Patent Publication Number: US-6671156-B2

Title: Method for controlling electromagnetic actuators for operating induction and exhaust valves of internal combustion engines

Description:
The present invention relates to a method for controlling electromagnetic actuators for operating induction and exhaust valves of internal combustion engines. 
     BACKGROUND OF THE INVENTION 
     As is known, there are currently under development propulsion units in which the operation of the induction and exhaust valves is managed by means of the use of electromagnetic actuators which replace the purely mechanical distribution systems (camshafts). Whilst, in fact, conventional distribution systems require the definition of a valve lifting profile which represents an acceptable compromise for all the possible operating conditions of the engine, the use of an electromagneticaly controlled distribution system makes it possible to vary the phase as a function of the operating point of the engine in such a way as to obtain an optimal efficiency in all operating conditions. 
     Therefore various control methods have been developed which allow the valves to be operated by means of the electromagnetic actuators in dependence on the desired timing and position and velocity profiles. Moreover they must avoid the possibility that, during time intervals when the valve is stationary, in which the valves are maintained shut in the closure position or in the fully open position, possible disturbing forces may cause unwanted displacements of the valves themselves. In fact, even partial unwanted opening or closing, if not rapidly opposed, can significantly alter the design flow of air from the induction manifold towards the cylinders, thereby degrading the performance and efficiency of the engine. 
     The known methods, moreover, have several disadvantages. According to these methods, in fact, for the purpose of opposing the disturbing forces which act on the valves and retaining or rapidly returning the valves themselves into the respective desired positions, during the time periods when the valves are stationary the electromagnets must be supplied with electrical currents which are significantly greater than the minimum currents required in nominal conditions. Moreover, the overall duration of the time period for which each valve is stationary is in one engine cycle, significantly greater than the time period for which it is in motion. There is, therefore, a high consumption of electrical energy caused by the fact that, for almost the entire duration of each engine cycle the current consumed by the electromagnets must be sufficient not only to maintain the valves in the desired nominal conditions, but also to guarantee a margin of safety with respect to possible unwanted displacements. This high consumption detrimentally affects the overall efficiency of the engine, reducing it disadvantageously. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a method for the control of electromagnetic actuators which will be free from the described disadvantages and, in particular, which will allow the overall consumption of electrical energy to be reduced. 
     According to the present invention there is provided a method for controlling electromagnetic actuators for operating induction and exhaust valves in internal combustion engines, where an actuator connected to a control unit is coupled to a respective valve having a real position and comprising a magnetically actuated element, moveable by means of a resultant force to control the movement of the said valve between a closure position and a fully open position; the said control unit being connected to piloting means and comprising supervision means, open loop control means, closed loop control means and selector means controlled by a switching signal generated by the said supervision means; the said first selector means being operable to connect the said piloting means selectively to the said open loop control means and the said closed loop control means; the method being characterised by the fact that it comprises the steps of: 
     a) operating in an open loop real position control mode; 
     b) operating in a closed loop real position control mode; and 
     c) alternatively selecting the said open loop control mode and the said closed loop control mode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention a preferred embodiment will now be described purely by way of non-limitative example with reference to the attached drawings, in which: 
     FIG. 1 is a partially sectioned side view of an induction or exhaust valve and the corresponding electromagnetic actuator; 
     FIG. 2 is a simplified block diagram relating to the method of control according to the present invention in a first embodiment; 
     FIG. 3 is a detailed block diagram of the block diagram of FIG. 2; 
     FIG. 4 is a table relating to the first embodiment of the present method; 
     FIG. 5 is a graph showing quantities utilised in the present method; 
     FIG. 6 is a detailed block diagram of a second detail of a block diagram of FIG. 2; 
     FIG. 7 is a graphical representation of the distance-force-current characteristics of the electromagnetic actuators; 
     FIG. 8 is a simplified block diagram relating to the control method according the present invention in a second embodiment; 
     FIG. 9 is a detailed block diagram of a first detail of the block diagram of FIG. 8; 
     FIG. 10 is a table relating to the second embodiment of the present invention; 
     FIG. 11 is a detailed block diagram of a second detail of the block diagram of FIG. 8; and 
     FIG. 12 is a partially sectioned side view of a second type of induction or exhaust valve and the corresponding electromagnetic actuator. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, an electromagnetic actuator  1 , controlled by the control system according to the present invention, is coupled to an induction or exhaust valve  2  of an internal combustion engine and comprises: a rocker arm  3  of ferromagnetic material having a first end pivoted to a fixed support  4  in such a way as to be able to reciprocate about a horizontal axis A of rotation perpendicular to a longitudinal axis B of the valve  2 , and a second end connected by means of a pivot  5  to an upper end of the valve  2 ; a valve-opening electromagnet  6   a  and a valve-closing electromagnet  6   b  disposed on opposite sides of the body of the rocker arm  3  in such a way as to be able to act when controlled alternatively or simultaneously, exercising a net force F on the rocker arm  3  to make it turn about the axis of rotation; and finally a resilient element  7  operable to maintain the rocker arm  3  in a rest position in which it is equidistant between the pole pieces of the two electromagnets  6  in such a way as to maintain the valve  2  in an intermediate position between a closure position Z SUP  (upper contact) and a fully open position Z INF  (lower contact) which positions the valve  23  assumes when the rocker arm  3  is disposed in contact with the upper pole of the electromagnet  6  and the lower pole of the electromagnetic  6  respectively. 
     For simplicity, hereinafter in this discussion reference will be made to a single valve-actuator unit and, furthermore, the valve-opening electromagnet  6   a  and valve-closure electromagnets  6   b  will be indicated as the upper electromagnet and the lower electromagnet respectively. It is, naturally, intended that the method explained is utilised for simultaneous control of the movement of all the induction and exhaust valves present in an engine. 
     Reference will now be made to the position of the valve  2  in a direction parallel to the longitudinal axis B with respect to the rest position originally assumed; moreover, by “motion phase” it will be intended to identify the time intervals in which the valve  2  is moving between the closure position and the fully open position, whilst the term “stationary phase” will indicate the time intervals during which the valve  2  must be held stationary in either the closure position or the fully open position. 
     In FIG. 2 there is shown a control unit  10  comprising a supervision block  11 , an open loop control block  12 , a closed loop control block  13  and a first selector  14 . The control unit  10  is interfaced with a measurement and piloting device  15  which delivers an upper current I SUP  and a lower current I INF  to the upper electromagnets  6   a  and, respectively, to the lower electromagnets  6   b  to exert on the rocker arm  3  a resultant force F of predetermined value. Moreover the measurement and piloting device  15  provides at its output, in a known manner, a measurement of the real position Z of the valve  2  and a measurement I MSUP  and I MINF  of the upper current I SUP  and lower currents I INF . 
     The supervision block  11  receives at its input, from the control unit  10 , a control signal COM generated according to a known strategy, an estimate or equivalently a measurement, of the real velocity V and, moreover, the measurement of the real position Z provided by the measurement and piloting unit  15 . In particular, the control signal COM can assume alternatively a first control value (“UP”) and a second control value (“DOWN”) to determine the closure and, respectively, the opening of the valve  2 . 
     As will be explained hereinafter, the supervision block  11  updates a control state (“STATE”) of the actuator  1  and provides at least five signals at its output, among which are: a first switching signal SW 1  having a first switching value (“OPEN”) and a second switching value (“CLOSED”); a state signal ST, representative of the control state (“STATE”); an objective position signal Z T  indicative of the position which the valve T must assume and corresponding alternatively to the closure position Z SUP  and fully open position Z INF ; an upper exhaust signal F DSUP  and a lower exhaust signal F DINF , having a first exhaust value (“SLOW”) and a second exhaust value (“FAST”) for selection between two different modes of operation of the upper electromagnets  6   a  and lower electromagnets  6   b  respectively. 
     The open loop control block  12  receives at it input the first state signal ST 1  from the supervision block  11  and provides at its output a first and second open loop objective current value I OLSUP  and I OLINF  (hereinafter simply indicated as “objective open loop current values”), which must be supplied to the upper electromagnets  6   a  and lower electromagnets  6   b  to retain the valve  2  in the fully open and closure positions respectively during the stationary phases. 
     During the motion phases the closed loop control block  13  acts in a first closed loop control mode, or motion control mode, for controlling the motion of the valve  2  as illustrated in detail hereinafter. For this purpose it receives at its input the measurements of the upper and lower current I SUP  and I INF  and the real position Z, the estimate of the real velocity V, the objective position signal Z T  and a plurality of parameters indicative of the operating conditions of the engine such as, for example, the load L and the velocity of rotation RPM. The closed loop control block  13  generates at its output first and second closed loop objective current values I CLSUP  and I CLINF  (hereinafter simply indicated as “closed loop objective current values”) which must be supplied to the upper and lower electromagnets  6   a  and  6   b  during the motion phases of the valve  2 . 
     The first selector  14  is controlled by the first switching signal SW 1  in such a way as selectively to connect the open loop control block  12  or the closed loop control block  13  to the piloting and measurement block  15 . In particular, when the first switching signal SW 1  assumes the first switching value (“OPEN”), the first selector  14  connects the output of the closed loop control block  12  to the input of the measurement and piloting block  15 , which, therefore receives the open loop objective current values I OLSUP  and I OLINF . When, on the other hand, the first switching signals SW 1  has the second switching value (“CLOSED”), the measurement and piloting block  15  receives, via the first selector  14 , the closed loop objective current values I CLSUP  and I CLINF  from the closed loop control block  13 , the measurement and piloting block  15  delivers an upper current I SUP  and, respectively, a lower current I INF  to the upper and lower electromagnets  6   a  and  6   b , having values equal to the objective current values received at its input. 
     Moreover, the measurement and piloting block  15  receives at its input the upper exhaust signals F DSUP  and the lower exhaust signal F DINF  and determines the mode of operation of the electromagnets  6   a ,  6   b . In detail, if the upper and lower exhaust signals F DSUP  and F DINF  are set to the first exhaust value (“SLOW”) a slow exhaust mode is selected, which is obtained by supplying the upper and lower electromagnets  6   a  and  6   b  between a supply source providing a voltage equal to about 15 volts, for example, and ground. When the upper and lower exhaust signal F DSUP  and F DINF  assume the second exhaust value (“FAST”) a rapid exhaust mode is selected by connecting the upper and lower electromagnets  6   a ,  6   b  respectively between supply sources of, for example, plus 15 v and minus 15 v. 
     FIG. 3 illustrates the operation of the supervision block  11  which implements a finite state machine  20  comprising four states from which the control state (“STATE”) can be selected, defined by sets of values of the command signal COM, the real position Z and the real velocity V. 
     In detail, in a first state  21  (“STAY UP”) the command signal is set to the first command value (“UP”), the real position Z is not less than an upper threshold position Z UP  and the estimate of the real velocity is less, in absolute value, than an upper threshold value V UP . In the first state  20 , moreover, the first state signal ST 1  has assigned to it a first state value (“S1”), the objective position Z T  is set equal to the closure position Z SUP , the first switching signal SW 1  is at the first switching value (“OPEN”), whilst the upper and lower exhaust signal F DSUP  and F DINF  both assume the first exhaust value (“SLOW”). 
     From the first state  20  it passes to a second state  22  (“MOVE UP”), if the real position Z, for example because of a disturbance, falls below the upper position threshold Z UP  or if the real velocity V is in absolute value, greater than the upper velocity threshold V UP ; on the other hand it passes to a third state  23  (“MOVE DOWN”) if the command signal COM assumes the second command value (“DOWN”). 
     When the finite state machine  20  is in the second state  22  the command signal COM is at the first command value (“UP”), whilst the real position Z lies between the upper threshold position Z UP  and a lower threshold position Z DOWN . Moreover, the first state signal ST 1  assumes a second state value (“S2”), the objective position is set equal to the closure position Z SUP  the first switching signal SW 1  is set equal to the second switching value (“CLOSED”) and the upper and lower exhaust signal F DSUP  and F DINF  assume the second exhaust value (“FAST”). 
     From the second state  22  the finite state machine  20  goes to the first state  21  if the real position Z rises above the upper threshold position Z UP  and, simultaneously the real velocity V is less, in absolute value, than the upper threshold velocity V UP ; if the command signal COM assumes the second command value (“DOWN”) it passes to the third state  23 . 
     In the third state  23  the command signal COM is at the second command value (“DOWN”) and the real position Z lies between the upper threshold position Z UP  and a lower threshold position Z DOWN . In the third state  23  the first state signal ST 1  assumes a third state value (“S3”), the objective position Z T  is equal to the fully open position Z INF , the switching signal SW is set to the second switching value (“CLOSED”), whilst the upper and lower exhaust signal F DSUP  and F DINF  assume the second exhaust value (“FAST”). 
     From the third state  23  it passes to a fourth state  24  (“STAY DOWN”) if the real position Z falls below the lower threshold position Z DOWN  and simultaneously the real velocity V falls in absolute value below a lower velocity threshold V DOWN ; if the command signal COM assumes the first command value (“UP”) the state machine  20  goes to the second state  22 . 
     The fourth state  24  is defined by the second command value (“DOWN”) for the command signal COM and by values of real position Z and real velocity V less than the lower threshold position Z DOWN  and respectively (in absolute value) the lower velocity threshold V DOWN . In the fourth state  24  the first state signal ST 1  assumes a fourth state value (“S4”), the objective position Z T  is set equal to the fully open position Z INF , the switching signal SW is at the first switching value (“OPEN”) and the upper and lower exhaust signals F DSUP  and F DINF  are assigned the first exhaust value (“SLOW”). 
     From the fourth state  24  the finite state machine  20  goes to the third state  23  if the real position Z goes above the lower threshold position Z DOWN  or if the real velocity V exceeds in absolute value the lower velocity threshold V DOWN ; otherwise, it goes to the second state  22  if the command signal COM assumes the first command value (“UP”). 
     For greater clarity, in FIG. 4 there is shown a table which illustrate the values assumed by the command signal COM, the first switching signal SW 1  and the exhaust signals F DSUP , F DINF  for each possible value of the state signal ST. 
     Moreover, FIG. 5 shows the closure position Z SUP , fully open position Z INF  and the upper and lower position threshold Z UP , Z DOWN , with respect to an axis of the real position Z parallel to the longitudinal axis B of the valve  2  and orientated along the direction of closure of the valve  2  itself. In FIG. 5 there are also shown an opening threshold Z OPEN  and a closure threshold Z CLOSE , the significance of which will be explained hereinafter. 
     In the proposed method it is therefore possible to alternate the open loop control mode and the first closed loop or motion control mode. In particular, the open loop control mode is performed during the stationary phases of the valve  2  when the control state (“STATE”) selected is the first state  21  or the fourth state  24  and the first switching signal SW 1  has the first switching value (“OPEN”); the first closed loop control mode is performed, on the other hand, during the motion phases, in which the control state is the second state  22  or the third state  23  and the first switching signal SW 1  is assigned the second switching value (“CLOSED”). 
     As previously indicated, during the stationary phases in which the open loop control mode is selected and corresponding to the first state  21  or the fourth state  24  of the finite state machine  20 , the first selector  14  connects the measurement and piloting block  15  to the open loop control block  12  which provides the open loop objective current values I OLSUP  and I OLINF . In particular, if the valve  2  is in the closure position Z SUP  the finite state machine  20  is in the first state  21  and, consequently, the first state signal ST 1  assumes the first state value (“S1”). In this case, the open loop control block  12  sets the open loop objective current values I OLSUP  and I OLINF  equal to an upper maintenance value I HUP  and zero respectively. On the other hand, if the valve  2  is disposed in the fully open position Z INF  and thus the finite state machine  20  is in the fourth state  24 , the state signal is set to the fourth state value (“S4”) and the open loop control block  12  sets the open loop objective current values I OLSUP  and I OLINF  equal to zero and, respectively, a lower maintenance value I HDOWN . 
     The upper and lower maintenance values I HUP  and I HDOWN  represent the minimum current values to be supplied to the actuator  1  to maintain the valve  2  in the desired position. 
     During the motion phase, corresponding to the second and third state ( 22 , 23 ) of the finite state machine  20 , the first closed loop control mode is selected. In particular, the first switching signal SW 1  is at the second switching value (“CLOSED”) and the first selector  14  connects the measurement and piloting block  15  to the closed loop control block  13  which operates for example as shown in Italian patent application no. BO99A 000594 Filed by the applicant on May 11, 1999. 
     As illustrated in detail in FIG. 6, the open loop control block  13  comprises a reference generation block  13  which receives at its input the objective position signal Z T  and the engine parameters (that is to say the load L and the velocity of rotation RPM) and provides at its output a position reference profile Z T  and a velocity reference profile V R  representing the position and the velocity which, instant by instant, it is desired to impose on the valve  2  during the motion phases; a fourth control block  31  receiving at its input the measurements of the upper current I SUP , the lower current I INF  and the real position Z, the estimate of the real velocity V, the position reference profiles Z R  and velocity reference profiles V R  and providing at its output an objective force value F O  indicative of the resultant force F to be applied to the rocker arm  3  for the purpose of minimising disturbances to the real position Z and the real velocity V with respect to the position reference profile Z R  and, respectively, the velocity reference profile V R ; and a conversion block  32  receiving at its input the objective force value F O  and providing at its output the pair of closed loop objective current values I CLSUP  and I CLINF  which must be applied to the upper and the lower electromagnets  6  to generate the objective force value F O . 
     During operation of the engine the reference generation block  31  determines the position reference profile Z R  and the velocity reference profile V R  on the basis of the values of the objective position signal Z T , the load L and the velocity of rotation RPM. These profiles can be, for example, calculated starting from the objective position signal Z T  by means of a non-linear two state filter implemented in a known manner generated by the reference generation block  30 , or extracted from tables defined in a calibration phase. 
     The force control block  31  then utilises the position reference profile Z R  and velocity reference profile V R , together with values of the real position Z and the real velocity V to determine the objective force value F O  of the resultant force F which must be applied to the rocker arm  3  according to the following equation: 
     
       
           F   o =( N   1   Z   R   +N   2   V   R )−( K   1   Z+K   2   V )  (1) 
       
     
     In equation (1) N 1 , N 2 , K 1 , and K 2  are gains which can be calculated by applying well known robust control techniques to a dynamic system which represents the motion of the valve  2  and is described by the matrix equation:                  [           Z   .               V   .           ]     =         [         0       1             K   /   M           B   /   M           ]                [         Z           V         ]     +       [         0             1   /   M           ]                   F         .           (   2   )                         
     in which {dot over (Z)} and {dot over (V)} are the time derivatives of the real position Z and the real velocity V respectively, K is an elastic constant, B is a viscosity constant and M is a total equivalent mass. In particular, the resultant force F and the real position Z represent an input and output respectively of the dynamic system. 
     The value of the objective force F O  calculated by the force control block  31  according to equation (1) is utilised by the conversion block  32  to determine the closed loop objective current values I CLSUP  and I CLINF . These current values can be derived 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 such characteristics is illustrated in the graph of FIG. 7 with reference to the electromagnet-valve unit as described. 
     In detail, along the abscissa is plotted the real position Z of the valve  2 , indicative of the position of the rocker arm  3  with respect to the upper and lower electromagnets  6   a ,  6   b ; the origin is the rest point at which the rocker arm  3  is at equal distance from the pole pieces of the two electromagnets, whilst the points Z UP  and Z INF  represent the closed and fully open positions respectively. Upon variation of the current I SUP  and I INF  consumed by the upper and lower electromagnets  6   a ,  6   b  the forces generated by these on the rocker arm  3  are illustrated by the first family of curves represented by solid lines and indicated F SUP  and, respectively a second family of curves represented by broken line indicated F INF . 
     It is important to underline that, according to the above mentioned patent application, both the electromagnets  6  can be supplied repeatedly, simultaneously or in sequence during the motion phase of the valve  2 , to allow the resultant force F exerted on the rocker arm  3  to have a value equal to the value of the objective force F O . 
     A second embodiment of the present method will now be described hereinafter with reference to FIGS. from  7  to  10 , in which those parts which are the same as those already illustrated in FIGS. from  2  to  5  are indicated with the same reference numerals. 
     In detail, in FIG. 8 there is shown a control unit  10 ′ similar to the control unit  10  of FIG.  2  and differing in the fact that the closed loop control block  13  receives at its input the state signal ST and a second switching signal SW 2  generated by the supervision block  11 . 
     In the variant, moreover, the supervision block  11  implements the second finite state machine  36  (FIG. 9) comprising six states from among which can be selected the control state (“STATE”) defined by sets of values of the command signal COM for the real position Z and the real velocity V. In particular, the finite state machine  36  comprises the first, second, third and fourth state  21 , 22 . 23  and  24  of the finite state machine  30  and, in addition a fifth state  37  (“DOCKING UP”) and a sixth state  38  (“DOCKING DOWN”). 
     Moreover, the state signal ST has a separate value for each of the states of the finite state machine  36 . 
     In the first state  21  the command COM is set to the first command value (“UP”) and the real position Z is equal to the closure position Z SUP ; moreover, the state signal ST has assigned to it the first state value (“S1”), the objective position Z T  is set equal to the closure position Z SUP , the first switching signal SW 1  is at the first switching value (“OPEN”), whilst the upper and lower exhaust signal F DSUP  and F DINF  both assume the first exhaust value (“SLOW”). 
     From the first state  20  it passes to the second state  22  if the valve  2  tends to open for example because of a disturbance, that is to say if the real position Z falls below the open threshold Z OPEN  lying between the closure position Z SUP  and the upper threshold position Z UP  (FIG. 5) or if the real velocity V exceeds in absolute value the upper velocity threshold V UP . Moreover, from the first state  20  it passes to the third state  23  if the command signal COM assumes the second command value (“DOWN”). 
     When the finite state machine  20  is in the second state  22  the command signal COM is at the first command value (“UP”) whilst the real position Z lies between the upper position threshold Z UP  and the lower position threshold Z DOWN . Moreover, the first state signal ST 1  assumes the second state value (“ST”), the objective position Z UP  is set equal to the closure position Z SUP , the first switching signal SW 1  is set equal to the second switching value (“CLOSED”), the second switching signal SW 2  assumes a third switching value (“CL1”) whilst the upper and lower exhaust signals F DSUP  and F DINF  are set to the second exhaust value (“FAST”). 
     From the second state  22  the finite state machine moves then to the fifth state  37  if the real position Z rises above the upper position threshold Z UP  and, simultaneously, the real velocity V is less in absolute value than the upper velocity threshold V UP ; if the command signal COM assumes the second command value (“DOWN”) it passes to the third state  23 . 
     In the third state  23  the command signal COM is at the second command value (“DOWN”) and the real position Z lies between the upper position threshold Z UP  and the lower position threshold Z DOWN . In the third state  23  the first state signal ST 1  assumes the third state value (“S3”), the objective position Z T  is equal to the fully open position Z INF , the first and seconds switching signals SW 1 ,SW 2  are set to the second and third switching value respectively (“CLOSED”,“CL1”), whilst the upper and lower exhaust signals F DSUP  and F DINF  both assume the second exhaust value (“FAST”). 
     From the third state  23  it passes to the sixth state  38  if the real position Z falls below the lower position threshold Z DOWN  and, simultaneously, the velocity V falls in absolute value beneath the lower velocity threshold V DOWN ; if the command signal COM assumes the first command value (“UP”), the state machine  20  goes to the second state  22 . 
     The fourth state  24  is defined by the second command value (“DOWN”), by the command signal COM and by the fully open value Z INF  for the real position Z. in the fourth state  24  the first state signal ST 1  assumes the fourth state value (S 4 ), the objective position Z T  is set equal to the fully open position Z INF  and the first switching signal SW 1  is assigned the first switching value (“OPEN”), whilst the upper and lower exhaust signals F DSUP  and F DINF  both assume the first exhaust value (“SLOW”). 
     From the fourth state  24  the finite state machine  20  goes to the third state  23  if the valve  2  tends to close, that is to say if the real position Z rises above the opening threshold Z DOWN , lying between the filly open position Z INF  and the lower position threshold Z DOWN  (FIG. 5 ), or if the real velocity V exceeds in absolute value the lower velocity threshold V DOWN . Moreover, from the fourth state  24  it passes to the second state  22  if the command signal COM assumes the first command value (“UP”). 
     In the fifth state  37  the command signal COM is at the first command value (“UP”), the real position Z is not less than the upper position threshold Z UP  and the estimate of the real velocity V is less in absolute value than the upper velocity threshold V UP . Moreover, the objective position Z T  is equal to the closure position Z SUP , the first and second switching signals SW 1 , SW 2  are at the second switching value (“CLOSED”) and, respectively, at a fourth switching value (“CL2”), whilst the upper and lower exhaust signals F DSUP  and F DINF  assume the second exhaust value (“FAST”) and the first exhaust value (“SLOW”) respectively. 
     From the fifth state  37  the following transitions can be made: towards the first state  21  if the condition that real position Z is not less than the upper position threshold Z UP  and the estimate of the real velocity V is less in absolute value than the upper velocity threshold V UP  remains at least for a predetermined time interval; towards the second state  22  if the real position Z goes to a value less than the upper position threshold Z UP  or if the absolute value of the real velocity V exceeds the upper velocity threshold V UP ; and towards the third state  23  if the command signal COM assumes the second command value (“DOWN”). 
     In the sixth state  38  the command signal COM is at the second command value (“DOWN”), the real position Z is not greater than the lower position threshold Z DOWN  and the real velocity V is less than the lower position threshold Z DOWN  and, respectively, (in absolute value) the lower velocity threshold V DOWN . Moreover, the objective position Z T  is equal to the fully open position Z INF , the first and second switching signals SW 1 , SW 2  are at the second and the fourth switching value (“CLOSED”, “CL2”) respectively; moreover, the upper and lower exhaust signals F DSUP  and F DINF  assume the first exhaust value (“SLOW”) and the second exhaust value (“FAST”) respectively. 
     From the sixth state  38  the following transitions can be made: towards the fourth state  24  if the condition that the real position Z is not greater than the lower position threshold Z DOWN  and the real velocity V is lower in absolute value than the lower velocity threshold V DOWN  remains at least for a predetermined time interval; towards the third state  23  if the real position Z goes to a value greater than the lower position threshold Z DOWN  or if the absolute value of the real velocity V exceeds the upper velocity threshold V DOWN ; and towards the second state  22  if the command signal COM assumes the first command value (“UP”). 
     In FIG. 10 there is shown a table which illustrates the values assumed by the command signal COM, the first and second switching signal SW 1 ,SW 2 , and the upper and lower exhaust signals F DSUP  and F DINF  in correspondence with each possible value of the state signal ST. 
     With reference to FIG. 11, the closed loop control block  13  comprises, according to the variant, the reference generation block  30 , the force control block  31 , the conversion block  32  connected together as illustrated in FIG. 6, and, further, a position control block  33  and a second selector  34 . 
     The position control block  33  receives at its input the real position Z, the reference position Zr and a second state signal ST 2 , and at its output provides a first and a second docking current I DSUP  and I DINF  (hereinafter simply indicated as “docking current values I DSUP  and I DINF ”. 
     The second selector  34  is controlled by the second switching signal SW 2  in such a way as to connect its output  35 , defining the output of the closed loop control block  13 , selectively with the output of the conversion block  32  and with the output of the position control block  33 . 
     In the variant, the state signal ST determines the mode on the basis of which the position control block  33  makes the calculation of the current docking values. In particular, if the state signal is to assume the fifth state value S 5  the docking current values I DSUP  and I DINF  are provided on the basis of the equations; 
     
       
           I   DSUP   =I   NOM   +I   G   |Z   SUP   −Z |  (3) 
       
     
     
       
           I   DINF =0  (4) 
       
     
     Where I NOM  is a nominal current value and I G  is a current gain, both predetermined. If, on the other hand, the state signal ST assumes the sixth state value S 6  the position control block  33  calculates the docking current values I DSUP  and I DINF  on the basis of the equations: 
     
       
           I   DINF =0  (5) 
       
     
     
       
           I   DINF   =I   NOM+   I   G   |Z   SUP   −Z|   (6) 
       
     
     In all other cases both the docking current values I DSUP  and I DINF  are set equal to 0. In particular, the nominal current value I NOM  and the current gain I G  can be chosen during the design stage in a manner known per se such that the docking current values I DSUP  and I DINF , calculated as a function only of the real position Z using linear relations, are on average less than the closed loop objective current values I CLSUP  and I CLINF  and have more gradual variation times than these. 
     Moreover, the second selector  34  connects the output  35  to the output of the conversion block  32  when the second switching signal is at the third switching value (“CL1”) and the output of the position control block  33  when the second switching signal is at the fourth switching value (“CL2”). 
     In this way a first and second closed loop mode are defined in practice which are selected alternatively on the basis of the value of the second switching signal SW 2 . 
     In particular, the first control mode, or motion control mode, coincides with that described with reference to FIGS. from  2  to  5  and is selected when, during the motion phases, the second switching signal is at the third switching value (“CL1”). In this case the closed loop control block  13  provides at its output the closed loop objective current values I CLSUP  and I CLINF  according to the method previously described. On the other hand, the second closed loop control mode or docking control mode, is selected during docking phases in which the second switching signal SW 2  assumes the fourth switching value. These docking phases are defined when the real position Z is greater than the upper position threshold Z UP  or less than the lower threshold Z DOWN  and therefore the valve  2  is close to the closure position or fully open position. Therefore, when the docking control mode is operated the closed loop control block  30  provides at its output the docking current values I DSUP  and I DINF . 
     The advantages offered by the present invention are clear from the above explanations. In particular, the method proposed makes it possible to optimise the efficiency of the engine, reducing electrical power consumption during the stationary phases and effecting a precise control of the movements of the valves during the motion phases. In fact, the upper and lower maintenance values I HUP  and I HDOWN  provided in the stationary phases in which the open loop control mode is selected, are very much lower, it being enough to maintain the valves in the desired positions only in the absence of disturbances. However, when disturbing forces intervene causing unwanted opening or closure, a closed loop control mode is selected in such a way as rapidly to bring the valves into the respective objective positions preventing the flow of air to the cylinders from becoming significantly altered. During the motion phases, on the other hand, the closed loop control mode makes it possible to give the valves optimal movement profiles in dependence on the operative conditions of the engine. Moreover, it is possible to damp the velocity of the valves close to the ends of their strokes thus avoiding impacts against fixed parts which would drastically reduce the useful life of the valve itself. 
     A further advantage is achieved by means of the second embodiment described, which makes it possible to select different closed loop control modes during the motion phases and during the docking phases. In fact, the docking control allows the motion of the valves to be controlled with a lower expenditure of energy given that smaller currents are delivered. On the other hand, during the motion phases the motion control mode makes it possible to obtain greater precision and velocity. 
     They are further advantages in the use of different operating modes for the actuators during the motion and stationary phases. During the motion phases, in particular, the rapid exhaust mode makes it possible quickly to pilot the electromagnets and therefore to make the control more robust. During the stationary phase, the slow exhaust mode makes it possible further to reduce the consumption of electrical power. 
     Moreover, the method proposed can be utilised even for the control of different sets of valve actuators from those described with reference to FIG.  1 . For example, as shown in FIG. 12, an actuator  40  co-operates with an induction or exhaust valve  41  and comprises: a core  42  of ferromagnetic material securely fixed to a rod  43  of the valve  41  and disposed perpendicularly to its longitudinal axis B; an upper electromagnet  44   a  and a lower electromagnet  44   b  both at least partially surrounding the stem  43  of the valve  41  and disposed on opposite sides with respect to the core  42  in such a way as to be able to act when commanded, alternatively or simultaneously, by exerting a resultant force F on the core  42  to make it translate parallel to the longitudinal axis B; and a resilient element  45  operable to maintain the core  42  in a rest position in which it is equidistant from the pole pieces of the lower and upper electromagnets  44   a  and  44   b  in such a way as to maintain the valve in an intermediate position between the closure position Z SUP  and the fully open position Z INF . 
     Finally, it is evident that the method described can have modifications and variations introduced thereto without departing from the ambit of the present invention.