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Title:
CONTROL AND MONITORING METHOD FOR A VALVE
Document Type and Number:
WIPO Patent Application WO/2020/207832
Kind Code:
A1
Abstract:
A method of monitoring functionality of an electromagnetic flexure valve (1), the valve including a flexure assembly (40) having a moveable part (40a) which is moveable between a pair of pole pieces (50a, 50b), and a coil which is configured to receive electrical power and to actuate the flexure assembly (40), the method including: providing a monitoring device, and at least one of: one or more strain gauges (80), the or each strain gauge (80) being configured to provide a strain gauge output (320) to the monitoring device, one or more field effect transistors, the or each field effect transistor being configured to provide a field effect transistor output (310) to the monitoring device, providing a supply voltage to the coil to drive the moveable part (40a) of the flexure assembly (40) towards one of the pole pieces (50a, 50b), monitoring at least one of the strain gauge output (320), the field effect transistor output (310), and a rate of change of current (300) through the coil during a monitoring period to determine whether the moveable part (40a) of the flexure assembly (40) is stuck.

Inventors:
PRICE GARY ALAN (GB)
MELLINGS CARL (GB)
Application Number:
PCT/EP2020/058858
Publication Date:
October 15, 2020
Filing Date:
March 27, 2020
Export Citation:
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Assignee:
HALDEX BRAKE PROD AB (SE)
International Classes:
G01M13/003; B60T8/88; B60T13/68; B60T17/22; F16K31/06; F16K31/08; F16K37/00; G01M17/00
Foreign References:
EP2756215A12014-07-23
US20180088166A12018-03-29
EP2756215A12014-07-23
GB201719309A2017-11-21
Attorney, Agent or Firm:
FORRESTERS IP LLP (GB)
Download PDF:
Claims:
CLAIMS

1. A method of monitoring functionality of an electromagnetic flexure valve, the valve including a flexure assembly having a moveable part which is moveable between a pair of pole pieces, and a coil which is configured to receive electrical power and to actuate the flexure assembly, the method including:

providing a monitoring device, and at least one of:

one or more strain gauges, the or each strain gauge being configured to provide a strain gauge output to the monitoring device,

one or more field effect transistors, the or each field effect transistor being configured to provide a field effect transistor output to the monitoring device,

providing a supply voltage to the solenoid to drive the moveable part of the flexure assembly towards one of the pole pieces,

monitoring at least one of

the strain gauge output,

the field effect transistor output, and

a rate of change of current through the coil

during a monitoring period to determine whether the moveable part of the flexure assembly is stuck.

2. A method according to claim 1 wherein the step of monitoring whether the moveable part of the flexure assembly is stuck includes determining whether the moveable part of the flexure assembly has moved and/or is moving, and/or the flexure assembly has not moved and/or the flexure assembly is stationary.

3. A method according to claim 1 or claim 2 wherein the supply voltage is provided in accordance with a predetermined current profile of the coil, to actuate the flexure assembly. 4. A method according to any preceding claim including providing the supply voltage such that there is a first increase in current through the coil and a second increase in current through the coil.

5. A method according to any of the preceding claims including comparing the actual current through the coil with an expected current through the coil, wherein detecting an absence of an expected disturbance in the rate of change of current through the coil is indicative of the moveable part of the flexure assembly being stuck.

6. A method according to any preceding claim including determining an expected variance in the strain gauge output, and in the event that the strain gauge variance is smaller than the expected strain gauge output, providing a signal that the moveable part of the flexure assembly is stuck.

7. A method according to any of the preceding claims including comparing an actual field effect transistor output with an expected field effect transistor output, wherein a modulation pattern of the actual field effect transistor output is different from a modulation pattern of the expected field effect transistor output.

8. A method according to claim 7 wherein a substantially continuous modulation pattern of the actual field effect transistor output is indicative that the moveable part of the flexure is stuck.

9. A method according to claim 7 or claim 8 wherein detecting a disturbance in the pulse width of the modulation pattern of the field effect transistor is indicative that the moveable part of the flexure is moving or has moved. 10. A method according to any of the preceding claims wherein the monitoring period is selected to be greater than an expected transition time for the moveable part of the flexure. 1 1. A method according to any of the preceding claims, wherein the electromagnetic flexure valve is part of a vehicle braking system, and in the event that the monitoring device detects that the moveable part of the flexure assembly is stuck, the monitoring device provides a failure signal to a control device of the vehicle braking system.

1 1. A valve assembly including an electromagnetic flexure valve which includes a flexure assembly having a moveable part which is moveable between a pair of pole pieces, and a coil which is configured to receive electrical power and to actuate the flexure assembly, the method including: providing a monitoring device, and at least one of:

one or more strain gauges, the or each strain gauge being configured to provide a strain gauge output to the monitoring device,

one or more field effect transistors, the or each field effect transistor being configured to provide a field effect transistor output to the monitoring device,

providing a supply voltage to the solenoid to drive the moveable part of the flexure assembly towards one of the pole pieces,

monitoring at least one of

the strain gauge output,

the field effect transistor output, and

a rate of change of current through the coil

during a monitoring period to determine whether the moveable part of the flexure assembly is stuck. 12. A valve assembly wherein the or each field effect transistor is a

MOSFET. 13. A valve assembly according to claim 1 1 or claim 12, wherein the valve assembly is incorporated in a vehicle braking system. 14. A valve assembly according to claim 13 wherein the vehicle braking system is an electro-pneumatic braking system.

Description:
Title: Control and monitoring method for a valve

Description of Invention

Embodiments of the invention relate to a control and monitoring method for a valve, in particular, but limited to, a valve which may be included in a fluid control system, for example a vehicle brake system.

Binary actuator valves suitable for use in fluid control systems are known. A good example of a significant development can be found in EP2756215.

With reference to figure 1 of the present application, EP2756215 teaches an electromagnetic valve comprising a yoke 10. A magnet 30a, 30b is provided having pole pieces 50a, 50b defining a gap 60. A flexure assembly 40 has one end attached to the yoke 10, such that part of the flexure assembly 40 extends into the gap 60. The flexure assembly 40 has at least one resilient portion 40a formed of a resilient material and at least one magnetisable portion, wherein the part of the flexure assembly 40 that extends into the gap is movable between the pole pieces 50a, 50b through an intermediate position towards which it is resiliently biased such that a resilient mechanical force is generated by deflecting the resilient portion from an undeflected position. A device 20 (e.g. a solenoid) is provided for polarising the magnetisable portion of the flexure assembly 40 so that the part 40a of the flexure assembly 40 that is movable between the pole pieces 50a, 50b is attracted towards a pole piece 50a, 50b by a magnetic force, thereby defining a valve state. The magnetisable portion and the resilient portion of the flexure assembly 40 are configured such that the magnetic force defining the valve state is greater than the resilient mechanical force. EP2756215 and GB 1719309.5 disclose forms of flexure assembly which can be used in valves of fluid control systems, such as vehicle braking systems, in which fast switching times are required with large pressure differentials across the valve.

If there is no electrical power provided to the device 20 (e.g. to the coil of the solenoid) of the valve of EP2756215, then the state of the valve can be unpredictable or, at least, dependent on the state of the valve immediately prior to termination of the supply of electrical power. In particular, the flexure assembly 40 may be retained in a position such that it abuts a particular pole piece 50a, 50b (e.g. if the flexure assembly 40 was abutting that pole piece 50a, 50b when electrical power was lost) or may return to the intermediate position (e.g. if the flexure assembly 40 had not reached a position close enough to a pole piece 50a, 50b to ensure that the elastic flexure force of the flexure assembly 40 could be overcome by the magnetic force and air pressure force) - see figures 2A-2D of EP2756215, for example).

In known configurations, when the flexure assembly 40 is energised in either direction, it is difficult to determine whether the moveable part of the flexure has“toggled” towards one of the pole pieces 50a, 50b. Therefore, a problem with known configurations is that the device 20 (e.g. the solenoid) is“over driven” during every operation with a current high enough to ensure that flexure assembly 40 toggles to the correct pole piece 50a, 50b. The only known means of detecting that the flexure assembly 40 is“stuck” when the valve is used in a brake assembly, for example, is by detecting an unexpected pressure change at one of a pressure sensor provided to detect pressure at or near a delivery port of the valve or a pressure sensor which is provided to detect supply pressure at or near a fluid reservoir. It is desirable to alleviate one or more problems associated with current flexure assemblies.

In accordance with embodiments of the invention, there is provided a method of monitoring functionality of an electromagnetic flexure valve, the valve including a flexure assembly having a moveable part which is moveable between a pair of pole pieces, and a coil which is configured to receive electrical power and to actuate the flexure assembly, the method including: providing a monitoring device, and at least one of:

one or more strain gauges, the or each strain gauge being configured to provide a strain gauge output to the monitoring device,

one or more field effect transistors, the or each field effect transistor being configured to provide a field effect transistor output to the monitoring device,

providing a supply voltage to the solenoid to drive the moveable part of the flexure assembly towards one of the pole pieces,

monitoring at least one of

the strain gauge output,

the field effect transistor output, and

a rate of change of current through the coil

during a monitoring period to determine whether the moveable part of the flexure assembly is stuck.

The step of monitoring whether the moveable part of the flexure assembly is stuck may include determining whether the moveable part of the flexure assembly has moved and/or is moving, and/or the flexure assembly has not moved and/or the flexure assembly is stationary.

The supply voltage may be provided in accordance with a predetermined current profile of the coil, to actuate the flexure assembly. The method may include providing the supply voltage such that there is a first increase in current through the coil and a second increase in current through the coil. The method may include comparing the actual current through the coil with an expected current through the coil, wherein detecting an absence of an expected disturbance in the rate of change of current through the coil may be indicative of the moveable part of the flexure assembly being stuck. The method may include determining an expected variance in the strain gauge output, and in the event that the strain gauge variance is smaller than the expected strain gauge output, providing a signal that the moveable part of the flexure assembly is stuck. The method may include comparing an actual field effect transistor output with an expected field effect transistor output, wherein a modulation pattern of the actual field effect transistor output is different from a modulation pattern of the expected field effect transistor output. A substantially continuous modulation pattern of the actual field effect transistor output may be indicative that the moveable part of the flexure is stuck.

Detecting a disturbance in the pulse width of the modulation pattern of the field effect transistor may be indicative that the moveable part of the flexure is moving or has moved.

The monitoring period may be selected to be greater than an expected transition time for the moveable part of the flexure. The electromagnetic flexure valve may be part of a vehicle braking system, and in the event that the monitoring device detects that the moveable part of the flexure assembly is stuck, the monitoring device provides a failure signal to a control device of the vehicle braking system.

There is also provided a valve assembly including an electromagnetic flexure valve which includes a flexure assembly having a moveable part which is moveable between a pair of pole pieces, and a coil which is configured to receive electrical power and to actuate the flexure assembly, the method including:

providing a monitoring device, and at least one of:

one or more strain gauges, the or each strain gauge being configured to provide a strain gauge output to the monitoring device,

one or more field effect transistors, the or each field effect transistor being configured to provide a field effect transistor output to the monitoring device,

providing a supply voltage to the solenoid to drive the moveable part of the flexure assembly towards one of the pole pieces,

monitoring at least one of

the strain gauge output,

the field effect transistor output, and

a rate of change of current through the coil

during a monitoring period to determine whether the moveable part of the flexure assembly is stuck.

The or each field effect transistor may be a MOSFET.

The valve assembly may be incorporated in a vehicle braking system. The vehicle braking system may be an electro-pneumatic braking system. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which:

FIGURE 1 A is a cross-sectional view of an electromagnetic flexure valve; FIGURE 1 B is an illustrative cross-sectional view of a part of the valve of Figure 1A, showing additional components which may be used in a monitoring method;

FIGURE 2 is an illustration of an exemplary current profile for an electromagnetic flexure valve;

FIGURE 3 shows traces indicative of current though a coil of an electromagnetic flexure valve, a switching signal of a field effect transistor, and a strain gauge output, when a moveable part of the flexure valve is stuck in an “open” position;

FIGURE 4 shows traces indicative of current though a coil of an electromagnetic flexure valve, a switching signal of a field effect transistor, and a strain gauge output, when a moveable part of the flexure valve is stuck“in a “closed” position;

FIGURE 5 shows traces indicative of current though a coil of an electromagnetic flexure valve, a switching signal of a field effect transistor, and a strain gauge output, when a moveable part of the flexure valve is operating substantially normally;

FIGURE 6 is an illustration of an alternative current profile for an electromagnetic flexure valve;

FIGURE 7 shows traces indicative of current though a coil of an electromagnetic flexure valve, a switching signal of a field effect transistor, and a strain gauge output, when operated in accordance with the current profile of Figure 6 and moveable part of the flexure valve is stuck in an“open” position; FIGURE 8 shows traces indicative of current though a coil of an electromagnetic flexure valve, a switching signal of a field effect transistor, and a strain gauge output, when operated in accordance with the current profile of Figure 6 and a moveable part of the flexure valve is stuck“in a “closed” position; FIGURE 9 shows traces indicative of current though a coil of an electromagnetic flexure valve, a switching signal of a field effect transistor, and a strain gauge output, when operated in accordance with the current profile of Figure 6 and a moveable part of the flexure valve is operating substantially normally;

FIGURE 10 is a schematic illustration of an exemplary configuration of switching devices;

FIGURE 1 1 is a schematic illustration of an alternative exemplary configuration of switching devices;

FIGURE 12 is a schematic illustration of a further alternative exemplary configuration of switching devices; and

FIGURE 13 is a schematic illustration of a fourth exemplary configuration of switching devices. Referring to Figures 1A and 1 B, there is shown a part of a valve arrangement 1. The valve arrangement includes a flexure assembly 40, as described above. Figure 1 B shows the inclusion of a pair of photodiodes 70, one adjacent each of the pole pieces 50a, 50b. Figure 1 B also shows a pair of strain gauges 80, one positioned either side of the flexure. It will be appreciated that more or fewer strain gauges may be provided, but that a pair of strain gauges enables a differential signal to be obtained. Each strain gauge 80 has a nominal resistance R N , for example 120W, which varies by an amount ±RVAR, for example ±0.3W, as the flexure moves in one direction or the other away from its intermediate position. The response of each strain gauge 80 may be monitored by a monitoring device. For the purposes of researching the viability of the proposed detection method, the monitoring device was an instrumentation amplifier, with a visual output. The monitoring device may include a high gain amplifier. However, it will be understood that in a practical embodiment of the method and apparatus, the response of each strain gauge 80 may be monitored by a control device, for example a controller of a vehicle braking system. In order to determine the relationship between the output of each strain gauge 80 and an actual position of the moveable part 40a of the flexure assembly 40, for example to determine a delay between movement of the flexure and a response of the or each strain gauge 80, or a phase relationship between the gauge output and the actual position of the flexure, the light source 90 and photodiodes 70 may be used to determine the actual position of the moveable part 40a of the flexure assembly 40. This configuration may be used as a calibration process or step.

It will be understood that this comparison between the actual position of the moveable part 40a of the flexure assembly 40 and the output of the or each strain gauge 80 may not form part of the monitoring method. The actual position of the moveable part 40a of the flexure assembly may be provided as a feedback signal to the monitoring device. The actual position of the moveable part 40a of the flexure assembly 40 may be obtained by illuminating the valve (including the flexure assembly 40) with the light source 90. An output from each of the photodiodes 70 may be passed to the monitoring device. The output from each photodiode 70 may be amplified. As the moveable part 40a of the flexure assembly 40 transitions away from a first one of the pole pieces 50a, 50b, and towards a second one of the pole pieces 50a, 50b, the photodiode 70 associated with the first pole piece is more uncovered, and the photodiode associated with the second pole piece becomes more covered. A time difference between uncovering one of the photodiodes and covering the other photodiode enables determination of a transition time for the moveable part of the flexure assembly 40 to move between one pole piece 50a, 50b and the other. The transition time may be 800 microseconds.

Embodiments of the invention may include monitoring a strain gauge output of one or more stain gauges to detect a variance in one or more of the or each strain gauge output (or absence of a variance) to determine normal functioning (or otherwise) of the flexure assembly, and hence normal functioning (or otherwise) of a valve of a vehicle braking system. The strain gauge output may have an expected variance VAR which is indicative of the moveable part 40a of the flexure assembly 40 transitioning from one position to another, for example between the pole pieces 50a, 50b, or between the intermediate position and one of the pole pieces 50a, 50b. The flexure assembly may have a different expected variance VAR for different types of transition. The method may include comparing an actual variance with the expected variance for the intended transition, and if the actual variance is different from the expected variance, for example is smaller than the expected variance, then it may be determined that the flexure assembly is not functioning, for example the moveable part 40a of the flexure assembly 40 is stuck.

The output of each strain gauge 80 may be sampled frequently, for example every 15-20 microseconds.

In embodiments, a supply voltage (for example 24V) is applied to the coil of the solenoid 20 via one or more switching devices under software control from a controller, for example a microcontroller. The controller may be a 32-bit microcontroller, for example. The or each switching device may be a MOSFET. The resultant current through the coil is also monitored by a controller, preferably the controller. This enables the controller to control and maintain the current though the coil of the solenoid 20 by rapidly switching the switching devices on and off as required.

The switching devices may be arranged in one of a number of configurations, for example, as a“full H-Bridge” (Figure 10),“half H-bridge” (Figure 11 ), High Side MOSFET (Figure 12) or Low Side MOSFET (Figure 13). A preferred configuration may be a full H-Bridge. To toggle a fast acting binary valve (FABV) flexure, i.e. to move or transition the moveable part 40a, a specific current profile is applied to the coil. The current profile consists of three phases, as shown (not to scale and for illustrative purposes only) in Figure 2:

· Phase 1 shows a desired pull in current I P ; this may be 6A for 20ms and is required to force the flexure to toggle.

• Phase 2 shows a hold current I H ; this may be 500mA for 500ms and is required to prevent blow back and bounce back - where the resilient member flexure assembly 40 may return to its original position because of air flow or mechanical bounce.

• Phase 3 is an idle state, the current in phase 3 may be 0A; this is the default state between operations.

It will be appreciated that the current through the coil cannot exactly follow the vertical edges of the target profile as shown in Figure 2, this is because inductors oppose change and have a natural current rise time that is dependent upon their inductance and resistance.

Figures 3 to 5 show resultant current waveforms through the coil whilst the resilient portion 40a of the flexure assembly is being driven open (in accordance with the current profile shown in Figure 2) for three different scenarios:

Figure 3 shows a first resultant current waveform 300 when the flexure assembly 40 is stuck open;

Figure 4 shows a resultant current waveform 400 when the flexure assembly 40 is stuck closed; and

Figure 5 shows a resultant current waveform 500 when the flexure assembly is operating normally. Only the first approximately 10ms of phase 1 is shown in each of the waveform traces in figures 3 to 5 as this is where the significant details are visible. Figures 3 to 5 also each show a respective MOSFET switching signal 310, 410, 510 and a respective strain gauge signal 320, 420, 520. It will be understood that the strain gauge signals 320, 420, 520 are provided for comparative purposes only, and the strain gauge signals need not be used in the detection methods described hereinafter.

In figures 3 and 4, since the flexure 40 is stuck, the respective strain gauge signals 320, 420 are substantially constant. In Figure 5, the strain gauge signal 520 exhibits a change which that occurs when the flexure assembly 40 toggles. The strain gauge signal may decrease when the flexure 40 toggles, as shown in Figure 5.

It can be seen from Figure 3 that during the period between T 0 and T P in which the current rises to the desired pull in current I P (e.g. 6A), the MOSFET signal 310 is permanently high or“on”, and then once the desired pull in current I P is achieved at time T P , the MOSFET signal 310 modulates to maintain the desired pull in current I P at a predetermined level or within a predetermined range. It can be seen from Figure 4 that in a situation when the flexure is stuck closed, the period between T 0 and T P is greater, i.e. it takes longer for the desired pull in current I P to be achieved than in the situation where the flexure is stuck open (as shown in Figure 3). This is a repeatable and measurable effect.

It can be seen from Figure 5 that driving the flexure open whilst the flexure assembly 40 operates normally (i.e. the flexure is not stuck) gives rise to a different resultant current profile 510. There is a disturbance D in the current beginning at T D owing to the inductance of the solenoid coil and reluctance of the flexure assembly dynamically changing during the movement of the resilient portion 40a of the flexure assembly. This is also a measurable and repeatable effect.

In all three scenarios above, the or each MOSFET only begins to modulate the supply voltage (and hence the current) once the desired pull in current I P is achieved.

In embodiments, the method may include monitoring the current through the coil during a monitoring period, to detect the presence or absence of a disturbance in the current. The monitoring period may be predetermined and may be greater than an expected transition time for the moveable part 40a of the flexure assembly 40. The monitoring period may be 10ms for example.

The disturbance D may include a change in the rate of change in the current. For example, the disturbance may include a decrease in the rate of change of the current, in other words, the current may increase more slowly than it had been increasing during the period between T 0 and T D. The disturbance D may include more than one characteristic. The disturbance may include a decrease in the rate of change of the current, followed by an increase in the rate of change of current.

The applicant has deduced that a disturbance D in the current through the coil occurs when the moveable portion 40a of the flexure assembly 40 moves from one position to another. Detecting a disturbance between T 0 and T P may indicate that the flexure assembly 40 has behaved in the expected way, i.e. that the resilient portion 40a has moved and“toggled”. The absence of a disturbance between T 0 and T P may indicate that the flexure assembly has not behaved in the expected way, and that the resilient portion 40a of the flexure assembly 40 may be stuck. One or more characteristics of the disturbance D may be dependent upon the supply voltage. The disturbance D may be very subtle. The disturbance D may become more difficult to detect with increasing supply voltage. For example, at supply voltages above 28V, the disturbance D may be so slight as to be very difficult to detect.

In embodiments, the method may include monitoring the modulation signal of one or more of the or each MOSFET. A disturbance in the modulation signal 310, 410, 510 of one or more of the or each MOSFET may also indicate that the flexure assembly 40 is operating as expected, i.e. that the resilient portion 40a of the flexure assembly 40 has moved or toggled. The disturbance in the modulation signal may include a change in the pulse width of the modulation signal. The disturbance may include an increase in the pulse width of the modulation signal. A difficulty with this method is that the or each MOSFET does not ordinarily modulate during a transition of the moveable part 40a of the flexure assembly 40 between one pole and another.

In embodiments, the or each MOSFET is caused to modulate during transition (i.e. movement) of the resilient portion 40a of the flexure assembly 40. In order to cause the or each MOSFET signal to modulate as the flexure transitions the supply current is increased gradually to the desired pull up current I P . The current may be ramped linearly to the desired pull up current Ip. The desired pull up current I P may be reached in two stages, including a first current increase and a second current increase. The desired pull up current I P may be reached by a combination of a step increase l s in current and a gradual increase dl/dt in current. Where a step increase I s in current I is implemented, the step increase may constitute a predetermined proportion of the desired pull up current I P . The step increase may constitute approximately 50% of the desired pull up current I P . For example the desired pull up current may be 6A, and the step increase may be 3A. The rate of change of current dl/dt may be substantially constant during the period in which the current is gradually increasing. Such a relationship is illustrated in Figure 6.

Figures 7, 8 and 9 show resultant current waveforms 700, 800, 900 through the coil of the solenoid whilst driving the flexure open is shown below for three different scenarios:

• Figure 7 shows a scenario where the flexure is stuck open;

• Figure 8 shows a scenario where the flexure is stuck closed; and

• Figure 9 shows a scenario where the flexure is operating normally, i.e. the flexure is not stuck.

As with Figures 3-5, only the first 10ms of phase 1 is shown in Figures 7-9. The traces also include a corresponding MOSFET switching signal 710, 810, 910 and a corresponding strain gauge signal 720, 820, 920 that shows when the flexure toggles from one position to another. As mentioned above, the strain gauge signals 720, 820, 920 are provided for comparative purposes.

It will be appreciated that the step increase I s in current does not actually happen instantaneously and, as can be seen from figures 7 to 9, the time taken for the step increase in current to occur is finite, but when considering that the entire time period covered by each of Figures 7 to 9 is approximately 10ms, it will be understood that the time taken for the step change I s in current to occur is negligible, and the step change l s can be considered to be substantially instantaneous for the purposes of the invention. When the flexure 40 is driven open but the flexure 40 is stuck open, the MOSFET signal 710 is fully on or “high” whilst the supply current rises relatively sharply (i.e. during the step increase l s in current) and then modulates during the gradual change dl/dt in current to the desired pull up current I P . When the flexure 40 is driven open but the flexure 40 is stuck in the closed position, as in the situation of Figure 8, the pulse widths of the modulating MOSFET signal 810 are wider during the gradual change dl/dt in current than the corresponding pulse widths of the modulating MOSFET signal 710 when the flexure 40 is stuck in the open position (as shown in Figure 7). This is because more energy is required to achieve the change in current I, and can also be seen by comparing Figures 3 and 4. This is a measurable and repeatable effect. Figure 9 shows the effect of driving the flexure 40 open whilst the flexure 40 operates normally. The MOSFET signal 910 briefly stops modulating at T M s as the flexure 40 transitions. This is because the rate of change of the expected current through the coil during a transition is less than than a target rise rate and therefore the actual current I through the coil falls below the desired or expected current I though the coil. This causes the or each MOSFET to turn fully on, without modulation, as a result of the current in the coil not being able to achieve the target rate of increase in current.

The desired rate of change of the current dl/dt (i.e. the target rate of increase in current) may be predetermined. The desired rate of change of the current dl/dt, may be less than the natural rate of change of current of a stuck solenoid but greater than the natural current rise during a transition event. This is to ensure that the or each MOSFET only stops modulating during the time taken for the flexure 40 to transition, as this is when the natural rate of change of current (i.e. natural rate of increase in current) is less than the target rate of increase of the current dl/dt.

The desired rate of change of the current may be 1000A/S, for example. The supply voltage may be 24V. The desired rate of change of the current may be a function of supply voltage as the solenoid’s natural rate of change of current varies with the supply voltage. A microcontroller may monitor the supply voltage and determine the desired rate of change of the current in accordance with the supply voltage.

Monitoring the rate of change of current may enable a determination of whether the flexure 40 is stuck or functioning (i.e. moveable). The rate of change (rate of increase) of the current dl/dt through the coil of the solenoid may be lower when the flexure is functioning than when the flexure is stuck.

When the flexure 40 is stuck, the most gradual increase in current (i.e. smallest rate of change of the current dl/dt may occur during the last approximately 10% of the time taken for the current to reach the desired pull up current I P . The smallest rate of change of current dl/dt may be between approximately 1750A/S and approximately 1850A/S, with a supply voltage of 24V. The smallest rate of change of current dl/dt may be approximately 1831 A/s when the supply voltage is 24V.

During a transition, i.e. when the flexure 40 moves/toggles, rate of change of current dl/dt may be less than 1000A/S with a supply voltage of 24V. The rate of change of current dl/dt may be approximately 572 A/s with a supply voltage of 24V.

In embodiments, the method may include increasing the current through the coil to the desired pull up current I D by providing the supply voltage in accordance with a predetermined profile of the current through the coil. A first current increase and a second current increase may be effected. One of the first and second current increases may include ramping the current linearly, for at least a part of the time taken for the current to reach the desired pull up current I D . The method may also include monitoring the pulse widths of a modulating MOSFET signal, to detect the time at which the flexure transits (moves) from one position to another. This may be done by comparing the actual modulation pattern of the modulating MOSFET signal with an expected modulation pattern of the modulating MOSFET signal. In the event that the moveable part 40a of the flexure assembly 40 is stuck, the expected modulation pattern of the MOSFET signal may be substantially continuous, for example, once the MOSFET signal begins to modulate, the pulse widths of the modulating signal may be substantially equal, whereas an expected modulation pattern for a functioning flexure assembly may include a disturbance D. If, during the monitoring period, a disturbance D is not detected, then the monitoring device may assume that the flexure assembly is not functioning.

It will be understood that in embodiments, the purpose of the strain gauge output signal is for reference only, and that a determination of whether or not the flexure is functional and able to move may be made by monitoring one or more of the current through the coil, and the modulation signal through the or each MOSFET, without reference to the strain gauge output signal.

An advantage of the invention is that by detecting a transition, it may be possible to end Phase 1 of the current profile, and begin Phase 2 (i.e. to cease trying to attain the desired pull up current l P , and apply the (lower) hold current l H ) as soon as the transition has been detected. This has the advantage of reducing heating effects in the coil and reducing energy consumption. It may be possible for the monitoring device to monitor the current achieved at the moment of transition, and to use this information to adjust the desired pull up current l P . Monitoring the current at the moment of transition over time (i.e. over a series of transitions) enables any changes in this current to be detected. This may enable early indication of future potential failures. It may be possible to increase the desired pull up current to a value that is higher than a typical desired pull up current because whilst the flexure assembly is operational, i.e. the flexure is not stuck, the maximum desired pull up current would seldom, if ever, be reached, owing to the fact that once a transition is detected, the system may move to Phase 2 immediately. Having a greater than necessary maximum pull up current may enable stuck flexures to be freed in the event that the flexure is identified as being stuck, since the system will remain in Phase 1. If the maximum pull up current is reached, then this is an indication that the flexure is stuck and a failure signal may be provided. Remedial action may be taken to free the flexure.

The method may include driving (or attempting to drive) the flexure in both directions (i.e. towards an open condition and towards a closed condition) upon powering up the system. If the flexure assembly 40 is identified as being stuck (for example because it takes longer than expected for the desired pull up current I P to be reached), then a signal may be provided and/or remedial action may be taken.

An advantage of embodiments of the invention may be that wear of the valve 1 , in particular wear of the moveable part of the flexure assembly 40 and/or of the pole pieces 50a, 50b, may be reduced since the velocity of the moveable part of the flexure assembly 40 as it nears and/or contacts the pole pieces may be reduced by virtue of reaching the desired pull up current by increasing the current in two stages. Providing a more gradual current increase dl/dt in a second current increase step of Phase 1 (as shown in Figure 6) may cause the impact velocity of the moveable part 40a of the flexure assembly 40 to be lower than known impact velocities (resulting from a single step increase in current). Representative features are set out in the following claims, which stand alone or may be combined, in any combination, with one or more features disclosed in the text and/or drawings of the specification. When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components. The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Although certain example embodiments of the invention have been described, the scope of the appended claims is not intended to be limited solely to these embodiments. The claims are to be construed literally, purposively, and/or to encompass equivalents.