WO2007046714A2

WO2007046714A2 - Method, apparatus and system for operating a valve

Info

Publication number: WO2007046714A2
Application number: PCT/NO2006/000367
Authority: WO
Inventors: Håkon Holm SOLBERG
Original assignee: Kongsberg Automotive As
Priority date: 2005-10-20
Filing date: 2006-10-20
Publication date: 2007-04-26
Also published as: SE531289C2; WO2007046714A3; SE0502345L

Abstract

The invention relates to a method, a control device and a system for operating a valve, the valve comprising a valve element which is moveable between a first, stable position representing a closed valve position, a second, transient position representing a semi-open valve position and a third stable position representing a open valve position, and an actuating element for causing the valve element to move between said first and third positions as a function of a pulsed actuating signal. The method comprises the step of varying the duty cycle of said pulsed actuating signal in accordance with a reference input in order to operate the valve. The reference input is a desired effective orifice area (ad) of the valve. Advantageously, the method further comprises varying he frequency of the pulsed actuating signal in accordance with the reference input.

Description

METHOD, APPARATUS AND SYSTEM FOR OPERATING A VALVE

FIELD OF THE INVENTION

The present invention relates generally to the field of operating fluid valves.

More specifically, the present invention relates to a method, a device, and a system for operating a valve with a valve element which is moveable between a closed position, a semi-open position and an open position.

BACKGROUND OF THE INVENTION

In fluid valve systems there is often a demand for both large and small flow rates, corresponding to, e.g., slow or fast movements of a piston. For cost reasons, leakage prevention and desired robustness (e.g. dirt, vibrations etc.), on/off valves are generally preferred to proportional valves. To emulate a proportional valve function, multiple on/off valves are used. A minimum number of on/off valves should preferably be used in order to utilize existing space and to minimize costs. In particular, there is a need for a method, a control device and a system for operating a three-state, bistable valve in an efficient and reliable way. A three-state, bistable valve is a valve with a first, stable, closed state, a second, transient, semi-open state and a third, stable, fully open state.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved method, a device and a system for operating a fluid valve.

A particular object of the invention is to provide a method, a device and a system for operating a fluid valve with a valve element which is moveable between a first, stable position representing a closed valve position, a second, transient position representing a semi-open valve position and a third stable position representing a open valve position.

At least some of the above objects and other advantages are achieved by a method, a device, and a system as set forth in the appended, independent claims.

Further advantageous features are set forth in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be described in detail below in conjunction with the appended drawings in which: Fig. 1 is a schematic block diagram illustrating a control device connected to a valve, the device operating in accordance with the invention.

Fig. 2 (a), (b) and (c) are schematic block diagrams illustrating three states of a valve having a transient third valve element position, the valve being particularly convenient for use in a preferred embodiment of the invention.

Fig. 3 is a schematic timing diagram illustrating the characteristics of the valve illustrated in fig. 2,

Fig. 4 is a schematic timing diagram illustrating further characteristics of the valve illustrated in fig. 2, Fig 5 is a schematic block diagram illustrating a preferred embodiment of a control device according to the invention, and

Fig. 6 is a schematic flow chart illustrating a method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 is a schematic block diagram illustrating a control device, connected to a valve and operating in accordance with the invention.

The valve 110 is generally arranged for opening or closing a fluid flow from the fluid input 122 to the fluid output 124, as function of a time-varying actuating signal 104 supplied to the valve 110.

In particular, the resulting flow through the valve is determined by an orifice area a of the flow restriction represented by the valve. The orifice area a is variable, set by the position of a valve element included in the valve. The valve element is moveable between three positions, each representing a particular orifice area: a first, stable position representing a closed valve position (orifice area a— Aj which typically equals zero), a second, transient position representing a semi-open valve position (orifice area a=Ai), and a third, stable position representing an open valve position (orifice area a=As). In general, Aι<A₂<As.

The position of the valve element is influenced by an actuating element, in particular a solenoid, which basically causes the valve element to move between the first and third positions of the valve as a function of the actuating signal 104. The actuating signal is typically a pulsed, digital (on/off) signal. Under normal operation, the valve element will thus switch between its first, closed position and its third, open position, while the second, semi-open position will be a non-stable, transitory or transient state of the valve. The control device 100 is arranged for receiving an input signal 102 representing a desired valve action. In particular, the input signal 102 represents a value representative of the resulting flow characteristics of the valve, such as a desired average flow value. In general, the value of the flow through the valve is highly dependent on pressure conditions and fluid characteristics. Therefore, in a preferred embodiment, the input signal 102 represents a desired average orifice area a_d of the flow restriction represented by the valve.

Fig. 2 (a), (b) and (c) are schematic block diagrams illustrating an exemplary valve 110 applicable for use with the invention, shown in three different states. The valve 110 is a bistable on/off solenoid valve, arranged for opening or closing the fluid flow from a fluid input 122 to a fluid output 124. The constructional features of the valve 110 are the same in each illustrated state (a)-(c).

The valve 110 comprises a movable valve element 112, in particular a substantially cylindrical plunger, made at least partly of a ferromagnetic material. The valve element 112 is slidingly moveable within a hollow cylindrical sleeve which is open at one, lower end and closed by an attached iron core 118 at the opposite, upper end.

More particularly, the valve element 112 is movable between a first, stable position, a second, transient position and a third, stable position, dependent on the energizing of the actuating element 114, which is typically an electromagnetic coil, embodied as the cylindrical sleeve and concentrically arranged with the valve element 112.

The first, normal position of the valve element 112, as illustrated in fig. 2 (a), represents a closed valve position, wherein the input flow 122 and thus the output flow 124 are zero. In this position the actuating element 114 is non-energized. A spring 120 is arranged between the valve element 112 and the iron core 118, causing the valve element 112 to be pushed towards the valve seat 116, thus keeping the valve closed.

Upon energizing of the actuating element 114 by means of the actuating signal 104, the valve element 112 will be retracted towards the iron core 118 to its third, open position as illustrated in fig. 2 (c). The valve element 112 is thus adapted to move between the first, closed position in fig. 2 (a) and the third, open position in fig. 2 (c) as a function of the actuating signal 104.

Fig. 2 (b) illustrates the second, transient state of the valve 110, corresponding to a semi-open valve position. The lower end of the valve element 112 is equipped with a pilot chamber 111 at its lower end. The pilot chamber 111 comprises an upper inlet orifice and a vertically movable disc 113 with a lower orifice. The arrangement of the pilot chamber 111 attached to the lower part of the valve element 112 results in that the valve 110 will enter its second, transient state, wherein the flow through the valve is partly restricted through the pilot chamber 111 and the orifice in the disc 113.

The actuating element 114 is energized by the actuating signal 104. When the actuating element 114 is an electromagnetic coil, the actuating signal 104 may be represented by the voltage signal applied to the coil.

The skilled person will realize that numerous other valve types than the example valve illustrated in fig. 2 may be used with the present invention. Suitable valves have a valve element which is moveable between a first, stable position representing a closed valve position, a second, transient position representing a semi-open valve position and a third stable position representing a open valve position. They also have an actuating element for causing the valve element to move between said first and third positions as a function of a pulsed actuating signal.

Fig. 3 is a schematic timing diagram illustrating certain timing and area characteristics of valves applicable for use with the invention, such as the valve 110 illustrated in fig. 2 (a)-(c).

The diagram comprises two graphs with a common time base at the horizontal axis.

The upper graph of fig. 3 illustrates an exemplary instance of an actuating signal 104 supplied to the valve 110. The lower graph of fig. 3 illustrates the resulting flow-through orifice area of the valve 110 when the actuating signal 104 is applied.

The area Aj corresponds to the orifice area of the first, closed position, as illustrated in fig. 2 (a). Typically, Ai equals zero.

A₂ is the orifice corresponding to the second, intermediate position as illustrated in fig. 2 (b), wherein the flow 122, 124 through the valve 110 is partly restricted by the pilot chamber 111 with the disc 113.

A₃ is the orifice area corresponding to the third, fully open position, as illustrated in fig. 2 (C).

The exemplary actuating signal 104 makes a unit step from passive to active state at time To. After that the actuating signal 104 is held steady at the active state until time T ₄, where the activating signal steps back to its passive state.

The resulting flow through orifice area starts at Ai, which is typically zero. After To the area changes, and it will reach its second value A₂ at time Ti. In the interval between To and Ti the area will not be clearly defined.

Thus, Ti is the semi-opening time of the valve 110. It will be understood that T] as well as other characteristics shown in fig. 3 may easily be determined by measurements and/or very simple experiments. Such characteristics may alternatively be provided by a manufacturer as pre-given ratings of the valve.

The valve orifice is maintained at A₂ as long as the vertically moveable disc 113 abuts the valve seat 116. When the valve element 112 starts elevating the movable disc 113, at time T₂, the effective orifice area changes again. In the interval between T₂ and T₃ the area will not be clearly defined.

At time T₃ the valve is fully open, and the orifice area equals A₂.

At T ₄ the actuating signal returns to passive state. The spring 120 will retract the valve element 112, and the resulting orifice area will return to its closed value A_\ (typically zero) at time T ₅.

For simplicity, it is assumed that the closing time of the valve equals T₅-T₄ irrespective of its state (i.e., its second, transient state or its third, steady state) at the time T ₄ when the actuating signal 104 is deactivated.

Fig. 4 is a schematic timing diagram illustrating further characteristics of the valve illustrated in fig. 2. Contrary to the diagrams in fig. 3, the actuating signal is switched back to its passive state before the valve has arrived at its third, fully open state.

The diagram in fig. 4 comprises two graphs with a common time base at the horizontal axis. The upper graph of fig. 4 illustrates an exemplary instance of an actuating signal 104 supplied to the valve 110. The lower graph of fig. 4 illustrates the resulting flow-through orifice area of the valve 110 when the actuating signal 104 is applied.

The QXQdiAi corresponds to the first, closed position, as illustrated in fig. 2 (a). Typically, Ai equals zero. A₂ is the orifice corresponding to the second, intermediate position as illustrated in fig. 2 (b), wherein the flow 122, 124 through the valve 110 is partly restricted by the pilot chamber 111 with the disc 113.

The exemplary actuating signal 104 makes a unit step from passive to active state at time To. Then the actuating signal 104 is held steady at the active state until time Te, where the activating signal steps back to its passive state.

The resulting flow through orifice area starts at Ai, which is typically zero. After To the area changes, and it will reach its second value A₂ at time Ti. In the interval between To and Ti the area will not be clearly defined. Thus, Ti -To is the semi- opening time of the valve 110. This is similar to the example illustrated in fig. 3. The valve orifice is maintained at A2 as long as the vertically moveable disc 113 abuts the valve seat 116.

In this case, the actuating signal 104 returns to its passive state at TV, which is earlier than the point T ₂ (cf. also fig. 3) when the valve element 112 would start elevating the movable disc 113 if the actuating signal had still been active. The spring 120 will retract the valve element 112, and the resulting orifice area will return to its closed value Ai (typically zero) at time Ty.

For simplicity, it is assumed that the closing time of the valve Tγ-Tβ equals T₅-T₄ (as in fig. 3) irrespective of the valve's state at the time (Tg) when the actuating signal 104 was deactivated.

Fig 5 is a schematic block diagram illustrating the structural hardware design of a preferred embodiment of a control device in accordance with the invention.

The control device 100 is advantageously implemented as a functional part of a device known in the automotive industry as an Electronic Control Unit (ECU). An Electronic Control Unit (ECU) is a centralized digital control unit arranged in a vehicle, which may be used for controlling various functions in the vehicle. For instance, the ECU may control engine functions such as fuel injection, or braking functions, such as the operation of anti-locking breaks. Alternatively or in addition, the ECU may control vehicle transmission functions. For instance, in the case of an automatic transmission, the ECU may set parameters associated with automatic transmission modes.

In the present invention, a special ECU may be provided and configured in accordance with the invention for controlling a valve in the vehicle. The valve may, e.g., be a clutch actuator valve. The control device 100 comprises an internal bus 510, operatively connected to a processing unit 550, in particular a microprocessor. A memory 520 is operatively connected to the bus 510. The memory 520 comprises a random access memory (RAM) portion 530, for storing temporary data during processing, and a nonvolatile memory portion 540 such as a Flash memory portion, for storing program instructions and fixed data.

The input signal 102 is fed to the I/O adapter 560, which is operatively connected to the bus 510. This enables the processing unit 550 to read the input signal 102.

The output signal 104 provided by the I/O adapter 560 is operatively connected to the actuating element input of the valve 110. In this example, it is supposed that the I/O adapter 560 also comprises necessary drive circuits for driving the solenoid coil of the valve 110. Alternatively, such drive circuits are provided external to the control device 100.

Although the signals 102, 104 are shown as separate signal lines for simplicity of illustration, the skilled person will realize that special digital bus technology commonly used in the automotive industry, such as the CAN bus, may advantageously be used for the communication between the ECU and the external components such as the valve.

In accordance with the present invention, the memory 520, and in particular the non-volatile memory portion 540, comprises processor instructions that causes the processing unit 550 to perform a method according to the present invention, as described in detail with particular reference to fig. 6 below.

Advantageously, a shared ECU is used. In this case, the hardware structure of fig. 5 may represent the overall ECU. An operating system included in the memory 520 is provided for low-level control of the hardware and for enabling higher-level computer program modules or portions held in the memory 520 to implement various control functions related to the operation of the vehicle. The present invention may in this case be put into effect by the described shared hardware structure and a computer program portion that performs the method in accordance with the invention, working in conjunction with the operating system. Alternatively, the control device 100 may be implemented as a separate unit, e.g. as a separate unit similar to the ECU illustrated in fig. 5.

The purpose of the method is to operate a three-state fluid valve, such as a valve as described with reference to figure 2 (a)-(c) and fig. 3 above, in accordance with the reference input signal a^ In particular, the valve should be operated in order to utilize the potential in such a valve, especially the second, transient position of the valve element.

The method has the desired effective (resulting) valve orifice area a^ as input. The output of the method is the actuating signal 104, which is a digital pulse signal which is advantageously characterized by a pulse frequency/ [Hz] and a duty cycle level dc [%].

For simplicity, it is assumed that the effective valve orifice area equals A]=O at time To and during the time interval To to Ti, i.e. in the transition from the first to the second state of the valve. Actually, the area is not clearly defined in this time interval, as illustrated with broken, sloped line in figures 3 and 4. Further, also for simplicity, it is assumed that the effective valve orifice area equals A₂ during the time interval T₂ to T₃. Actually, the area is not clearly defined in this time interval, as illustrated with broken, sloped line in fig. 3.

Further, also for simplicity, it is assumed that the effective valve orifice area equals A₃ during the time interval T₄ to Ts Actually, the area is not clearly defined in this time interval, as illustrated with broken, sloped line in fig. 3.

Further, also for simplicity, it is assumed that the effective valve orifice area equals A3 during the time interval Te to Tγ Actually, the area is not clearly defined in this time interval, as illustrated with broken, sloped line in fig. 4. Further it is assumed that the pressure drop over the valve is constant and hence do not affect the response times of the valve.

The following constants are defined in the method:

ΔT_st_atei-2 is the time necessary for bringing the valve from the first, open state to the second, semi-open state, i.e. AT_statei-2-Tι-To ΔTdose is the time necessary for the valve to return to closed state, i.e. AT_Chse ^~ T ₅- T₄ (or similarly, AT_dose = T₇-T₆).

ΔT_statei-3 = T3 - T0 [ms] is the time interval from the closed state {To) to the fully open state (T₃) of the valve.

T _small is the period time of the PWM pulse signal 104 when the valve is operated in the semi-open state, i.e. by means of the small valve orifice A₂. Advantageously, this value is used for the signal period: T_smau = AT_statei-₃ = T₃ - To.

/_small ⁼ 1/T_smaiι [Hz] is the PWM frequency of the pulse signal 104 when the valve is operated in the semi-open state, i.e. by means of the small valve orifice A₂.

T _large [ms] is the period time of the pulse signal 104 in case the valve is operated in the fully open state, i.e. by means of the large valve orifice A₃. Advantageously, the value of Tι_arge is selected as substantially twice the value of T_smau i.e.

J- large ⁼ * J- small fiarge = 1/Tiarge [Hz] is the PWM frequency of the pulse signal 104 in case the valve is operated in the fully open state, i.e. by means of the large valve orifice A₃. A₂ [mm²] is the small orifice area of the valve, i.e. a predetermined valve parameter. A₃ [mm²] is the large orifice area of the valve, i.e. a predetermined valve parameter.

It will be understood by the skilled person that all the parameters, characteristics or constants discussed above may easily be determined by measurements and/or very simple experiments, based on the present disclosure and the actual valve that shall be used with the invention. Some of the characteristics may alternatively be provided by a manufacturer as pre-given ratings.

In an advantageous valve for use with the invention, the value of A₃ is 4 to 8 times the value of A₂. The most appropriate value of A₃ may be determined as a trade-off between actual response time and accuracy requirements. As an example, A₃ may be set as 6-A2

The method starts at the initial step 600.

In step 610, the reference input signal a_d is provided as the desired average valve orifice area [mm²]. The reference input signal a_d may either be input as an actual, physical signal or provided as an output variable in another process performed by the control device.

In the comparison step 620, the process determines if the reference input signal a_d is less than a predetermined, first limit value aj_ow. Advantageously, this limit value is predetermined, based on characteristics of the valve, including area and timing characteristics. In particular, the limit value is advantageously established as α_/Ow=

A 2^T cl_ose/T _smaιι

If this test is true, the process is reiterated at step 610. Alternatively, the process continues at the comparison step 630. Advantageously, the duty cycle of the pulsed actuating signal 104 is substantially set to zero, or the signal 104 is substantially set to zero in any other appropriate way, if the test is true.

In the comparison step 630, the process determines if the input signal a_d is greater than a second, predetermined limit value am_gh. The second limit value is advantageously predetermined based on characteristics of the valve, including area and timing characteristics. In particular, the second limit value is advantageously established as a_high= (A₂- (ΔT_statei-3-ΔT_statei.2)/ AT_statei-3)-

If this test is false, the process continues at the small orifice control step 640. If the test is true, the process continues at the large orifice control step 650.

In step 640, the actuating signal 104 is formed in order to obtain small orifice control. The actuating signal 104 is then established as a pulse signal with duty cycle calculated as a function of the reference input a_d. Advantageously, the duty cycle is calculated as a first linear function of the reference input a_d- The function advantageously has constant parameters that are based on characteristics of the calce, including area and timing characteristics. Particularly advantageously, the duty cycle is calculated substantially as follows: dc = [(ΔT_statei-₂-ΔT_cι_ose)/ ^' T_smau + d_d/A₂] ' 100% In this case, in step 640, it is presupposed that the frequency of the actuating signal is set to f=f _smaii, which is an advantageous feature of the invention.

The actuating signal is computed and generated directly by the control device 100 or by providing the frequency value and the duty cycle value to a PWM controller. The small orifice control enables fine control of the resulting fluid flow, in particular in a lower subrange of the total valve flow range.

The resulting actuating signal is supplied to the valve. Then the process reiterates at step 610.

In step 650, the actuating signal 104 is formed in order to obtain large orifice control. The actuating signal 104 is then established as a pulse signal with duty cycle calculated as a function of the reference input a^ Advantageously, the duty cycle is calculated as a second, linear function of the reference input a^ The function advantageously has constant parameters that are based on characteristics of the valve, including area and timing characteristics. Particularly advantageously, the duty cycle is calculated substantially as follows:

dc = 100%

Of course, the duty cycle is also delimited to max. 100%.

In this case, in step 650, it is presupposed that the frequency of the actuating signal is set to f=f ι_αrge ,which is an advantageous feature of the invention. As may be derived from the above discussion, fι_arge (the frequency used for large orifice control) will typically be less than f_smaιι (the frequency used for small orifice control). Typically, fι_{arge =}f_smaιι/2.

The actuating signal is computed and generated directly by the control device 100 or by providing the frequency value and the duty cycle value to a PWM controller. The large orifice control enables coarse control of the resulting fluid flow, in particular in a higher range of the total valve flow range.

The actuating signal is supplied to the valve. Then the process reiterates at step 610.

Although not explicitly shown in fig. 6, the process may of course be terminated by appropriate operating conditions or operating means. Although the invention has been particularly described with reference to a three- state fluid valve, the skilled person will realize that the principles of the invention may also be used with valves wherein a valve element has more than one transient position between its stable outermost (closed and fully opened) positions. In accordance with the detailed example embodiment, the frequency of the actuating signal is altered between two distinct values in the two operation modes (small orifice - large orifice) of the valve. The skilled person will however realize that alternative embodiments of the invention exist wherein the frequency may be varied further than the two suggested values. The present invention is especially applicable for clutch actuation valves. However, the skilled person will realize that the invention may be used in a large variety of hydraulic and pneumatic applications.

Claims

1. Method for operating a fluid valve, c h a r a c t e r i z e d i n that the valve comprises

- a valve element which is moveable between - a first, stable position representing a closed valve position, a second, transient position representing a semi-open valve position and a third stable position representing an open valve position, and

- an actuating element for causing the valve element to move between said first and third positions as a function of a pulsed actuating signal, the method comprising the step of varying the duty cycle of said pulsed actuating signal in accordance with a reference input in order to utilize the second transient position of the valve element during the operation of the fluid valve.

2. Method according to claim 1, wherein said reference input is a desired effective orifice area (a_d) of the valve, and wherein the pulsed actuating signal is substantially set to zero if the desired effective orifice area (a£) is less than a first limit value (α/_ow).

3. Method according to claim 1 or 2, wherein the duty cycle (dc)of the pulsed actuating signal is calculated as a first function of the reference input if the desired effective orifice area (ad) is less than a second limit value (amφ).

4. Method according to claim 3, wherein said first function of the reference input is a linear function of the reference input, the function having constant parameters that are predetermined, based on characteristics of the valve, including area and timing characteristics.

5. Method according to one of the claims 3 or 4, wherein the duty cycle (dc) of the pulsed actuating signal is calculated as a second function of the reference input if the desired effective orifice area (a_d) is larger than said second limit value (aι,igh)-

6. Method according to claim 5, wherein said second function of the reference input is a linear function of the reference input, the function having constant parameters that are predetermined and based on characteristics of the valve, including area and timing characteristics.

7. Method according to one of the claims 1-6, further comprising varying the frequency of said pulsed actuating signal in accordance with the reference input in order to operate the valve.

8. Method according to claim 7, wherein a first frequency (f s_mal_l) is used for the pulsed actuating signal if the desired effective orifice area (ad) is less than a said second limit value (phigh)

9. Method according to one of the claims 7 or 8, wherein a second frequency (/ }_arge) is used for the pulsed actuating signal if the desired effective orifice area (ad) is larger than said second limit value (α_/,,_g/,).

10. Method according to one of the preceding claims, wherein said limit values (α/_ow, ct_hig_h) are predetermined, based on characteristics of the valve, including area and timing characteristics.

11. Control device for operating valve a fluid valve, c h a r a c t e r i z e d i n that the valve comprises

- a valve element which is moveable between - a first, stable position representing a closed valve position, a second, transient position representing an semi-open valve position and a third stable position representing a open valve position, and

- an actuating element for causing the valve element to move between said first and third positions as a function of a pulsed actuating signal, the control device comprising a generator for said pulsed actuating signal, configured for varying the duty cycle of said pulsed actuating signal in accordance with a reference input in order to utilize the second transient position of the valve element during the operation of the fluid valve.

12. Control device according to claim 11, wherein said reference input is a desired effective orifice area (a_d) of the valve, and wherein the control device is configured for setting the pulsed actuating signal substantially to zero if the desired effective orifice area (ad) is less than a first limit value (a_hw).

13. Control device according to claim 11 or 12, wherein the control device is configured for calculating the duty cycle (<ic)of the pulsed actuating signal as a first function of the reference input if the desired effective orifice area (aj) is less than a second limit value (aι,i_gh)-

14. Control device according to claim 13, wherein said first function of the reference input is a linear function of the reference input, the function having constant parameters that are predetermined and based on characteristics of the valve, including area and timing characteristics.

15. Control device according to one of the claims 11-14, wherein the control device is configured for calculating the duty cycle (dc) of the pulsed actuating signal as a second function of the reference input if the desired effective orifice area (ad) is larger than a second limit value (ah,_gh)-

16. Control device according to claim 15, wherein said second function of the reference input is a linear function of the reference input, the function having constant parameters that are predetermined and based on characteristics of the valve, including area and timing characteristics.

17. Control device according to one of the claims 11-16, wherein the control device is further configured for varying the frequency of said pulsed actuating signal in accordance with the reference input in order to operate the valve.

18. Control device according to claim 17, wherein a first frequency (f_smaιι) is used for the pulsed actuating signal if the desired effective orifice area {ad) is less than a said second limit value (ajύg_h)

19. Control device according to one of the claims 17 or 18, wherein a second frequency (fia_rge) is used for the pulsed actuating signal if the desired effective orifice area (α^) is larger than said second limit value (ct_hig_h)-

20. Control device according to one of the claims 11-19, wherein said limit values (α/_ow, α_/%/0 are predetermined and based on characteristics of the valve, including area and timing characteristics.

21. System for operating a fluid valve, c h ar a c t e r i z e d i n that the system comprises

- a fluid valve which includes a valve element which is moveable between a first, stable position representing a closed valve position, a second, transient position representing a semi-open valve position and a third stable position representing a open valve position, and

- an actuating element for causing the valve element to move between said first and third positions as a function of a pulsed actuating signal, and

- a control device as set forth in one of the claims 11-20 for operating the valve.