WO1994013991A1

WO1994013991A1 - Electromagnetic valves

Info

Publication number: WO1994013991A1
Application number: PCT/GB1993/002508
Authority: WO
Inventors: John Colin Mumford
Original assignee: Pi Research Ltd.
Priority date: 1992-12-08
Filing date: 1993-12-08
Publication date: 1994-06-23
Also published as: GB9225622D0

Abstract

An electromagnetic valve (1) is opened by application of a drive voltage to solenoid (8) which attracts the valve (1) to lift the valve from a valve seat (5) to a stop position in which the valve is open. A system for detecting opening of the valve (1) includes means for monitoring the current flowing in the solenoid (8) and means for detecting a time variation in the solenoid current which corresponds to the stop position of the valve (1). A system for detecting closing of the valve includes means for monitoring the voltage across the solenoid (8) and means for detecting the time variation of the solenoid voltage which corresponds to the closed position of the valve (1).

Description

ELECTROMAGNETIC VALVES

The present invention relates to a system for detecting opening or closing of an electromagnetic valve. Electromagnetic valves are used in many systems, for example, as fuel injection valves or in hydraulic systems. In simple terms, most electromagnetic valves have a head which is pressed by a spring against a valve seat to close the valve. The head is withdrawn from the valve seat to open the valve against the force of the spring by a solenoid which attracts a plate of ferromagnetic material on the valve on application of a drive current to the solenoid. In some systems, the operation of the spring is replaced by the head itself being a permanent magnet which is attracted into the valve seat by coils.

In an adaptation, a separate armature floats on the valve stem and, when the solenoid coil is energised, the armature is attracted and moved towards the coil against the action of a spring. The armature strikes the flared end of the valve stem and then carries the valve to lift the valve from the valve seat. Thus, in this adaptation, the armature has a certain amount of pre-travel before the valve is lifted, which can reduce the delay between initial opening and fully open, and gives better consistency from cycle to cycle if the valve shows any tendency to stick to the seat, or if there are variations in supply pressure.

In all of these systems, there is a great deal of uncertainty in the precise moment at which the valve is fully open, at which point fluid flow through the valve is at a maximum. This problem is exacerbated because there is always a delay from the time at which the opening voltage is applied to the time at which the valve opens, as the current in the solenoid rises in accordance with the inductance and possibly the resistance of the system including the solenoid. Moreover, the delay between application of voltage to the solenoid and the valve reaching its fully open position varies in accordance with the supply voltage, the strength of the spring (which varies in use with time due to ageing) , the initial gap in the solenoid, the solenoid inductance, the fluid pressure and surface tension, and friction within the system generally. The spring strength and the solenoid gap are particularly susceptible to variation during manufacture of the valve. Temperature variation of the solenoid resistance and the fluid viscosity are particularly pronounced and the spring strength may vary with temperature also. Where the valve is a fuel injection valve in a motor vehicle, the battery voltage can vary considerably. Also, the properties of the fluid may alter as the fluid composition is varied. All of these factors make it difficult to anticipate accurately the time when the valve will be fully open.

According to a first aspect of the present invention, there is provided a system for detecting opening of an electromagnetic valve, the valve being opened by application of a drive voltage to a solenoid which attracts the valve to lift the valve from a valve seat to a stop position in which the valve is open, the system comprising: means for monitoring the current flowing in the solenoid; and, means for detecting a time variation in the solenoid current which corresponds to the stop position of the valve.

It has been found that when the valve reaches its fully open position, at which point the valve conventionally hits a mechanical stop (for example, by means of the armature hitting the solenoid coil) , the drive current ramps upwards sharply. The present invention monitors for this abrupt change.

The means for detecting a change in the drive current may comprise means for monitoring the first derivative of the drive current with respect to time. However, preferably, the means for detecting a change in the drive current comprises means for monitoring the second derivative of the drive current with respect to time. The second derivative of the drive current shows a dramatic "spike", corresponding to the valve hitting its mechanical stop. Such a large spike is easily monitored. In the valve which has an armature having a certain amount of pre-travel described above, there is a first spike in the second time-derivative of solenoid current, which corresponds to the armature hitting the valve to lift the valve, and which is quickly followed by a second spike corresponding to the valve itself hitting the mechanical stop which signifies that the valve is fully opened. Where the mass of the armature is greater than that of the valve assembly, the first spike tends to be significantly smaller but is dependent on a number of factors including the fluid supply pressure.

By means of the present invention, it is possible to determine with great accuracy the point in time when the valve is fully open. This is particularly advantageous in a fuel injection system as it allows the fuel delivered per cycle to be controlled very precisely and to meet the fuel mass requirement determined by a processor controlling the overall system.

The present invention also includes a method for detecting opening of an electromagnetic valve, the valve being opened by application of a drive voltage to a solenoid which attracts the valve to lift the valve from a valve seat to a stop position in which the valve is open, the method comprising the steps of: monitoring the current flowing in the solenoid; and, detecting a time variation in the solenoid current which corresponds to the stop position of the valve.

The value of the drive current may be differentiated with respect to time. Preferably, however, the valve of the drive current is double-differentiated with respect to time, and the second derivative is monitored. A constant voltage may be supplied across the solenoid as it is opened. According to a second aspect of the present invention, there is provided a system for detecting closing of an electromagnetic valve, the valve being opened by application of a drive voltage to a solenoid which attracts the valve to lift the valve from a valve seat to a stop position in which the valve is open, the system comprising: means for monitoring the voltage across the solenoid; and, means for detecting the time variation of the solenoid voltage which corresponds to the closed position of the valve.

The invention also includes a method of detecting closing of an electromagnetic valve, the valve being opened by application of a drive voltage to a solenoid which attracts the valve to lift the valve from a valve seat to a stop position in which the valve is open, the method comprising the steps of: monitoring the voltage across the solenoid; and, detecting the time variation of the solenoid voltage which corresponds to the closed position of the valve.

The first derivative of the solenoid voltage with respect to time may be monitored. Alternatively, the second derivative of the solenoid voltage with respect to time may be monitored. A constant current may be supplied to the solenoid as the valve is closed.

An example of the present invention, when applied to fuel injection valves, will now be described with reference to the accompanying drawings, in which:

Fig. 1 is a schematic diagram of a first example of an electromagnetic valve;

Fig. 2 is a schematic diagram of a second example of an electromagnetic valve;

Figs. 3A-C are graphs showing the variation of the control signal, the drive current, and the fuel flow rate with respect to time for the valve shown in Fig. 1;

Figs. 4A-C are graphs corresponding to those in Figure 3 for the valve shown in Figure 2; Fig. 5 is a block diagram showing circuit components for the valve drive system and valve-opening detector;

Fig. 6A and B are circuit diagrams of the solenoid drive circuit and a variation respectively; Fig. 7 is a circuit diagram for the current control circuit;

Fig. 8 is a circuit diagram for the detector;

Fig. 9 is a diagram of the circuit providing the threshold voltages for the circuits of Fig. 7 and Fig. 8; Fig. 10 is a diagram of the opening detector logic;

Fig. 11 shows the variation of high side drive to the solenoid;

Fig. 12 shows the variation of the low side drive to the solenoid; Fig. 13 is a graph showing -dl/dt;

Fig. 14 shows the variation of d 2I/dt2;

Fig. 15 shows the logic sequence for the detector logic of Figure 10;

Fig. 16 shows the output Initial Opening XOR Fully Open for the detector logic;

Fig. 17 shows the current waveform obtained using the Fully Open detection to switch from "Pull" mode to "Hold" mode;

Fig. 18 shows a modified solenoid drive circuit (added to the circuit shown in Fig. 6A or 6B) ;

Fig. 19 shows a closure detection and current control circuit;

Fig. 20 shows a circuit for producing the threshold voltages for the circuit shown in Fig. 19 (added to the circuit shown in Fig. 9) ;

Fig. 21 shows the closure detection and current control logic circuit;

Fig. 22 shows the variation of low side drive voltage (VLO) and dV/dt at turn-off for first example of valve closing monitoring;

Fig. 23 shows the closure detection signal corresponding to Fig. 22; Fig. 24 shows the variation of low side drive voltage (VLO) and dV/dt at turn-off for a second example of valve closing monitoring;

Fig. 25 shows the closure detection signal corresponding to Fig. 24; and.

Fig. 26 the sequence of operations of the detector logic shown in Fig. 21.

In Figure 1 there is shown a fuel injection valve l which has a valve stem 2 at one end of which is a valve head 3 and at the opposite end of which is a ferromagnetic plate-like armature 4. The valve head 3 is urged against a valve seat 5 to close the valve 1 by a coiled compression spring 6 acting between shoulders on the valve head 3 and a fixed member 7. A solenoid 8 having a U-shaped core 9 is positioned with the end faces 10 of the core 9 in confronting relationship with the armature 4. Fuel is supplied under pressure into the region between the fixed member 7 and the valve seat 5 and which is indicated generally by "A" in the drawing. When the solenoid 8 is energised by applying a voltage across the solenoid coil, the armature 4 is attracted towards the core 9 which lifts the valve head 3 out of the valve seat 5 against the action of the compression spring 6. In Figure 2, there is shown a further example of a fuel injection valve. Those components which correspond to those shown in Figure 1 bear the same reference numerals. This second example differs in that, instead of the fixed armature 4 of Figure 1, a floating armature 11 is provided between the enlarged end 12 of the valve stem and a second fixed member 13. The floating armature 11 is biased away from the solenoid 8 by a second coiled compression spring 14.

As the solenoid 8 is energised, the floating armature 11 is attracted towards the solenoid core 9 against the action of the second compression spring 14. After moving a short distance, the floating armature 11 "picks up" the valve end 12 to lift the valve head 3 from the valve seat 5. The use of a floating armature 11 can reduce the delay between initial opening and fully open and gives better consistency if the valve shows any tendency to stick to the seat, or if there are variations in supply pressure.

It is to be understood that the valves are only shown schematically in Figures 1 and 2 and precise details will vary from design to design. The valves, and the system described below, have particular application in petrol engines.

In Figure 3, Figure 3A indicates the variation of the control signal applied to open the solenoid coil 8 for the first example of the valve shown in Figure 1. Figure 3B shows the variation of the current I in the solenoid coil 8 with respect to time and Figure 3C shows the rate of flow of the fuel through the valve. At T_start, the voltage is applied to the solenoid 8 and the current I builds up, the build up being in accordance with the inductance and possibly the resistance of the solenoid coil 8. At a point T₀, the magnetic force generated by the solenoid 8 is sufficient to overcome the compression spring 6 and the valve 3 begins to open. The current I continues to build until the point T_F at which point the armature 4 strikes the core 9 (or other mechanical stop in the valve) and the valve is fully open. As seen particularly clearly in Figure 3B, there is a change in the slope of the graph of I versus T at the point indicated "X". Once the current has reached the specified pull-in value (IPULL, Figure 3B) which in general will depend on the supply voltage, it can be reduced to the lower hold value (IHOLD, Figure 3B) until it is decided to close the valve, at which point in time the drive voltage is reversed to cause the current to drop to zero as rapidly as possible and the valve relaxes to the closed position under the action of the compression spring 6. In practice, the value of the current I can be dropped to the hold value as soon as it is detected that the valve is fully open, rather than waiting for the nominal pull-in value to be achieved as shown in the demonstration curves reproduced in the drawings.

The area under the curve shown in Figure 3C represents the amount of fuel which has flowed. It will be seen that almost all of the fuel flow is during the period indicated by "PW" during which the valve is fully open in the example shown where the pulse width is relatively large.

Figure 4 shows graphs corresponding to those in Figure 3 for the second example of the valve shown in Figure 2. On application of the control signal, the current I begins to rise. At the time T,, the floating armature 11 strikes the valve end 12, causing the valve head to lift from the valve seat 5. As the floating armature 11 strikes the valve end 12, there is an increase in the rate of change of current as indicated at the point χ. _

The current continues to rise and the armature 11 is drawn closer to the core 9 of the solenoid 8, further opening the valve. At the point T₂, the armature 11 strikes the core 9 (or some other mechanical stop) , which indicates that the valve is fully open. At this instant, there is another, larger increase in the rate of change of current as indicated at X₂. When the current has reached the specified pull-in value (IPULL, Figure 4B) , it is allowed to fall to the lower value (IHOLD, Figure 4B) required to hold the valve open as described above in respect of the first example. The valve is then subsequently closed by reversing the drive voltage. Again, the value of the drive current can in practice be dropped to the hold value as soon as it is detected that the valve is fully open.

The fully open position of the valve is detected by monitoring for the change in slope of the curve of I v T. As will be understood from the description below, it is simpler in practice to monitor for the corresponding sharp increase in d 2I/dt2 at the points X, X, and X₂ corresponding to the abrupt changes in dl/dt. The reason for the change in slope can be seen by considering the situation where a constant voltage V is applied to a solenoid coil having a resistance R and in which a current I flows, the solenoid attracting an armature which is a distance x away, and the system having an inductance L. The equation of state, assuming a linear response in which no part of the magnetic circuit is saturated, is:

and dL_ dL dx dt^" dx dt ^'

Now, dx/dt is the velocity of the armature. Accordingly, for a constant voltage V, an abrupt change in velocity of the armature (dx/dt) causes an abrupt change in dL/dt, which causes an abrupt change in dl/dt. The increase in dl/dt which occurs as the armature hits the solenoid core produces a corresponding large spike in the value of d²I/dt².

This can be seen in Figures 13 and 14. The upper trace in Figure 13 shows -dl/dt and the lower trace shows the solenoid current. There is an initial step from the change in dl/dt as the voltage is first applied (the overshoot is due to eddy currents) . There is then a first jump at T, corresponding to the armature 11 hitting the valve end 12 and a second jump at T₂ corresponding to the armature hitting the solenoid core or other mechanical stop.

As shown in Figure 14, in which the upper trace shows d 2I/dt2 and the lower trace shows the solenoid current, these jumps in dl/dt translate to a first spike and then a much larger spike in d²I/dt² at T, and T₂ respectively. Also, the use of d I/dt² is more secure in that it is more tolerant to variations in the vehicle supply voltage, and other variations which may occur in practice such as to the fuel pressure, viscosity, etc.

It should be noted that, in principle, it is possible to use yet higher derivatives of the current in order to monitor for the fully open position of the valve, such as d³I/dt³ and dI/dt⁴. Also, it may be possible to use d³I/dt³ or d⁴I/dt⁴ at constant voltage to monitor for the initial opening of the valve. These possibilities can be derived from the equation of state described above. However, in practice, it is believed that noise present in the current signal will preclude use of d I/dt and d I/dt in making accurate measurements, though it may be nevertheless suitable or preferred in some circumstances, depending on inter alia fuel pressure, any tendency of the valve to stick, and the geometry of the components used, particularly the valve, coil and/or armature.

Figure 5 is a block diagram of the components"used in the present system. The circuit is designed such that solenoids with non-overlapping duty cycles can share many components, and the diagrams have been drawn for the case of two solenoids sharing the same current control, detector and detector logic circuits. The solenoid 8 is driven by a solenoid drive circuit 20. The current I passing through the solenoid 8A or 8B is fed to earth through a sense resistor 21 having a resistance R_s. The voltage IR_S developed on the sense resistor 21 is fed to a detector circuit 22 which performs a double time-differentiation on the input signal, the results of which are then passed to a detector logic circuit 23. The detector logic circuit 23 detects the point X, of Figure 4B corresponding to the armature 11 hitting the valve end 12 (i.e. the initial opening of the valve) and also detects the fully open position of the valve corresponding to the point X₂. Where the valve is of the type shown in Figure 1, having the characteristics shown in Figure 3, there is only one jump in the value of d 2I/dt2 correspondi.ng to the position where 11 the valve is fully open and indicated X in Figure 3B, and for this type the circuit can be simplified somewhat.

The outputs of the detector logic circuit 23 are fed to a processor 24. The processor 24 provides overall control of the circuit and in particular determines the point in time when the solenoid 8 (A or B) is energised and deenergised to control valve opening based on fuel requirements and previous cycles.

A current control circuit 25 provides signals to control operation of the solenoid drive circuit 20, and is itself under control of the processor 24. The current control circuit 25 also has the voltage IR_S as an input.

Whilst the specific example shown has various logic circuits and other devices to implement the present invention, it will be understood that many of the circuits (for example, the current control and detector circuits) could be replaced by a processor performing the required functions entirely in software.

The solenoid drive circuit 20 is shown in more detail in Figure 6A. A high side drive logic signal (HSD) from the current control circuit 25 is supplied to an integrated high-side switch circuit 31 which, in a specific example, is a BTS410F Profet. This provides the output drive voltage to the solenoid 8 (A and B) which is termed herein the "solenoid high side signal" (voltage denoted by VHI) . A low side logic signal from the processor 24 is supplied to the gate of a transistor 32 (A or B) which, in a specific example, is a BUK555-100B. The drain of the transistor 32 receives the current from the solenoid low side (the "solenoid low side signal" with voltage denoted by VLO) . The sense resistor 21 connects the sources of the transistors 32 (A and B) to earth and the voltage IR_S across this component is fed into the detector and current control circuits. A (Schottky) diode 33 connects the output of the switch circuit 31 to earth.

The switch circuit 31 accepts the logical input HSD and supplies a high side drive voltage. The BTS410F Profet has various useful features, particularly short-circuit and over-temperature protection and a status output (STAT) which can be used to monitor fault conditions (open circuit load, shorted load, etc) . The low side drives use respective logic level MOSFETS 32 (A and B) rated at lOOv VDS. These are bypassed with Transzorbs ZD1 (A and B) (in this example type SA51A which clamp at approximately 60v) which are used to absorb the energy stored in the solenoids at turn-off. An alternative arrangement, using a conventional Zener diode, is indicated in Figure 6B. The diode 33 ensures that the output voltage to the solenoid high side does not fall more than a fraction of a volt below earth, thus keeping down the dissipation of energy in the Profet 31. If the circuit is to be operated in an electrically noisy environment, the leads to the solenoids will require capacitors CIA, C1B and C2 connected to the circuit ground to prevent high frequency interference reaching the digital logic. A resistor Rl is then required to discharge the capacitors between cycles. (It may also be required for the closing detection, to be described in more detail below.)

The logic signals HSD and LSD (A or B) are provided by the current control circuit 25 and the processor 24 respectively. An "enable" signal used for resetting the digital logic between cycles and gating the HSD signal is formed by IC3A as LSD (A) OR LSD (B) .

The current control circuit 25 is shown in greater detail in Figure 7. At the start of the cycle, the Q output of the D-type flip-flop IC4A is low ("pull" mode). The HSD signal becomes high when either LSD(A) or LSD(B) ("enable") is switched high by the processor 24. With switch SI in position A, the current in the solenoid ramps up until the specified pull-in current is achieved (IR_S reaches V2 on the comparator IC2B) , thus clocking IC4A and setting the Q output high ("hold mode"). With the switch SI in position B, switching to hold mode occurs either when IR_S reaches V2 or when the fully open condition is detected (whichever occurs first) . In hold mode, the current ramps between two limits set by VI (on comparator IC2A) and the resistors R3 and R4. The lower of these limits is set slightly above the specified solenoid hold current. At the end of the cycle, the processor returns the LSD signal to the low state (setting enable and HSD low also) , causing the solenoid current to flow through the Transzorb (ZD1A, ZD1B, Figure 6A) and producing a large reverse voltage (in this example about 60V) across the coil, and a rapid (in this example about 60 μsec) decay of the current to zero.

The voltage waveforms appearing on the solenoid high side and low side, together with the corresponding current waveform, are shown in Figs 11 and 12 respectively, the upper trace being the voltage and the lower trace being the current. The drive signal to the solenoid is oscillated as shown, which is extremely effective in reducing the power dissipated in the components in the drive circuit.

The detector circuit, shown in Figure 8, is supplied with the voltage IR_S across the sense resistor. This voltage is differentiated twice by IC6A and IC6B to produce a signal proportional to d 2I/dt2. The bandwidth of the differentiators is selected to optimize the signal to noise ratio at the output. The second derivative is compared with the threshold voltages V3 and V4 by comparators IC2C and IC2D. The outputs from these are fed into the detector logic (Figure 10) , which in this example has been implemented using positive edge triggered D-type flip- flops. The sequence of operations occurring for the eight edges (Figure 14) derived from the two comparators during the opening part of the cycle is indicated in Figure 15.

The four threshold voltages VI, V2, V3 and V4 are generated by the circuit in Figure 9, and these may be shared by a number of separate current control and detector circuits. In cases where the range of supply voltages is significant, it is advantageous to vary V2 and V3 with VBAT, as indicated in this example. The two detected outputs corresponding to initial opening and fully open are returned to the processor 24 to be used for the calculation of pulse width and pulse start on subsequent cycles, and in some applications may be used for the calculation of the timing of the end of cycle on the current cycle. The two detections are shown in Figure 16 where the signals from the detector logic (Figure 10) have been combined using an exclusive OR gate. The current waveform obtained by switching from pull mode to hold mode when the fully open condition is detected (SI in position B) is shown in Figure 17.

The present invention provides a remarkably simple system for determining the instant when the valve is fully open or closed with an extremely high degree of accuracy. The present invention has wide applicability, but is particularly useful in determining the opening and/or closing of fuel injection valves in vehicle engines, particularly petrol engines, so that the fuel flow rate can be monitored and controlled more precisely. By knowing precisely when the valve is fully open and/or closed, the delay between application of the drive voltage and full opening of the valve can be determined, in real time if required, so that the drive voltage for the next cycle can be applied at the instant required to achieve full opening of the valve for the desired amount of time for the pulse width required. Similar considerations apply to valve closing.

The present invention is tolerant to relatively large variations in the supply voltage. Vehicle supply voltages are often irregular and ill-defined and thus the present invention can be used in many vehicles easily, without requiring the use of a regulated power system. The component count is also low, which also helps to keep down costs and also size. In order to provide drive for multiple fuel injectors, a single Profet 31 is described above for use when the duty cycles of the injectors are non-overlapping, the solenoids of the fuel injectors effectively being connected in parallel between the Profet 31 and ground, and being operated sequentially. If the duty cycles overlap, a separate drive needs to be used for any overlapping injectors.

In order to detect the valve closure, it is necessary for some flux to be present in the magnetic circuit (core 9 and armature 4 in Figure l and core 9 and floating armature 11 in Figure 2) at the instant when the valve 3 strikes the seat 5. For solenoids constructed using "ideal" magnetic materials (no hysteresis) , a small current will have to be supplied to achieve this. For many solenoids, however, the magnetic materials may have sufficient remnant magnetism for the closure event to be detected without applying current through the coil.

For solenoids without pretravel (Figure 1) , there is an abrupt change in the velocity of the armature 4 as the valve 3 contacts the seat 5. For solenoids with pretravel (Figure 2), provided the end of the valve 12 is in contact with the armature 11 as the valve closes, there will in general be an abrupt change in the armature acceleration as the valve 3 contacts the seat 5. There may be cases, however, where the components 11 and 12 tend to adhere, in which case the closure event can yield an abrupt change in armature velocity, as in the case of the solenoid without pretravel.

For the case of ideal magnetic materials, the voltage V (= VHI - VLO) across the coil is given by:

where R is the coil resistance, L the inductance and I is the current, and dL_ dx dL dt^~ dt dx

so that at constant I V=IR+I dx dL dt dx

At constant current, a stepwise change in the armature velocity dx/dt will yield a stepwise change in V and a spike in dV/dt.

Also at constant current

and consequently a stepwise change in the armature acceleration d²x/dt² will yield a stepwise change in dV/dt and a spike in d v/dt .

For the more general case where the magnetic materials have some hysteresis and consequently exhibit some remnant magnetism

where N is the number of turns on the coil and F is the flux in the magnetic circuit given by

F = Magnetomotive Force/Total Circuit Reluctance

where M is the magnetomotive force from the remnant magnetism and x,, μ_} and A,- are respectively the magnetic path length, the permeability and the cross-sectional area of the components in the magnetic circuit. As the armature moves, the value of the x_k corresponding to the gap (or, more commonly, two gaps) between the solenoid and the armature changes, so that at constant current the coil voltage is given by

which is of the same form as the equation for the ideal magnetic materials with I.dL/dx replaced with N.dF/dx_k. Consequently, the qualitative effect of stepwise changes in the armature velocity and armature acceleration is the same as in the previous analysis.

In Fig 21, a switch S2 is shown which is arranged to allow the option of using either the solenoid remnant magnetism (position A, to which Figures 22 and 23 relate) or the constant current drive (position B, to which Figures 24 and 25 relate) according to the properties of the solenoid. In the case where only the former option is required, the circuit can be simplified considerably.

Note that the logic is reset during the active part of the cycle, i.e. when ENABLE is high.

At the end of the cycle, both HSD and LSD signals are set low, causing Ql (Figure 6) to turn off and the solenoid current to flow through ZDl. When the current has fallen to a low value (<lmA) , the voltage VLO across ZDl falls below the rated breakdown voltage (51v for the SA51A) , and, at a predetermined lower value set by R33, R34 and V5 (Figure 19) , the output of the comparator IC9A goes high. This clocks the flip-flop IC11A and triggers the two monostables IC12A and IC12B.

With the switch S2 in position B (i.e. to use a constant current drive, CCD) , IC12A switches both HSD and CCD signals high for a time (set by R47 and C26) which is long enough for the closure event to occur. The CCD signal drives the base of Q2 (Figure 18) which provides a small current (in this case 20mA) through the coil. The monostable IC12B and the flip-flop IC13A prevent spurious detections from the transient caused by switching the high side drive (HSD) and the constant current drive (CCD) . After a delay set by R46 and C27, IC13 is clocked, setting the D input of the detection flip-flop IC13B high.

The solenoid low side voltage, VLO, is differentiated by the amplifier IC10A (Figure 19) . This is fitted with zener diodes ZD6 and ZD7 to clip the output, and clamping diodes D6 and D7 at the input to ensure rapid recovery from the large values of dVLO/dt occurring at the initial turn-off. At the closure event, the negative jump in VLO causes a positive spike in dV/dt (= -dVLO/dt at constant VHI) at the output of IC10A. When this exceeds the threshold V6 on comparator IC9B, the C0MP6 signal clocks the flip-flop IC13B. The output from this is returned to the processor 24 for the calculation of pulse widths on subsequent cycles.

When the monostable IC12A resets, the HSD and CCD signals are returned to the low state, which causes the solenoid current to flow through ZDl (again) . The presence of the flip-flop IC11A prevents the sequence being repeated until another opening cycle has occurred.

It should be noted that for the solenoids used in testing, which in fact have sufficient remnant magnetism for the closure to be detected without applying a current through the coil, the addition of the 20mA current increased the closure time by about 50μsec.

A list of the components used in the examples described above is set out below:

Ql BUK555-1

Claims

1. A system for detecting opening of an electromagnetic valve (1) , the valve being opened by application of a drive voltage to a solenoid (8) which attracts the valve (1) to lift the valve from a valve seat (5) to a stop position in which the valve is open, the system comprising: means for monitoring the current flowing in the solenoid (8) ; and, means for detecting a time variation in the solenoid current which corresponds to the stop position of the valve

(1).

2. A system according to claim l, wherein the means for detecting a change in the drive current comprises means for monitoring the first derivative of the drive current with respect to time.

3. A system according to claim 1, wherein the means for detecting a change in the drive current comprises means for monitoring the second derivative of the drive current with respect to time.

4. A system according to any of claims l to 3, comprising a valve (1) having an armature (4) and a valve head (3) and which reaches a fully open position at which the valve (1) strikes a mechanical stop.

5. A system according to claim 4, comprising a coiled compression spring (6) for urging the valve (l) to a closed position.

6. A system according to any of claims 1 to 3, comprising a valve (1) having a floating armature (11) between a valve end (12) and a fixed member (13), wherein, on opening of the valve (1) , the floating armature (11) strikes the valve end (12) causing the valve to lift, the valve (1) reaching a fully open position at which the valve (1) strikes a mechanical stop.

7. A system according to claim 6, comprising a first coiled compression spring (6) for urging the valve (1) to a closed position and a second coiled compression spring (14) for urging the floating armature (11) away from the solenoid (8) .

8. A system according to claim 7, comprising means for detecting a first time variation in the solenoid current which corresponds to the floating armature (11) striking the valve end (12) prior to detecting a second time variation in the solenoid current which corresponds to the stop position of the valve' (1) .

9. A system according to any of claims 1 to 8, comprising a solenoid drive (20) for driving the solenoid (8) with a drive current (I) which is fed to earth through a sense resistor (21) ; a detector (22) for monitoring the voltage (IR_S) developed on the sense resistor (21) and performing a double time-differentiation on the input signal; a processor (24) for receiving an output from the means for detecting a time variation in the solenoid current; and, a current control (25) having the voltage (IR_S) across the sense resistor (21) as an input, the current control (25) being under control of the processor (24) for providing signals to control operation of the solenoid drive (20) .

10. A system according to claim 9, wherein the means for detecting a time variation in the solenoid current (I) which corresponds to the stop position of the valve (1) comprises a detector logic circuit (23) which provides an output to the processor (24) .

11. A system for detecting closing of an electromagnetic valve (1) , the valve being opened by application of a drive voltage to a solenoid (8) which attracts the valve (1) to lift the valve from a valve seat (5) to a stop position in which the valve is open, the system comprising: means for monitoring the voltage across the solenoid (8) ; and, means for detecting the time variation of the solenoid voltage which corresponds to the closed position of the valve (1) .

12. A system according to claim 11, comprising means for monitoring the first derivative of the solenoid voltage with respect to time.

13. A system according to claim 11, comprising means for monitoring the second derivative of the solenoid voltage with respect to time.

14. A system according to any of claims 11 to 13, comprising means for supplying a constant current to the solenoid (8) as the valve (1) is closed.

15. A system according to claim 14, comprising a switch (S2) to enable the means for supplying a constant current to the solenoid (8) as the valve (1) is closed to be selectively enabled and disabled.

16. A system according to any of claims 11 to 15, comprising a valve (1) having an armature (4) and a valve head (3) .

17. A system according to any of claims 11 to 15, comprising a valve (1) having a floating armature (11) between a valve end (12) and a fixed member (13) , wherein, on opening of the valve (1) , the floating armature (11) strikes the valve end (12) causing the valve to lift.

18. A method for detecting opening of an electromagnetic valve (1) , the valve being opened by application of a drive voltage to a solenoid (8) which attracts the valve to lift the valve (1) from a valve seat (5) to a stop position in which the valve (1) is open, the method comprising the steps of: monitoring the current flowing in the solenoid (8) ; and, detecting a time variation in the solenoid current which corresponds to the stop position of the valve (1) .

19. A method according to claim 18, comprising the step of differentiating the value of the drive current with respect to time.

20. A method according to claim 18, comprising the steps of double-differentiating the value of the drive current with respect to time and monitoring the second derivative of the value of the current.

21. A method according to any of claims 18 to 20, comprising the step of supplying a constant voltage across the solenoid (8) as the valve (1) is opened.

22. A method according to any of claims 18 to 21, wherein the valve (1) has a floating armature (11) between a valve end (12) and a fixed member (13) , wherein, on opening of the valve (1) , the floating armature (11) strikes the valve end (12) causing the valve to lift, the valve (1) reaching a fully open position at which the valve (1) strikes a mechanical stop, comprising the step of detecting a first time variation in the solenoid current which corresponds to the floating armature (11) striking the valve end (12) prior to detecting a second time variation in the solenoid current which corresponds to the stop position of the valve (1).

23. A method of detecting closing of an electromagnetic valve (1) , the valve being opened by application of a drive voltage to a solenoid (8) which attracts the valve (1) to lift the valve from a valve seat (5) to a stop position in which the valve is open, the method comprising the steps of: monitoring the voltage across the solenoid (8) ; and, detecting the time variation of the solenoid voltage which corresponds to the closed position of the valve (1) .

24. A method according to claim 23, comprising the step of monitoring the first derivative of the solenoid voltage with respect to time.

25. A method according to claim 23, comprising the step of monitoring the second derivative of the solenoid voltage with respect to time.

26. A method according to any of claims 23 to 25, comprising the step of supplying a constant current to the solenoid (8) as the valve (1) is closed.