EP1099828B1 - Method for oscillating an electromagnetic actuator - Google Patents

Description

The invention relates to a method for oscillating an electromagnetic actuator actuating a switching element, in particular an internal combustion engine lift valve, in which an armature acting on the switching element oscillates between two electro-magnetic coils in each case against the force of at least one return spring by alternating energization of the electromagnetic coils is, so that the switching element with the armature and the return springs is an oscillatory spring-mass system, and starting from the idle state of the system, in which the armature is held by the return springs essentially in the middle between these solenoids, to oscillate this Spring-mass system, the electro-magnetic coils are alternately excited by the application of electrical alternating voltage of a certain frequency. Regarding the technical environment, in addition to EP 0 118 591 B1, reference is made to DE 33 07 070 C2.

A preferred application for an electromagnetic actuator with the features of claim 1 is the electromagnetically actuated valve train of internal combustion engines, ie the gas exchange stroke valves of a reciprocating piston internal combustion engine are desired by such actuators Actuated, that is, oscillatingly opened and closed. In such an electromechanical valve train, the globe valves are moved individually or in groups via electromechanical actuators, the so-called actuators, the timing for the opening and closing of each globe valve being essentially entirely free to choose. In this way, the valve timing of the internal combustion engine can be optimally adapted to the current operating state (this is defined by speed and load) and to the respective requirements with regard to consumption, torque, emissions, comfort and response behavior of a vehicle driven by the internal combustion engine.

The essential components of a known actuator for actuating the lift valves of an internal combustion engine are an armature and two electromagnets for holding the armature in the "lift valve open" or "lift valve closed" position with the associated solenoid coils, and also return springs for the movement of the armature between the positions "lift valve open" and "lift valve closed". For this purpose, reference is also made to the attached FIG. 1 , which shows such an actuator with an associated lift valve in the two possible end positions of the lift valve and actuator armature, and the course of the armature stroke between the two states or positions of the actuator / lift valve unit shown z or armature path between the two solenoid coils over time t is shown in simplified form.

As can be seen in FIG. 1, the closing process of an internal combustion engine lift valve is shown, which is designated by reference number 1 and which in this case moves in the direction of its valve seat 30. As usual, a valve closing spring or first return spring 2a acts on this lifting valve 1, furthermore acts on the stem of the lifting valve 1 - here with the interposition of a (not absolutely necessary) hydraulic valve lash compensation element 3 - the whole number 4 Actuator on. In addition to two electromagnetic coils 4a, 4b, this consists of a push rod 4c acting on the stem of the lift valve 1, which carries an armature 4d which is guided in an oscillating, longitudinally displaceable manner between the electromagnetic coils 4a, 4b. A valve opening spring or second return spring 2b also acts on the end of the push rod 4c facing away from the stem of the lifting valve 1.

This is therefore an oscillatory spring-mass system for which the valve closing spring 2a and the valve opening spring 2b form a first and a second return spring, for which consequently the reference numbers 2a, 2b are also used in the following. The first end position of this oscillatory system is shown on the left in FIG. 1, in which the lift valve 1 is fully open and the armature 4d rests on the lower electromagnetic coil 4b. The second end position of the oscillatory system is shown on the right in FIG. 1, in which the lift valve 1 is completely closed and the armature 4d bears against the upper electromagnetic coil 4a. The armature 4d is moved into these two end positions by suitable excitation or de-excitation of the respective magnet coils 4a, 4b.

When the system is idle, i.e. if neither of the solenoids 4a, 4b is energized, the armature 4d is located substantially in the middle between the two solenoids 4a, 4b and is held in this position by the suitably designed return springs 2a, 2b. Starting from this rest position for a desired operation of this system, i.e. for a desired oscillating actuation of the lift valve 1, the entire spring-mass system can be swung.

A possible method for starting this spring-mass system is described in the second mentioned DE 33 07 070 C2. There, too, the electromagnetic actuator with the globe valve and with the Return springs as a mechanical spring-mass system and it is applied to the two electro-magnetic coils with periodic current pulses, the natural frequency of the vibrating spring-mass system is selected for the frequency of the exciting periodic current pulses.

Building on this known prior art, EP 0 118 591 B1, which was mentioned at the outset, proposes that the excitation frequency, viewed over a longer period of time, initially be chosen to be higher than the resonance frequency of the system and then slowly decreased. This frequency change should take place slowly, so that the stroke valve is excited for a sufficiently long time when the resonance frequency is reached so that the armature can oscillate one after the other into its two end positions.

A general disadvantage of this known prior art is that due to the mutual interactions between the electromagnetic part of the actuator and the mechanical spring-mass system, the decoupled approach of these two subsystems that has been practiced so far only reflects the true facts relatively imprecisely. With this previous approach, the highly non-linear electromagnetic properties of the actuator are not taken into account. As a result, there is usually an unnecessarily long start-up period for the exciting current pulses and thus an excessive power requirement of the electromagnetic actuator. This problem is particularly blatant in the prior art according to EP 0 118 591 B1, which was mentioned at the outset, since it is precisely the slow change in the excitation frequency proposed there that means that a rapid change in the resonance frequency cannot be taken into account due to nonlinear effects.

The object of the present invention is to provide a remedial measure for the problems described.
The solution to this problem is characterized in that an essentially sinusoidal or similar AC voltage is applied to the electro-magnetic coils instead of the previously generally substantially rectangular shape of the AC voltage over time, either in the form of a variable one constant voltage curve or in the form of a corresponding changeable pulse width modulated voltage curve. Advantageous further developments are the content of the subclaims.

According to the invention, the voltage profile of the exciting AC voltage, that is to say alternating voltage applied to the two electromagnetic coils at a certain frequency, is changed or made changeable compared to the known prior art, since further influencing factors can hereby be taken into account. In the known state of the art, only an essentially rectangular course of the applied AC voltage U over time t has been realized, as is shown by way of example in FIG. 3 . In contrast, the present invention proposes an essentially sinusoidal or similar course of the applied alternating voltage U, as is shown by way of example in FIG. 2a over time t. This can be a constant voltage curve (as in FIG. 2a ), but alternatively also a corresponding pulse-width-modulated, clocked voltage curve, as shown by way of example in FIG. 2b , and what the end result is directly comparable to the curve in FIG. 2a . This voltage curve can also be changed, so that a more realistic model than previously customary can be used for its determination.

Thus, it is possible to consider not only the oscillating spring-mass system, but the entire actuator, which, as explained, from the mechanical spring-mass system and the associated electromagnetic subsystem. This entire actuator represents a non-linear system which, according to the theory of anharmonic vibrations (see, for example, LDLandau and EMLifschitz, Textbook of Theoretical Physics, Volume I: Mechanics, Chapter V, Sections 28, 29, pp. 103-106) has typical nonlinear effects. The resonance frequency of the entire actuator system is shifted compared to the natural frequency of the linear spring-mass oscillator. Furthermore, the location and speed profile of the vibrating element, here the armature, generally has no exactly sinusoidal profile. Furthermore, the oscillation frequency depends on the oscillation amplitude.

Based on this, it is - as already explained - proposed to no longer excite an electromagnetic actuator according to the preamble of claim 1 with periodic rectangular voltage pulses and also no longer necessarily with the natural frequency of the spring-mass system, but rather a suitable input signal with a non-linear one Vibration system adapted periodic course - essentially a sinusoidal or similar course that is changeable - to use to start. The amplitude or the pulse width duty cycle of the selected course of the alternating voltage results from the theory of anharmonic vibrations (and generally does not match the natural frequency of the spring-mass system).

It can be taken into account that the frequency of the overall system generally depends on the vibration amplitude, so that the frequency of the AC voltage can consequently be changed as a function of the vibration amplitude of the armature.

Furthermore, it was recognized that the damping of the spring-mass oscillator contained in the entire actuator can be temperature-dependent, in particular if viscosity influences of a lubricating oil coming into contact with a part of the overall system play a role. This results in a shift in the system resonance frequency, with the result that the electromagnetic actuator must be swung at different frequencies at different ambient temperatures or can only be swung at different frequencies. Such temperature influences can therefore be taken into account when determining the frequency of the proposed periodic, substantially sinusoidal AC voltage. This avoids an unnecessarily high energy consumption of the electromagnetic actuator. For example, in the event of a cold start of an internal combustion engine equipped with such electromagnetic actuators, the excitation frequency then no longer has to be reduced over an extremely long period of time, as suggested in the aforementioned EP 0 118 591 B1, until it finally becomes of the order of magnitude due to the temperature or cold-influenced increased damping reduced natural frequency of the system. Rather, the alternating voltage can be applied to the electro-magnetic coils at a suitable frequency, so that rapid oscillation is achieved, so that there is no unnecessarily high energy consumption.

Finally, with the changeable, essentially sinusoidal or similar exciting AC voltage, the following further idea can be taken into account. It is in fact desirable, even on the basis of the linear spring-mass system, that excitation is excited when starting with a periodic force curve adapted to the system capable of oscillation. According to a Fourier analysis, the previous rectangular profile of the AC voltage already mentioned and shown in FIG. 3 has a relatively high harmonic content, so that in particular also on a real actuator, which, as already explained represents a strongly non-linear system, possibly unwanted harmonics are excited in the actuator, so that the energy consumption is unnecessarily high. To remedy this problem, it is now proposed to select the waveform or sinusoidal shape of the exciting voltage in such a way that the fundamental vibration alone, but not an harmonic of the spring-mass system, is excited.

The method according to the invention is thus characterized in particular by a lower energy consumption and thus a lower need for electrical power. Advantageously, fewer amplitudes are required to vibrate the actuator, although it should also be pointed out that details, for example of the actuator actually used, may of course differ from the above description or from the simplified figure representation, without leaving the content of the claims.

Claims (5)

1. A method for setting into oscillation an electromagnetic actuator (4), which actuates a switching element, especially an internal-combustion engine lift valve (1), in which an armature (4d), which acts on the switching element, oscillates between two electromagnet coils (4a, 4b), in each case against the force of at least one return spring (2a, 2b) by means of alternating energising of the electromagnet coils (4a,4b), so the switching element with the armature (4d) and the return springs (2a, 2b) represents an oscillatory spring-mass system and wherein starting from the rest state of the system, where the armature (4d) is held by the return springs (2a, 2b) substantially in the centre between these magnet coils (4a, 4b), in order to set into oscillation this spring-mass system, the electromagnet coils (4a, 4b) are excited alternately by loading with an electric alternating voltage of a specific frequency, characterised in that instead of the previously customary substantially rectangular curve of the alternating voltage (U) over the time (t) an essentially sinusoidal alternating voltage (U) is applied to the electromagnet coils (4a, 4b) and, especially, either in the form of a variable constant voltage curve or in the form of a correspondingly variable pulse-width modulated voltage curve.
2. A method according to claim 1, characterised in that the frequency of the alternating voltage is produced from the theory of anharmonic oscillations and generally does not agree with the natural frequency of the spring-mass system.
3. A method according to claim 1 or 2, characterised in that the frequency of the alternating voltage is changed as a function of the oscillation amplitude of the armature.
4. A method according to any preceding claim, characterised in that an especially temperature-dependent change in the damping of the spring-mass system and therefore its resonance frequency is taken into account in the determination of the frequency of the alternating voltage.
5. A method according to any preceding claim, characterised in that the sinusoidal form or waveform of the alternating voltage is selected such that only the fundamental oscillation and not the harmonics of the spring-mass system is excited.