Method of operating a fuel injector, a control unit that performs the method, and a system that includes the control unit

1. A method of operating a fuel injector comprising a solenoid actuator comprising an armature, pole piece(s), electromagnet coil(s) arranged, in response to energisation, to cause travel of the armature between first and second positions along a direction of travel, and spring(s) arranged to bias the armature, wherein in the first position the fuel injector is closed and wherein quasi-static position of the armature is at least monotonic with the coil current over all or some of length of travel of the armature, the method comprising:

applying, in a cycle, drive waveform(s) to the electromagnet coil(s), the drive waveform(s) including turn-on drive waveform(s) having profile(s) and duration so as to position the armature at a position between the first and second positions and at which the armature is at rest, and turn-off drive waveform(s) having profile(s) and duration so as to cause the armature to move towards the first position;

obtaining measurement(s) of at least one of back emf, current, and resistance from the electromagnet coil(s); and

adjusting the drive waveform(s) applied to the electromagnetic coil(s) in at least one of the cycle and a subsequent cycle using the measurement(s).

2. A method according to claim 1 wherein the current towards the end of the turn on drive waveform is adjusted to cause forces on the armature to come in to balance at the end of the turn on drive waveform.

3. A method according to claim 1 , comprising:

applying hold drive waveform(s) to the electromagnet coil(s) having profile(s) and duration so as to hold the armature at the position between the first and second positions and to drive current(s) through the electromagnet coil(s).

4. A method according to claim 1 , wherein at least one of the turn-off drive waveform(s) includes a bias profile portion of opposite polarity to the turn-on drive waveform(s).

5. A method according to claim 1 , comprising:

applying anti-bounce drive waveform(s) to the electromagnet coil(s) so as to decelerate the armature returning to the first position.

6. A method according to claim 1 , comprising:

providing predefined drive waveform(s); and

adapting the predefined drive waveform(s) based on a previously-delivered drive waveform(s).

7. A method according to claim 1 , comprising:

providing a boost waveform so as to cause the armature to reach a desired hold position and then providing a constant low impedance voltage so as to hold the armature stable in the desired hold position for each injection.

8. A method according to claim 7 , comprising:

maintaining voltage using a PWM supply.

9. A method according to claim 1 comprising after driving current in a first direction, driving current in a second, reverse direction through the electromagnet coil(s).

10. A method according to claim 1 comprising:

applying a negative current through the electromagnet coil(s) to hold the armature at the first position.

11. A method according to claim 1 comprising:

applying a negative current through the electromagnet coil(s) to enable fast movement of the armature towards the first position.

12. A method according to claim 1 comprising:

applying a first and second pulses to enable the armature to achieve a stable position at the second position or at a position in between the first and second positions.

13. A method according to claim 1 comprising:

adapting impedance so as to match the electromagnetic coil(s) to reduce bounce when the armature stops travelling.

14. A method according to claim 1 comprising:

generating a voltage in dependence upon a fixed mark space ratio produced by a controller.

15. A method according to claim 1 comprising:

measuring emf(s) from the electromagnet coil(s) when driving current in constant current mode.

16. A method according to claim 15 comprising:

controlling the drive waveform(s) applied to the electromagnetic coil(s) in dependence upon the measured emf(s).

17. A method according to claim 1 comprising:

measuring current through electromagnet coil(s) when in constant voltage mode.

18. A method according to claim 1 comprising:

controlling the drive waveform(s) applied to the electromagnetic coil(s) in dependence upon measured current(s).

19. A method according to claim 1 comprising:

determining armature position in dependence upon measured emf(s).

20. A method according to claim 1 comprising:

determining armature position in dependence upon measured current(s).

21. A method according to claim 1 comprising:

analysing measured emf(s).

22. A method according to claim 1 comprising:

analysing current(s) through the electromagnet coil(s).

23. A control unit configured to perform the method according to claim 1 .

24. A system comprising:

a fuel injector comprising a solenoid actuator comprising:

an armature;

pole piece(s);

electromagnetic coil(s) arranged, in response to energisation, to cause travel of the armature between first and second positions along a direction of travel;

spring arranged to bias the armature; and

a control unit according to claim 23 operatively connected to the actuator.

25. The system of claim 24 , further comprising permanent magnet(s) positioned and orientated for latching the armature in at least the first position.

26. A system according to claim 24 , wherein the solenoid actuator further comprises:

a pintle;

wherein the pintle is disengageably coupled to the armature.

27. The method of claim 1 , wherein permanent magnet(s) are positioned and orientated for latching the armature in at least the first position.

28. A method according to claim 1 , wherein the solenoid actuator further comprises a pintle which is disengageably coupled to the armature.

29. A method according to claim 1 , comprising:

using pulse width modulation to form the hold drive waveform(s).