using a shape memory alloy member

Description

The present invention relates to a valve assembly for controlling a flow of a fluid, and to a method for controlling a flow of a fluid using such a valve assembly. The valve assembly comprises a valve housing defining a cavity for receiving and expelling the fluid, a spool disposed in the cavity and movable therein between a first position and a second position, wherein the fluid is permitted to flow through a first port of the cavity when the spool is in the first position and wherein the fluid is permitted to flow at least through a second port of the cavity when the spool is in the second position. An actuator is attached to the spool, the actuator including a shape memory alloy member, wherein the actuator is configured to move the spool between the first position and the second position. The shape memory alloy member is configured to be in a deactivated state when the spool is in the first position and to change into an activated state during a transition of the spool into the second position, wherein an electric circuit is provided for conducting an electric current through at least a segment of the shape memory alloy member.

Heating and cooling circuits in a hydraulic system, for example, require thermally activated valves for the distribution of a fluid flow into separate circuits depending on the fluids temperature. In a cooling circuit of an engine of a vehicle, in particular, the fluid is guided into a short cooling circuit after starting the cold engine. After a certain operating time has elapsed the hot engine will require sufficient cooling. This is achieved by switching the fluid flow from the short to a large cooling circuit. Temperature dependent actuators like wax actuators or bimetals are known in this regard. Adding additional heaters like electrical heaters to these actuators, further allows to control the switching.

As a further example, it is useful in diesel fuel supply systems, especially during start up at low environmental temperatures, to warm the fuel before it reaches a fuel pump in order to minimize the risk of wax formation and pressure drops within the fuel supply system. In some applications, the heat energy gained by the fuel as it passes through a fuel pump is used to warm the feed to the fuel pump by partial recirculation of excess fuel into the feed. This facilitates starting and initial running of the engine. However, once an acceptable fuel temperature is reached the excess fuel is returned to the fuel tank as excessive fuel tem- perature within the fuel pump may lead to degradation of the fuel pump. Such heated fuel recirculation is as well achieved using a thermostatic diverting valve to feed some, all or none of the fuel leaving the fuel pump back into the feed to the fuel pump. Such thermostatic valves may also use bimetallic components that change configuration as their temperature changes.

As the mechanical movement of a bimetallic component in response to a temperature change may not be sufficient to open large apertures valves for controlling the liquid flow are used, which comprise a valve member being movable in response to the condition of a shape memory alloy (SMA) component having a transformation temperature, the SMA component biasing the valve member to a first position, wherein the SMA component is in thermal contact with the liquid, and as the temperature of the SMA component reaches the transformation temperature the SMA component expands or contracts and biases the valve member to a second position. In US 4 973 024 A, for example, a valve is disclosed, the valve comprising a valve casing provided with a valve port; a valve element guided within said casing for movement in directions for opening the valve port and closing the valve port; biasing means for biasing said valve element in a direction to close the valve port; a wire-shaped element being constructed of shape memory alloy which reverts to an original length when heated to or above a critical temperature, the wire-shaped element being operatively connected to said valve element so as to undergo an elongation deformation from its original length when said valve element is subjected to the biasing force of the biasing means at a temperature below the critical temperature. An electric circuit is provided for conducting an electric current through the wire-shaped element to thereby heat the wire-shaped element to pro- duce a lengthwise contraction thereof from its elongated deformed state and an attendant movement of the valve element in a direction to open the valve port.

For a reasonable distribution of a fluid flow, an adequate knowledge about the actual posi- tion of the valve, or the temperature and flow rate of the fluid would be desirable. Though there are feasible sensors for such purpose, this option is usually not pursued due to reliability or economic reasons. It is an objective of this invention to provide a valve assembly and a method for controlling a flow of a fluid using SMA material to actuate the valve, wherein a feedback about at least one of the valve position, the fluid temperature and the fluid flow rate are obtained in a facile and reliable way. The objective is achieved by a valve assembly and a method for controlling a flow of a fluid according to claims 1 and 9. The respective dependent claims refer to preferred embodiments.

The valve assembly for controlling a flow of a fluid according to the invention comprises a valve housing defining a cavity for receiving and expelling the fluid, a spool disposed in the cavity and movable therein between a first position and a second position, wherein the fluid is permitted to flow through a first port of the cavity when the spool is in the first position and wherein the fluid is permitted to flow at least through a second port of the cavity when the spool is in the second position. The valve assembly according to the invention further comprises an actuator attached to the spool and including a shape memory alloy (SMA) member, wherein the actuator is configured to move the spool between the first position and the second position, wherein the shape memory alloy member is configured to be in a deactivated state when the spool is in the first position and to change into an activated state during a transition of the spool into the second position, and an electric circuit for conducting an electric current through at least a segment of the shape memory alloy member. Preferably, a full length of the shape memory alloy member is part of the electric circuit. For the following description, however, it is noted that only a segment of the shape memory alloy member may be included in the circuit, even if it is not expressly declared. According to the invention, a resistance meter is connected to the electric circuit and configured for measuring a development over time of an ohmic resistance of the segment of the shape memory alloy member. Shape memory alloys have been found to provide an inherent sensory property, of which the invention makes use in an advantageous way. Connecting a resistance meter to the electrical circuit allows to obtain active feedback about the valve position, as well as the fluid temperature and the fluid flow rate, which may be derived from the measured devel- opment of the ohmic resistance of the shape memory alloy member over time.

A shape memory alloy, herein further also referred to in short as SMA, is a material with the property of returning to a previously defined shape or size when subjected to an appropriate thermal procedure. In many cases the appropriate thermal procedure is simply to raise or lower the temperature of the SMA past a transformation temperature at which the crystal structure of the alloy changes, changing the mechanical properties of the material. An example of a shape memory alloy is Nickel-Titanium alloy, which is commonly used in a binary way. The SMA member of the valve assembly may be made substantially of SMA only, or may include other materials that do not behave in the same way as an SMA, but do not substantially affect the function of the valve.

In the valve assembly according to the invention the temperature of the SMA member is preferably dependent, at least in part, upon the temperature of the fluid within the cavity. If the temperature of the fluid changes and the temperature of the SMA member passes the transformation temperature of the SMA, the mechanical properties of the SMA member change. This may cause the SMA member to extend or contract and cause the spool to move. The SMA member is preferably provided in the form of a wire, though the form of the SMA member may as well be that of a spring, the spring preferably being a helical spring or any other suitable spring made from SMA. A further preferred embodiment refers to the valve with the actuator being partly or fully immersed within the fluid flow, thus enabling an advantageously fast response to the fluids actual temperature. The extension or contraction of the SMA member may be due to the member expanding or contracting as the mechanical properties of the SMA change. It may also be a result of the elastic properties of the member changing and the member being expanded or contracted by the force of a co-operating resilient biasing means. The SMA member may change configuration during transformation such that the resultant movement of the spool is sufficient to move the spool from the first position to the second position, or vice versa. However, it is preferred that the actuator includes a resilient biasing means that co-operates with the SMA member to bias the spool to the first or second position and the valve will be described hereinafter with reference to this preferred embodiment.

The properties of the SMA member preferably change quickly. A faster reaction is advanta- geously achieved by a low thermal mass of the SMA member and by the direct immersion of the SMA member within the fluid. Whenever the expression transition temperature is used herein, in the sense of the invention, it is not meant to define a closely confined temperature at which the SMA member suddenly changes into its respective other state, because the changing process advances continuously. It rather denotes a first temperature range, in which a temperature change causes a substantial deflection of the SMA member, and distinguishes from a second temperature range in which the SMA member's deflection is rather minor. Advantageously, the SMA member's direct and fast reaction to temperature changes of the surrounding in the first range is improved over a bimetallic valve actuator.. The SMA member also allows an advantageously great mechanical movement of the spool at the first temperature range and thus advantageously enables larger ports to be opened than would be possible with a bimetallic valve actuator. The advantageous mechanical movement is due to a large strain of typically about 4% without any kind of gearing or transmission, in combination with a typically elongate shape of the SMA member, which thus preferably is a wire. An SMA member in the form of a spring, which acts like a trans- mission, an effective strain of 50% can be reached.

According to a preferred embodiment of the valve assembly a control logic comprising a processor is provided, the processor being adapted to perform an analysis of the measured development over time of the ohmic resistance of the shape memory alloy member. The control logic preferably further comprises a data storage for storing historical data of the ohmic resistance measurement.

According to a further preferred embodiment of the valve assembly the control logic com- prises a model of the shape memory alloy member for determining a current deflection of shape of the shape memory alloy member correlated to the measured development over time of the ohmic resistance of the shape memory alloy member. The ohmic resistance of the shape memory alloy member depends upon a number of conditions including the tem- perature and the shape, in particular a length and a cross-sectional area, of the SMA member. Further, a basic resistance also depends upon the actual crystal structure of the SMA member. Thus, a model representing the correlation between the ohmic resistance and deflection of shape of the SMA member is advantageous. Further, any property changes of the SMA member including the ohmic resistance are subject to hysteresis. Preferably, the model of the shape memory alloy member represents a material hysteresis characteristic of the shape memory alloy member when changing from the deactivated state into the activated state and vice versa. In particular, the model covers an outer hysteresis, as well as an inner hysteresis, the latter referring to an inversion of a direction of the change without fully undergone transformation into the activated or into the inactivated state. Then, ad- vantageously, the model of the shape memory alloy member represents the hysteretic behaviour of the SMA material in reaction to temperature changes. In particular, from the deactivated state into the activated state and vice versa, but also including a transition from any intermediate state to any one of those states, or to another intermediate state. The hysteresis requires a consideration of the development over time of the ohmic resistance, as the currently measures resistance values alone could not be correlated to a deflection of shape unambiguously. Furthermore, the SMA member is subjected to wear over a certain number of work cycles. Thus, the model of the SMA member may advantageously be adapted to a certain wear state using and analyzing the historical data of the ohmic resistance measurement. Preferably the control logic is configured to determine a position or at least a state of the spool from the deflection of shape of the shape memory alloy member. The position or state of the spool is a necessary input value for controlling a fluid flow through the valve assembly according to the invention.

Rather than implementing a model of just the valve itself, the microprocessor may fur- thermore preferably include a model of a cooling circuit, in which the valve according to the invention is usually applied to control the fluid flow, hence enabling an advantageously fast and fully autonomous control of the temperature of the cooling circuit. According to a yet further preferred embodiment of the valve assembly the control logic is configured to determine from the measured development over time of the ohmic resistance of the shape memory alloy member at least one development over time of a temperature of the fluid, a strain of the shape memory alloy member and a flow rate of the fluid. The tem- perature and flow rate of the fluid are further essential input values for controlling the fluid flow with regard to a heating or cooling fluid circuit. The strain of the SMA member is preferably determined as an auxiliary variable to obtain the temperature and flow rate of the fluid. The flow rate and temperature of the fluid may alternatively be calculated directly from a sufficiently accurate model, independently and without prior knowledge of the strain.

According to yet a further preferred embodiment of the valve assembly the control logic is provided for controlling the flow of the fluid in dependency of at least one of the following input values: a position or state of the spool, a temperature of the fluid, and a flow rate of the fluid.

The method for controlling a flow of a fluid, according to the invention, also solves the problem stated above, using a valve assembly as referred to in here before. According to the invention, the method comprises a step of measuring a development over time of an ohmic resistance of the segment of the shape memory alloy member. As discussed before, at least one of a position or state of the spool, a temperature of the fluid, and a flow rate of the fluid may advantageously be obtained from the recorded development over time of the ohmic resistance. An adequate knowledge of said values is able to greatly enhance the control of the fluid flow. As the recorded development of the ohmic resistance has to be re- garded in order to obtain said values, it is preferred that historic data of the measured development over time of the ohmic resistance of the shape memory alloy member is stored and analyzed. In particular, the measured development over time of the ohmic resistance of the shape memory alloy member is analyzed, taking into account a hysteresis of the development between a transition from the deactivated state into the activated state and a tran- sition vice versa of the SMA member. As noted before, in the sense of the invention, only a segment of the shape memory alloy member needs to be included in the electric circuit for conducting the electric current through and measuring the ohmic resistance. It is thus also understood that the complete shape memory alloy member may be included in the electric circuit, which applies to any embodiment described herein, unless it is expressly declared otherwise.

According to a preferred embodiment of the method, a model of the shape memory alloy member is used to determine a current deflection of the shape memory alloy member. A position or a state of the spool is then, preferably, determined from the current deflection of the shape memory alloy member.

According to a further preferred embodiment of the method, a strain of the shape memory alloy member is determined from the measured development over time of the ohmic resistance of the shape memory alloy member. Then, furthermore preferably, a current temperature of the shape memory alloy member is determined from a historic development of the strain and the ohmic resistance of the shape memory alloy member. The temperature of the shape memory alloy member is, however, not that important, and it is rather used as an auxiliary variable. Advantageously, from the shape memory alloy member's temperature the temperature of the fluid can be concluded. The current temperature of the shape memory alloy member corresponds to the current temperature of the surrounding fluid, which is advantageously determined, according to the embodiment, as the fluid temperature is an important influencing value for the control of the fluid flow.

According to a preferred embodiment of the method, a signal is sent through the electric circuit and at least one parameter that influences a thermal response of the shape memory alloy member is determined from a response of the measured development over time of the ohmic resistance of the shape memory alloy member to the signal through the electric circuit. The determined parameter is, for example, one of a flow rate of the fluid near the shape memory alloy member, a concentration of a substance in the fluid, like, in particular, a percentage of ethanol or other cooling agents in the circuit, a current phase of the fluid, and a heat conductivity of the fluid, which correlates, for example, with an amount of salt in the fluid.

According to a particularly preferred embodiment of the method, the flow rate of the fluid is determined by the following steps: a thermal time constant of the shape memory alloy member is determined from the response to heating the shape memory alloy member us- ing the signal through the electric circuit, subsequently, a convection coefficient is calculated from the determined thermal time constant, and the flow rate near the shape memory alloy member is calculated from the convection coefficient. The invention will further be described with respect to an exemplary embodiment of the valve assembly according to the invention, as shown in the attached drawings. The explanations are exemplary and do not limit the scope of the invention. They refer to the method according to the invention, as well.

In the drawings

Figure 1 shows a schematic illustration of a sectional view of an embodiment of a valve assembly according to the invention;

Figures 2A, 2B and 2C show the embodiment of Figure 1 in reduced detail, each illustrating a different state of the valve.

A construction of the exemplary embodiment of the valve assembly is described with reference to Figure 1. The valve assembly is adapted for controlling a flow of a fluid, the flow into a cylindrical valve housing 1 defining a cavity 2 being illustrated by an arrow P. Inside the cavity 2, a spool 3 is disposed and movable axially therein between a first position, which is depicted in Figure 1 and a second position, which will be referred to later with respect to Figure 2C. It is noted, however, that the spool 3 may as well take any position between the first and second positions, as shown exemplary in Figure 2B. In the first position of Figure 1, the fluid is permitted to flow through the spool 3 and a first port 4 out of the cavity 2, which flow is illustrated by an arrow R. When the spool 3 is in the first position the fluid is not permitted to flow through a second port 5 of the cavity 2, as the second port 5 is closed by a side wall of the spool 3. The fluid may flow through the second port 5 of the cavity 2 when the spool 3 is in the second position, which flow is illustrated by an arrow Q, although it is understood that with respect to the embodiment illustrated in Figure 1, the flow through the second port 5 is zero. An actuator 6 is attached to the spool 3 and includes a shape memory alloy member 7 in the exemplary form of a wire, which is secured to the valve housing 1 at an end stop portion 16 with both ends of the SMA member 7. Midway of the wire length, the SMA member 7 is secured to the spool 3. The actuator 6 further comprises a resilient spring 8, which abuts the end stop portion 16 on one end and the spool 3 on the other end, thus biasing the spool 3 towards the first position. The actuator 6 is configured to move the spool 3 between the first position and the second position, wherein the shape memory alloy member 7 is configured to be in a deactivated state when the spool is in the first position and, by changing into an activated state, moving the spool 3 against the resilient spring 8 towards the second position. An electric circuit 9 is provided for conducting an electric current through the shape memory alloy member 7, which is connected to a resistance meter 15 configured for measuring a development over time of an ohmic resistance of the shape memory alloy member 7.

The resistance meter 15, a processor 11 and a data storage 12 for storing historical data of the ohmic resistance measurement are illustrated as parts of a control logic 10, but may as well be standalone equipment connected in any suitable way including a network. The control logic 10 is adapted to control the actuator 6, in particular by applying a voltage to the electric circuit 9 for heating the SMA member 7, to or above its transitional temperature, thus initiating the SMA member's transition from the inactivated state to the activated state. The values of the ohmic resistance measured over time are further processed by the control logic 10 using, for example, the processor Hand the data storage 12. Preferably, a model 14 of the shape memory alloy member 7 is filed on the data storage 12 and used for determining a current deflection of shape of the shape memory alloy member 7 correlated to the measured development over time of the ohmic resistance of the shape memory alloy member 7. From that, the control logic 10 may easily determine a position of the spool 3 or at least a state of the spool 3. Further, the control logic is configured to determine from the measured development over time of the ohmic resistance of the shape memory alloy mem- ber 7 at least one of a current temperature of the fluid, a current strain of the shape memory alloy member 7 and a current flow rate of the fluid. Thus, the control logic 10 is advantageously endowed to control the flow of the fluid in dependency of at least one of said input values. By using an SMA wire 7 to actuate the spool 3 and exploiting the inherent sensory properties of the SMA material, it is possible to obtain active feedback about the position of the spool 3, as well as the fluid temperature and flow rate. Instead of adding any additional sensors, these variables are determined from the measurement of the resistance of the SMA wire 7 in connection with eligible signal processing.

The fluid flow control function is referred to as follows with reference to Figures 2A, 2B and 2C, which are described together. The reference numerals of Figure 1 are used, even if they do not appear in the Figures 2A, 2B and 2C. By moving the spool 3, the fluid flow may be freely distributed between the first and second ports 4, 5, which may, for example be connected to a long cooling circuit and to short cooling circuit of a vehicle motor. The spool 3 is connected to a combination of the preloaded spring 8 and the SMA wire 7, which are arranged in such a way, that heating the SMA wire 7 causes the SMA wire 7 to contract and open the passage to the second port 5 of the long cooling circuit, as depicted in Figure 2B illustrating a fluid flow by arrow T, which flow is divided to the first and second ports 4, 5 and in Figure 2C illustrating an undivided fluid flow by arrow U to the second port 5. Cooling the SMA wire 7 leads to an elongation of the SMA wire 7 and the first port 4 of the short cooling circuit passage is opened, as depicted in Figure 2A, illustrating a fluid flow by arrow S to the first port 4. It is noted that the SMA member 7 does not deliver the power to move the spool 3 by the transition back from the activated to the inactivated state. The necessary power is provided by the resilient spring 8.

In an unpowered state of the actuator 6, the temperature dependent behaviour of the SMA material is exploited for passive valve actuation, while in an energized state of the actuator 6, the supply of additional heating power via the circuit 9 offers further control over the valve assembly and, for example, the cooling circuits connected thereto. However, the main advantage of using SMA and the focus of the invention lies in the possibility of estimating system parameters by exploiting the inherent sensory properties of SMA. It is possible to estimate the current deformation of the SMA wire 7 from the resistance measurement. This means - at least in the context of the applications described herein - that the state of the spool 3 and respectively the distribution of the fluid flow can be determined only from the measurement of the resistance of the SMA wire 7. Furthermore, a response of the SMA wire 7 to an applied power signal depends on an amount of convective power loss, which itself depends on the fluid temperature and flow rate. Since both parameters influence the response of the SMA wire 7 in a different manner, they may advantageously be calculated independently from the temporal behaviour of the resistance of the SMA wire 7. Though the example of a closed hydraulic system has been referred to, the valve assembly and method according to the invention may as well be applied for the control of gaseous flows, for example in an air conditioning, or of open circuits, for example for overheating protection. By using the SMA member 7 for the spool actua- tion, a thermally activated valve assembly is provided, which allows for - but does not necessarily require - external control. By exploiting the inherent sensory properties of SMA, it is possible to determine the valves current position or state as well as the fluid temperature and flow rate without adding any additional sensors.

Reference numerals

1 Valve housing

2 Cavity

3 Spool

4 First port

5 Second port

6 Actuator

7 Shape memory alloy member, SMA member

8 Resilient spring

9 Electric circuit

10 Control logic

11 Processor

12 Data storage

14 Model

15 Resistance meter

16 Stop portion

P, Q, R Flow illustrating arrows

S, T, U Flow illustrating arrows