the following paragraphs. The computer program code is stored on a non-transient computer-readable medium which is connected to the control device 1 in a fixed or removable fashion. One skilled in the art will understand, however, that in alternative embodiments, functional modules configured to perform said functions can be implemented partly or fully by way of hardware components. Moreover, in alternative embodiments, the processor 14 is arranged, fully or partly, in different components of the HVAC system 100, for example in the actuator 11, the flow sensor 13, or the building control system 4.

[0069] It is further noted that FIG. 2, as well as the flow diagrams of FIGS. 3 to 5, only consider mode switching in the context of faulty sensor signals. Additionally, however, user-controlled mode switching to a user-selected control mode may be available, for example as an override control mode. In such an override control mode, the control unit is forced to control the opening of the valve 11 according to the override control mode, independently of the default control mode. In some embodiments, a fallback control mode to which control may switch automatically, in case of a faulty sensor signal, may additionally serve as a manually selectable override control mode.

[0070] As is illustrated in FIG. 1, the flow sensor 13 is connected to a main unit of the control device 1 for providing timely or current-time measurement values of the flow to the control device 1. Furthermore, the control device 1 is connected to the actuator 11 for supplying a control signal Z to the actuator 11 for controlling the actuator 11 to open and/or close the valve 10, i.e. to adjust the opening (or position or size of orifice) of the valve 10.

[0071] Moreover, the temperature sensors 21, 22 of the heat exchanger 2 are connected to a main unit of the control device 1 for providing to the control device 1 timely or current-time measurement values of the supply temperature T.sub.in and the return temperature T.sub.out of the fluid entering or exiting the heat exchanger 2, respectively.

[0072] Optionally, the control device 1 is further connected to the building control system 4 for receiving from the building control system 4 control signals s and/or parameters, for example user settings for a desired room temperature, and/or measurement values, such as the load demand (for example from zero BTU to maximum BTU) or a transport energy per time, E.sub.T, i.e. the transport power, that is currently used by the pump 3 to transport the fluid through the fluid circuit 101. The transport power energy per time, E.sub.T, can be measured by an optional power measurement unit 31. Based on the transport energy per time, E.sub.T, that is used by a plurality of pumps 3 and received at the building control system 4 from a plurality of fluid circuits 101 (through transmission in push mode or retrieval in pull mode), the building control system 4 may be configured to optimize the overall efficiency of the HVAC system 100, for example by setting the flow through the valve 10 of one or more fluid circuits 101 based on the total value of the transport energy per time, E.sub.T, that is used by all the pumps 3 of the HVAC system 100. In an alternative or additional embodiment, a power sensor arranged at the pump 3 is connected directly to the control device 1 for providing the current measurement value of the transport time, E.sub.T, to the control device 1.

[0073] In the example of FIG. 1, the building control system further provides, as input to the control device 1, a set point value E.sub.S for the energy per time, setpoint E, that is exchanged by the heat exchanger 2.

[0074] In a variant, the HVAC system 100 further comprises sensors which are arranged in the space where the heat exchanger 2 is located, for example a humidity sensor, for measuring the humidity of the air in the room where the heat exchanger 2 is arranged, a temperature sensor for measuring the air temperature in the space around the heat exchanger 2, and/or an air flow sensor, for measuring the air flow across the heat exchanger 2. The sensor signals that are generated by such sensors may be processed by the control device 1 and/or the building control system 4. Faulty sensor signals originating from those sensors may be handled in the same way as the signals from the flow sensor 11 and the temperature sensors 21, 22 in accordance with the present invention.

[0075] In the following paragraphs, reference is additionally made to FIG. 2, showing the major functional units that are hosted on the processor 14 together with main functional links between those units and additional associated functional and structural units and components.

[0076] It is to be noted, that the functional units which are hosted on the processor 14 may also be distributed over a number of hardware components and may further be implemented, fully or partly, by discrete analogue and/or digital hardware components. Those functional units, like other structural and functional units of the control device 1, may further be part of a common physical unit and be accordingly arranged at a common location. Alternatively, they may be distributed over different locations of the HVAC system 100.

[0077] A difference determination unit 141 determines the temperature difference signal T*, as difference of the sensor signals T*.sub.in, T*.sub.out, respectively. The temperature sensors 21, 22 and the difference determination unit 141 accordingly form, in combination, a temperature difference sensor (or a temperature difference sensor system).

[0078] As will be discussed further below in more detail, the exemplary control device 1 is configured to control the opening of the valve 10 according to a power control mode. The temperature difference signal T* and the flow signal * accordingly are sensor sub-signals which, in combination, form the default sensor signal.

[0079] Both the flow signal * and the temperature difference signal T* are routed to a sensor testing unit 142. It is to be noted that the temperature signals T*.sub.in, T*.sub.out may be routed to the sensor testing unit 142 in addition or alternatively to the temperature difference signal T*. The sensor testing unit 142 determines whether any of the sensor sub-signals *, T* are faulty, by assessing the signals based on fault indicating criteria. The fault indicating criteria that are applied by the sensor testing unit 142 may, at least partly, be specific for the single sensors and largely depend on their individual design and operational characteristics. Other fault indicating criteria are generic and may be used for all or at least a variety of sensors. Some major generic fault indicating criteria will be discussed below. Depending on the embodiment, the determination, for the individual sensors and sensor signals, is based on a single criterion or a combination of several criteria. Major generic criteria that are applicable by the sensor testing unit include: [0080] signal fluctuation: Since the measured parameters of the fluid flow can generally be expected to be stable in a steady state of the HVAC system, extensive fluctuation, in particular high-frequency fluctuation, may indicate a faulty sensor signal. A sensor signal may, for example, be considered to be faulty, if it varies over the full or a considerable portion of the sensor measurement range within a short time period. This may, occur, for example, in case of a wiring between a sensor element and subsequent circuitry, for example an instrumentation amplifier, being broken, resulting in the amplifier output floating over the hole output range in a substantially undefined way. [0081] output range limit: A sensor signal at the upper or lower output range limit, corresponding to the sensed parameter being at the upper or lower limit of the measuring range, may, especially if occurring for an extended time period, indicate an interruption, a short circuit or some component of a sensor or the following circuitry being in a state of saturation. [0082] plausibility check/deviation from expected value: while the actual values of the sensed parameters are not pre-known, an expected value or value range is typically known as given by the design of the HVAC system 100 and the fluid circuit 101, as well as its operating point. Significant deviation from an expected value or value range may therefore indicate a faulty sensor signal. This is the case, especially if other sensors provide signals in the expected range. [0083] self testing routines: Some sensors, especially sensors with electronic signal conditioning and/or embedded capabilities, such as ultrasonic or inductive-magnetic flow sensors, provide integrated self testing and/or fault detection functionality. If such sensors are applied, the sensor testing unit may be configured to evaluate sensor self test results. The same applies for dedicated error signals that are provided by some sensors.

[0084] The following description only considers switching to fallback control modes that generally operate differently from the default control mode and use different control parameters. They may especially be long-term fallback control modes, as explained before in the general description. One or more additional short-term control modes in which a faulty sensor (sub-)signal is replaced may be present as well.

[0085] The sensor testing unit 142 is operatively coupled to a control mode selection unit 143. The control mode selection unit 143 is further operatively coupled to a number of control signal generators 144, 145, 146. Each of the control signal generators 144, 145, 146 is configured to alternatively generate the control signal Z for the actuator 11. The control signal generator 144 exemplarily is a standard control signal generator for generating a standard control signal Z.sub.0 for controlling the opening of the valve 10 according to the default control mode. The other control signal generators 145 146 are fallback control signal generators for generating fallback control signals Z.sub.1, Z.sub.2 for controlling the opening of the valve 1 according to fallback control modes. The standard control signal generator 144 is exemplarily considered as generating the control signal Z.sub.0 for operating the valve 10 in a power control mode as default control mode. Generation of the control signal Z is accordingly based on a setpoint energy per time value, E.sub.S, setpoint, as well as on the flow signal *, and the temperature difference signal T* as sensor sub-signals. As explained before, the set energy per time value, E.sub.S, is exemplarily received by the control device 1 from the building control system 4. It may, however, also be input directly on a user interface of the control device 1, via some wired or wireless remote programming device, or readily stored in non-volatile memory of the processor 14.

[0086] The fallback control signal generator 145 is configured to generate a fallback control signal Z.sub.1 for controlling the opening of the valve in a pressure-independent control mode as fallback control mode. Generation of the fallback control signal Z.sub.1 is accordingly based on a set flow value .sub.S as input setpoint, and the flow signal * as fallback sensor signal, but not the temperature difference signal T*.

[0087] The fallback control signal generator 146 is configured to generate a fallback control signal Z.sub.2 for controlling the opening of the valve in an opening control mode as fallback control mode. Generation of the control signal Z.sub.2 is based on a set valve opening, without considering either of the flow signal * or the difference temperature signal T*. The valve actuator 11 is here considered as actuation drive which is configured to receive the control value Z indicating the desired valve opening, and controls the opening of the valve autonomously in accordance with the control signal Z, for example, based on drive internal sensors and control routines. Therefore, the control signal generator 146 merely routes the setpoint value Z.sub.S to the valve actuator 11.

[0088] The sensor testing unit 142 and the control mode selection unit 143 favourably operate in a substantially continuous way to ensure that a faulty sensor signal is reacted on with no or only negligible time delay.

[0089] In the following paragraphs, reference is additionally made to FIGS. 3, 4, 5, illustrating operation of the exemplary control device 1 with respect to sensor testing and control mode switching as schematic operational flows.

[0090] FIG. 3 illustrates sensor testing and control mode switching when controlling the opening of the valve 11 in the power control mode, referred to as step S1, as default control mode.

[0091] In a step S11, the sensor testing unit 142 determines whether the flow signal * is faulty, applying, alone or in combination, any suited fault indicating criteria, such as the before-discussed exemplary criteria. In case the flow signal * is faulty, operation proceeds with step S12 where controlling the opening of the valve is switched to the opening control mode. If the flow signal * is valid, operation proceeds with step S13 where it is determined whether the temperature difference signal T* is faulty. In this case, operation proceeds with step S14 where control of the opening of the valve is switched to the pressure-independent control mode. If the temperature difference signal T* is valid, control of the valve opening is continued in the power control mode. Starting from the power control mode as standard operation mode, either of the pressure-independent control mode or the opening control mode may accordingly serve as fallback control mode.

[0092] In a hierarchical view, the power control mode is the default control mode, which may also be referred to as zero-rank control mode (L.sup.0 control mode). The pressure-independent control mode is a lower ranking 1.sup.st rank fallback control mode (L.sup.1 control mode). The opening control mode is a still lower ranking 2.sup.nd rank fallback control mode (L.sup.2 control mode). If the default sensor signal, including both the flow signal * and the temperature difference signal T* is faulty and operation in the L.sup.0 control mode cannot be continued, it is tested whether operation may be continued in the L.sup.1 control mode. In the affirmative case, operation is continued in this L.sup.1 control mode. Only if operation can neither be continued in the L.sup.0 control mode nor in the L.sup.1 control mode, it is continued in the L.sup.2 control mode

[0093] FIG. 4 illustrates sensor testing and control mode switching when controlling the opening of the valve 11 in the pressure-independent control mode, referred to as step S2, as L.sup.1 control mode.

[0094] In step S21, the sensor testing unit 142 determines whether the (previously valid) flow signal * is now faulty. If the flow signal * is faulty, operation proceeds with step S22 where control of the opening of the valve is switched to the opening control mode as L.sup.2 control mode. If the flow signal * is still valid, operation proceeds with step S23 where it is determined whether the (previously faulty) temperature difference signal T* is (still) faulty. If the temperature difference signal T* is still faulty, control of the opening of the valve is continued in the pressure-independent control mode as L.sup.1 control mode. If the (previously faulty) temperature difference signal T* is now valid, control of the opening of the valve is switched back to the power control mode as L.sup.0 control mode in step S24. Starting from the pressure-independent control mode or as fallback control mode, the opening control mode accordingly serves as further fallback control mode, for the case that the flow signal * as fallback sensor signal becomes faulty.

[0095] FIG. 5 illustrates sensor testing and control mode switching when controlling the opening of the valve 11 in the opening control mode, referred to as step S3, as L.sup.2 control mode.

[0096] In step S31, the sensor testing unit 142 determines whether the (previously faulty) flow signal * is still faulty. In case the flow signal * is still faulty, control of the opening of the valve is continued in the opening control mode as L.sup.2 control mode. If the (previously faulty) flow signal * is now valid, operation proceeds with step S32 where it is determined whether the (previously faulty) temperature difference signal T* is (still) faulty. In case the temperature difference signal T* is (still) faulty, control of the opening of the valve is switched to the pressure-independent control mode as L.sup.1 control mode in step S33. If the temperature difference signal T* is valid, control of the opening of the valve is switched back to the power control mode as L.sup.0 control mode in step S34.

[0097] It can be seen that the hierarchical view on the control modes as explained in the context of FIG. 3 also holds true with respect to FIGS. 4 and 5. In each of the steps S2 and S3, it is tested whether operation may be continued in a higher-ranking control mode. If this is the case, operation is continued in this control mode. It is in particular continued in the highest-ranking available control mode that is available based on the available sensor (sub-)signals. If switching to a higher-ranking mode is not possible, operation is generally continued in the same mode. Only if this mode is not further available due to a further faulty sensor (sub-)signal, operation is continued in a lower-ranking control mode.

[0098] In the illustrated example, however, it is to be noted thatwith three control modes being present in totalswitching to both a higher-ranking control mode and a lower ranking control mode is available only in the intermediate pressure-independent control mode as L.sup.1 control mode. For the highest ranking power control mode as L.sup.0 control mode, only switching to lower-ranking control modes is available. For the lowest-ranking opening control mode as L.sup.2 control mode, only switching to a higher-ranking control mode is available. In other embodiments, the number of control modes may be larger and a plurality of intermediate control modes may be available between the highest-ranking and the lowest-ranking control mode.

[0099] It is further noted that the hierarchy ranks are not necessarily run through in consecutive order. Starting from the highest-ranking L.sup.0 control mode, operation may, in case of the sensor signal being faulty, either be continued in the L.sup.1 control mode or the L.sup.2 control mode, in dependence of the faulty sensor (sub-)signal. Similarly, starting from the lowest-ranking L.sup.2 control mode, operation may be continued in either of the L.sup.1 control mode (pressure-independent control mode) or L.sup.0 control mode.

[0100] It is further noted that in the present example, only a single control mode exists on each rank, namely the power control mode as highest-ranking L.sup.0 control mode, the pressure-independent control mode L.sup.1 as intermediate-ranking control mode, and the lowest-ranking opening control mode L.sup.2. In alternative examples, however, more than one alternative control mode may share a common rank. In a modification of the before-described embodiment, two 1.sup.st rank fallback control modes may be available, which may be selected based on the faulty sensor (sub-)signals. For example, a further 1.sup.st rank fallback control mode may be available in addition to the pressure-independent control mode. In this further 1.sup.st rank fallback control mode controlling the opening of the valve may not rely on the flow signal *, but may exclusively rely on the temperature difference signal T*.

[0101] It is further noted that the power control mode is considered as only available default control mode for conciseness reasons. However, since a pressure-independent control mode is available as fallback control mode, the valve 10 may also be controlled to operate in the pressure-independent control mode as alternative standard operation mode, if desired. In this case, the opening control mode is available as only fallback control mode. Likewise, opening of the valve 10 may also be controlled to operate in the opening control mode as further alternative default control mode.

[0102] It should be noted that, in the description, the computer program code has been associated with specific functional modules and the sequence of the steps has been presented in a specific order, one skilled in the art will understand, however, that the computer program code may be structured differently and that the order of at least some of the steps could be altered, without deviating from the scope of the invention.