A method for the control of electromagnetic actuators for the actuation of intake and exhaust valves in internal combustion engines

Abstract: A method for the control of an electromagnetic actuator (1, 25) coupled to a respective valve (2, 26) and provided with a moving ferromagnetic member (3, 27) connected to at least one point of the valve, a pair of electromagnets (6, 29) disposed on opposite sides with respect to the moving ferromagnetic member (3, 27) and an elastic member (7, 30) adapted to maintain the valve in a rest position. The method comprises the stages of detecting an actual position (Z) and an actual velocity (V) of the valve (2, 26), determining a reference position (Z R ) and a reference velocity (V R ) of the valve (2, 26) and minimising differences between the reference position (Z R ) and the actual position (Z) and between the reference velocity (V R ) and the actual velocity (V) of the valve (2, 26) by means of a feedback control action.

Description: The present invention relates to a method for the control of electromagnetic actuators for the actuation of intake and exhaust valves in internal combustion engines. As is known, drive units are currently being tested in which the actuation of the intake and exhaust valves is managed by using actuators of electromagnetic type that replace purely mechanical distribution systems (camshafts). While conventional distribution systems make it necessary to define a valve lift profile that represents an acceptable compromise between all the possible operating conditions of the engine, the use of an electromagnetically controlled distribution system makes it possible to vary the phasing as a function of the engine point in order to obtain an optimum performance in any operating condition. A number of control systems enabling the valves to be moved by means of electromagnetic actuators according to desired timings have thus been developed. These control systems have, however, some drawbacks. They are based on open loop control systems and require, when each valve is opened or closed, the actuators to be supplied with corresponding currents and/or voltages of a value such as to ensure that the valve, irrespective of the resistance opposing it, reaches the desired position within a predetermined time interval. In this way, however, the valve is subject to an impact each time that it comes into contact with fixed members in the position of maximum opening (lower contact) or in the closed position (upper contact). This is particularly critical, since the valves are subject to an extremely high number of opening and closing cycles and therefore wear very rapidly. Moreover, drive units that use these known control system are undesirably noisy, in particular at low speeds, precisely because of the impacts that take place during the phases of movement of the valves. Another example of a known actuator is disclosed in DE-A-197. 39 840, which describes an electromagneticcally actuated servo drive for an intake/exhaust valve in an internal combustion engine. As displacement sensor arranges on the valve stem enables a control circuit to monitor the exact position of the valve an to calculate the rate at which the valve is : displaced. The object of the present invention is to provide a method for the control of electromagnetic actuators that is free from the above-described drawbacks and, in particular makes it possible to guide the movement of the valves during the contact phases corresponding to the open and closed positions. The present invention therefore relates to a method for the control of electromagnetic actuators for the actuation of intake and exhaust valves in internal combustion engines, as claimed in claim 1. The invention is set out in further detail below with reference to a non-limiting embodiment thereof, made with reference to the accompanying drawings, in which: Fig. 1 is a lateral elevation, partly in cross-section, of a first type of intake or exhaust valve and of the corresponding electromagnetic actuator; Fig. 2 is a simplified block diagram relating to the control method of the present invention; Fig. 3 shows examples of reference movement profiles implemented according to the present method; Fig. 4 is a simplified block diagram of a feedback-based dynamic system implementing the present method; Fig. 5 shows graphs relating to distance-force-current characteristics of electromagnetic actuators; Fig. 6 is a lateral elevation, partly in cross-section, of a second type of intake or exhaust valve and of the corresponding electromagnetic actuator. In Fig. 1, an electromagnetic actuator 1, controlled by a control system of the present invention, is coupled to an intake or exhaust valve 2 of an internal combustion engine and comprises an oscillating arm 3 of ferromagnetic material, having a first end hinged on a fixed support 4 so as to be able to oscillate about a horizontal axis of rotation A perpendicular to a longitudinal axis B of the valve 2, and a second end connected via a hinge 5 to an upper end of the valve 2, a pair of electromagnets 6 disposed on opposite sides of the body of the oscillating arm 3 so as to be able to act on command, simultaneously or alternatively, by exerting a net force F on the oscillating arm 3 in order to cause it to rotate about the axis of rotation A and an elastic member 7, adapted to maintain the oscillating arm 3 in a rest position in which it is equidistant from the polar heads of the two electromagnets 6, so as to maintain the valve 2 in an intermediate position between the closed position (upper contact, Z SUP ) and the position of maximum opening (lower contact, Z INF ) which the valve 2 assumes when the oscillating arm 3 is disposed in contact with the polar head of the upper electromagnet 6 and with the polar head of the lower electromagnet 6 respectively. For simplicity, reference will be made in the following description to a single valve-actuator unit. It will be appreciated that the method described is used for the simultaneous control of the movement of all the intake and exhaust valves present in a drive unit. Moreover, reference will always be made to the position of the valve 2 in a direction parallel to the longitudinal axis B, with respect to the rest position assumed to be the starting position. As shown in Fig. 2, a control unit 10 comprises a reference generation block 11, a force control block 12 and a conversion block 13 and is further interfaced with a guiding and measurement circuit 14. The reference generation block 11 receives as input an objective position signal Z T , generated in a known manner by the control unit, and a plurality of parameters indicative of the engine operating conditions (for instance the load L and the number of revolutions RPM). The reference generation block 11 also supplies as output a reference position profile Z R and a reference velocity profile V R and supplies them as input to the force control block 12 which also receives a measurement of the actual position Z and en estimate of the actual velocity V of the valve 2. The measurement of the position Z is supplied by the guiding and measurement circuit 14, as described below, and the estimate of the actual velocity V may be obtained, for instance, by providing the system with an accelerometer adapted to measure the acceleration of the valve 2 and integrating the signal supplied by this accelerometer over time or, as an alternative, recording successive measurement values of the actual position Z and carrying out a derivation of the time series obtained in this way. The force control block 12 calculates and supplies as output an objective force value Fo indicative of the net force F to be applied to the oscillating arm 3 by means of the electromagnets 7 in order to minimise the deviations of the actual position Z and of the actual velocity V with respect to the reference position Z R and reference velocity V R profiles respectively. The conversion block 13 receives as input the objective force value F o and supplies as output a pair of objective current values I OSUP and I OINF that need to be applied to the upper electromagnet 6 and the lower electromagnet 6 respectively in order to generate the objective force value F o . The guiding and measurement circuit 14, of known type, receives as input the objective current values I OSUP and I OINF and causes the corresponding upper and lower electromagnets 6 to be supplied with respective currents I SUP and I INF . It is connected, moreover, to a position sensor 15 of known type adapted to detect the position of the valve 2 or, in an equivalent way, of the oscillating arm 3. The position sensor 15 supplies a signal V Z indicative of the actual position Z of the valve 2 to the guiding and measurement circuit 14 which in turn supplies the measurement of the actual position Z to the control unit 10 and in particular to the force control block 12. During the operation of the engine, the control unit 10, using known strategies, determines the moments of opening and closing of the valve 2. At the same time, it sets the objective position signal Z T to a value representative of the position that the valve 2 should assume. The objective position signal Z T is in particular assigned an upper value Z SUP corresponding to the upper contact or a lower value Z INF corresponding to the lower contact, depending on whether the control unit 10 has supplied a closing or opening command to the valve 2. On the basis of the values of the objective position signal Z T , the load L and the number of revolutions RPM, the reference generation block 11 determines the reference position profile Z R and the velocity reference profile V R which respectively represent the position and the velocity which, as a function of time, it is desired to impose on the valve 2 during its displacement between the positions of maximum opening and closure. These profiles may for instance be calculated from the objective position signal Z T by means of a two-state non-linear filter, implemented in a known manner by the reference generation block 11, or taken from tables drawn up at the calibration stage. Fig. 3 shows an example relating respectively to a position profile Z R and a velocity profile V R generated, at a time T o , together with a command to close the valve 2. As will be seen, the profiles are defined such that the valve 2 slows down in the end section of its stroke, in order to avoid an abrupt impact on the fixed members. The force control block 12 therefore uses the reference position profiles Z R and velocity reference profiles V R , together with the values of the actual position Z and the actual velocity V, to determine the objective force value F o of the net force F that needs to be applied to the oscillating arm 3, according to the following equation: F o = (N 1 Z R + N 2 V R ) - (K 1 Z + K 2 V) In (1), N 1 , N 2 ,K 1 and K 2 are gains that can be calculated by applying well-known control techniques to a dynamic system 20 (shown in Fig. 4) that represents the movement of the valve 2 and is described by the matricial equation: in which Z and V are the time derivatives of the actual positions Z and respectively of the actual velocity V, K is an elastic constant, B is a viscous constant and M is an equivalent total mass. In particular, the net force F and the real position Z represent an input and respectively an output of the dynamic system 20. The force control block 12 therefore carries out, with respect to the dynamic system 20, the function of a feedback controller, shown by 21 in Fig. 4, which uses the net force F as the control variable in order to impose that the controlled variable, i.e. the real position Z, has a course that is as close as possible to a predetermined course provided by the reference position profile Z R . As mentioned above, the objective force value F o calculated by the force control block 12 according to equation (1) is used by the conversion block 13 to determine the objective current values I OSUP and I OINF of the respective currents I SUP and I INF that need to be supplied to the upper and lower electromagnets 6. These current values may be obtained in a manner known per se by inversion of a mathematical model or on the basis of tables representative of distance-force-current characteristics. An example of these characteristics is shown in the graph of Fig. 5, with reference to the valve-electromagnets unit as described. In detail, the position of the oscillating arm 3 with respect to the electromagnets 6 is shown on the abscissa; the origin is set at the rest point in which the oscillating arm 3 is equidistant from the polar heads of the two electromagnets 6, while the points Z SUP and Z INF represent the upper contact and the lower contact respectively. With the variation of the currents I SUP and I INF absorbed by the upper and lower electromagnets 6, the forces generated by these on the oscillating arm 3 are illustrated by a first family of curves, shown by continuous lines and indicated by F SUP and, respectively, a second family of curves, shown by dashed lines and indicated by F INF . It should be stressed that both the electromagnets 6 can be supplied during a same closing or opening stroke of the valve 2, to enable the net force F exerted on the oscillating arm 3 to have a value equal to the objective force value F o . For instance, during a closing stroke, in which the valve 2 moves between the position of maximum opening and the closed position, the upper electromagnet 6 is initially supplied; if the actual velocity V of the valve 2 exceeds the reference velocity V R , the force control bock 12 generates an objective force value F o such as to exert a braking action on this valve 2. This braking action is thus obtained by deactivating the upper electromagnet 6 and supplying the current I INF to the lower electromagnet 6 while the valve 2 is still moving towards the upper contact Z SUP . Vice versa, during an opening stroke, in which the valve 2 is moving between the closed position and the position of maximum opening, the upper electromagnet 6 is used to brake the valve 2, while the lower electromagnet 6 makes it possible to impose an acceleration thereon. The stages of supply and de-activation of the electromagnets 6 in order to accelerate or brake the valve 2 as described above may be repeated in sequence several times during each opening and closing stroke so as to minimise the deviations of the actual position Z and the actual velocity V of the valve 2 from the reference position profile Z R and the reference velocity profile V R respectively. The method described above has the following advantages. In the first place, the feedback control makes it possible to actuate the valves according to predetermined movement profiles. It is in particular possible to impose a desired velocity trend, moderating it at the end-of-stroke sections, so that the contact between the valves and the fixed members takes place gently. This makes it possible to obtain a so-called "soft touch", avoiding impacts that would substantially reduce the life of the valves and would make the use of electromagnetic actuation systems problematic for mass-produced vehicles. Moreover, the use of moderated velocity profiles makes it possible substantially to reduce the noise generated by the drive unit, thereby improving its silencing in particular at low speeds. Further advantages are provided by the use of the net force F as a control variable, making it possible to carry out accurate control and, at the same time, to optimise the currents absorbed by the electromagnets. These currents must ensure only that the net force F applied to the oscillating arm has a value equal to the objective force value F o . According to known methods, however, the electromagnets must absorb currents sufficient to ensure the displacement of the valve between the upper and lower contacts irrespective of the force actually required. A safety margin therefore has to be provided and high currents are therefore supplied to the electromagnets. It will therefore be appreciated that the method proposed advantageously makes it possible to reduce current consumption and substantially to improve the overall performance of the drive unit. As a result of the lower current absorption, there is less risk of damage to the windings of the electromagnets as a result of overheating. The method proposed may, moreover, also be used for the control of valve actuator units other than those described with reference to Fig. 1. For instance, as shown in Fig. 6, an actuator 25 cooperates with an intake or exhaust valve 26 and comprises an anchor of ferromagnetic material 27 joined rigidly to a stem 28 of the valve 26 and disposed perpendicular to its longitudinal axis C, a pair of electromagnets 29 at least partially bounding the stem 28 of the valve 26 and disposed on opposite sides with respect to the anchor 27, so as to be able to act, on command, alternatively or simultaneously, by exerting a net force F on the anchor 27 in order to cause it to move in translation parallel to the longitudinal axis C and an elastic member 30 adapted to maintain the anchor 27 in a rest position in which it is equidistant from the polar heads of the two electromagnets 29 so as to maintain the valve 26 in an intermediate position between the closed position (upper contact) and the position of maximum opening (lower contact) that the valve 26 assumes when the anchor 27 is disposed in contact with the polar head of the upper electromagnet 6 and respectively with the polar head of the lower electromagnet 6. It will be appreciated that modifications and variations may be made to the above description without departing from the scope of the present claim 1.

Claims: A method for the control of electromagnetic actuators for the actuation of intake and exhaust valves in internal combustion engines, in which an actuator (1, 25), connected to a control unit (10), is coupled to a respective valve (2, 26) and comprises a moving member (3, 27) actuated magnetically to control the movement of the valve (2, 26) between a closed position (Z SUP ) and a position of maximum opening (Z INF ) and an elastic member (7, 30) adapted to maintain the valve (2, 26) in a rest position, which method comprises the stages of: a) detecting an actual position (Z) and an actual velocity (V) of the valve (2, 26); b) determining a reference position (Z R ) and a reference velocity (V R ) of this valve (2, 26); c) minimising differences between this reference position (Z R ) and the actual position (Z) and between the reference velocity (V R ) and the actual velocity (V) of the valve (2, 26), by means of a feedback control action; the method being characterised in that the difference minimisation stage c) comprises the stage of: c1) determining an objective force value (F o ) to be exerted on the moving ferromagnetic member (3, 27). A method as claimed in claim 1, characterised in that the stage c1) of determining the objective force value (Fo) comprises the stage of: c11) calculating this objective force value (F o ) as a function of the reference position (Z R ), the actual position (Z), the reference velocity (V R ) and the actual velocity (V). A method as claimed in claim 2, characterised in that the stage c11) of calculating the objective force value (F o ) as a function of the reference position (Z R ), the actual position (Z), the reference velocity (V R ) and the actual velocity (V) comprises the stage of: c111) calculating the objective force value (F o ) according to the equation: F o = (N 1 Z R + N 2 V R ) - (K 1 Z + K 2 V) in which N 1 , N 2 , K 1 and K 2 are respectively a first, second, third and fourth predetermined gain. A method as claimed in claim 1, characterised in that the stage c1) of determining the objective force value (F o ) precedes the stage of: c2) exerting on the moving member (3, 27) a net force (F) of a value equal to the objective force value (F o ) A method as claimed in claim 4, in which the actuator (1, 25) further comprises at least a pair of electromagnets (6, 29) disposed on opposite sides with respect to the moving member (3, 27) and in which the valve (2, 26) travels an opening stroke when moving from the closed position (Z SUP ) to the position of maximum opening (Z INF ) and a closing stroke when moving from the position of maximum opening (Z INF ) to the closed position (Z SUP ), which method is characterised in that it comprises the stage c2) of exerting a net force (F) comprising the stage of: c21) supplying both the electromagnets (6, 29) during each opening and closing stroke of the valve (2, 26). A method as claimed in claim 5, characterised in that the stage c21) of supplying both the electromagnets (6, 29) comprises that stage of: c211) supplying the electromagnets (6, 29) repeatedly in sequence. A method as claimed in claim 5, characterised in that the stage c21) of supplying both the electromagnets (6, 29) further comprises the stages of: c212) calculating at least a first and second objective current value (I OSUP , I OINF ) as a function of the objective force value (F o ); and c213) supplying the pair of electromagnets (6, 29) with a first and a second current (I SUP , I INF ) having a value equal to the first and the second objective current value (I OSUP , I OINF ) respectively. A method as claimed in any one of the preceding claims, characterised in that the stage a) of detecting the actual position (Z) and the actual velocity (V) comprises the stages of: a1) measuring the actual position (Z), and a2) estimating the actual velocity (V). A method as claimed in any one of the preceding claims, characterised in that the stage b) of determining the reference position (Z R ) and the reference velocity (V R ) comprises the stages of: b1) generating an objective position signal (Z T ) indicative of position; and b2) processing the objective position signal (Z T ) by means of filtering means (11).

Inventors: BURGIO GILBERTO, DI LIETO NICOLA, FLORA ROBERTO, BURGIO, GILBERTO, DI LIETO, NICOLA, FLORA, ROBERTO
Applicants: MAGNETI MARELLI POWERTRAIN SPA, MAGNETI MARELLI POWERTRAIN S.P.A.
Application year: 2000
Main classification: F01L9
IPC codes: F01L9