A valve driving apparatus using an electromagnetic coil to move a valve body with reduced noise

Abstract: An electromagnetic force generated in a valve driving apparatus is rapidly decreased when a valve body (12) moves to a position close to the end of its stroke so that a shock generated when the valve body (12) reaches the end of the stroke is reduced. The valve body (12) is movable between opposite ends of its stroke so as to open and close a valve provided in an internal combustion engine. An electromagnetic coil (24, 26) generates an electromagnetic force exerted on the valve body (12). A current supplied to the electromagnetic coil (24, 26) is controlled in accordance with an operational condition of the internal combustion engine. The current flowing in the electromagnetic coil (24, 26) is rapidly decreased when the valve body (12) approaches the end of its stroke.

Description: The present invention relates to a valve driving apparatus of an internal combustion engine and, more particularly to a valve driving apparatus for driving an intake or an exhaust valve by using an electromagnetic force generated when a predetermined electric power is supplied to an electromagnetic coil. A valve driving apparatus is known in the art which drives an intake valve or an exhaust valve of an internal combustion engine by using an electromagnetic force generated by an electromagnetic coil. Japanese Laid-Open Patent Application No.2-181006 discloses an example of such a valve driving apparatus. Such a valve driving apparatus eliminates a valve driving mechanism such as a camming mechanism. Additionally, since an ideal timing of opening and closing of the valves can be easily provided based on an operational condition of the internal combustion engine, output characteristics and specific fuel consumption of the internal combustion engine can be improved. The above-mentioned conventional valve driving apparatus generates an electromagnetic force by supplying a predetermined current to an electromagnetic coil. A valve body is moved toward an open position or a closed position by the electromagnetic force. If the current is supplied for the purpose of merely moving the valve body, the valve body may be strongly moved toward the open position or the closed position. This may result in generation of noise and reduces service life of the valve body. However, in the conventional valve driving apparatus, the current supplied to the electromagnetic coil is controlled without any consideration with respect to a seating characteristics of the valve body. Accordingly, the conventional valve driving apparatus does not always provide a desired characteristics with respect to a low noise operation and a long service life of the valve body. Document Research Disclosure No. 352, 1. August 1993, page 518, XP 000 395246 "Electromagnetic engine valve actuator with low seating velocity" discloses an electromagnetic engine-valve actuator with low seating velocity, wherein a feed-back control concept is disclosed for controlling the current fed to the coils of the electromagnetic actuator. The electromagnetic actuator is preferably used for precisely positioning valves of an internal combustion engine and for adapting the control of the valves to the engine speed and load. To this end, an effective algorithm is defined on the basis of the sliding-mode control theory and in the respective functions for carrying out the control concept the valve position, valve velocity and the current to be fed to the electromagnetic coils is measured or determined, respectively. When the valve during its motion towards the opened or closed position reaches a position near the opened or closed position, the algorithm is activated for obtaining an appropriate current control when the valve moves closer to the opened or closed position. Alternatively, another algorithm is used as a modified version of the known adaptive feed forward control. The valve motion is continuously monitored and on the basis of a model a current control is then provided on the basis of calculations from a motion equation. Furthermore, document DE 43 00 666 Al discloses a valve actuator for internal combustion engines having two axially arranged electromagnets including ring-shaped poles as well as an electromagnetic core arranged on the valve stem. By means of two separately provided control units both electromagnetic coils of the ring-shaped poles which are arranged opposite to each other, are fed with a suitable current for obtaining a desired movement of the valve from a fully opened position to a fully closed position. When the valve moves towards one of the end positions or to an intermediate position, the electromagnetic coil generating a force in the opposite direction is excited for bringing the valve into the end position without undesired oscillations. Moreover, at the beginning of the movement of the valve both coils are fed with current and for starting the movement of the valve the coil generating a braking force is switched off. It is a general object of the present invention to provide an improved and useful valve driving apparatus in which the above-mentioned problems are eliminated. A more specific object of the present invention is to provide a valve driving apparatus in which an electromagnetic force generated therein is rapidly decreased when a valve body moves to a position close to either end of its stroke so that shock, generated when the valve body reaches the end of stroke, is reduced. According to the present invention, these objects are accomplished by a valve driving apparatus as claimed in the appended claims. According to the present invention, the valve body does not strongly collide with a valve seat at the end of its stroke. Thus, an improvement in low-noise operation of the engine is achieved. Additionally, the service life of the valve body is improved. In particular, the current decreasing means comprises: a flywheel circuit returning a current flowing in the electromagnetic coil; and a variable resistor circuit increasing a resistance of the flywheel circuit when the valve body approaches at least one of the first position and the second position. In this embodiment, a current generated in the electromagnetic coil due to a reverse electromotive force flows to the flywheel circuit. The current flowing in the flywheel circuit is rapidly decreased since the current is converted to heat by the increased resistance of the flywheel circuit. Thus, the current flowing in the electromagnetic coil when the valve body approaches the end of stroke is rapidly decreased. The valve driving apparatus according to the present invention may comprise current detecting means for detecting the current supplied to the electromagnetic coil so as to generate a current detection signal. The variable resistor circuit varies the resistance of the flywheel circuit in accordance with the difference between a value of a current designating signal supplied by an engine control unit and a value of a current detection signal supplied by the current control means. The current designating signal is supplied by an engine control unit for controlling the current supplied to the electromagnetic coil. When the valve body approaches the end of the stroke, the value of the current designating signal is decreased. At this time, an actual current flowing in the electromagnetic coil differs from the current designated by the current designating signal. However, in this embodiment, the current supplied to the electromagnetic coil is rapidly decreased since the resistance of the flywheel circuit is increased in accordance with a difference between the current designated by the current designating signal and the actual current flowing in the electromagnetic coil. Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. FIG.1 is a circuit diagram of a first embodiment of a valve drive apparatus according to the present invention; FIG.2 is a cross-sectional view of an electromagnetic actuator operated by the valve drive apparatus according to the present invention; FIG.3 is a graph showing a characteristic of the electromagnetic actuator shown in FIG.2; FIG.4 is a time chart for explaining an operation of the drive circuit shown in FIG.1; FIG.5 is a circuit diagram of a second embodiment of a valve drive apparatus according to the present invention; FIG.6 is a circuit diagram of a third embodiment of a valve drive apparatus according to the present invention; FIG.7 is a block diagram of a determining circuit used in the third embodiment; A description will now be given, with reference to FIG.1, of a first embodiment of the present invention. FIG.1 is a drive circuit of a valve driving apparatus according to the first embodiment of the present invention. FIG.2 is a cross-sectional view of an electromagnetic actuator 10 driven by the drive circuit shown in FIG.1. The electromagnetic actuator 10 shown in FIG.2 drives a valve body 12. The valve body 12 reciprocally moves to open or close an intake or an exhaust port 14 (hereinafter simply referred to as a port 14) by seating on a valve seat 16 or separating from the valve seat 16. A valve stem 18 extends from the valve body 12. The valve stem 18 is slidably supported by a valve guide 20 in the axial direction. A plunger 22 is fixed on an end of the valve stem 18. The plunger is made of a soft magnetic material and has a disk-like shape. A first electromagnetic coil 24 is arranged above the plunger 22 with a predetermined distance apart from the plunger 22. The first electromagnetic coil 24 is supported by a first core 28. A second electromagnetic coil 26 is arranged under the plunger 22 with a predetermined distance apart from the plunger 22. The second electromagnetic coil 26 is supported by a second core 30. The first and second cores 28 and 30 are made of a soft magnetic material. The first and second core 28 and 30 are arranged in a housing 32 made of a non-magnetic material. Each of first and second core 28, 30 has an annular shape having a center opening thereof which accommodates springs 34 and 36, respectively. In the structure shown in FIG.2, the plunger 22 is elastically supported by the springs 34 and 36 from both sides so that the plunger 22 can elastically move between the electromagnetic coils 24 and 26. When no force is applied to the plunger 22, the plunger 22 is positioned between the first core 28 and the second core 30 since the pressing forces generated by the springs 34 and 36 are balanced. This balanced position of the plunger 22 will be hereinafter referred to as a neutral position. When the plunger 22 is at the neutral position, the valve body 12 is positioned at an intermediate position between the open position and the closed position. The intermediate position of the valve body 12 will be hereinafter referred to as a half-open position. In the above structured electromagnetic actuator 10, a magnetic circuit is formed by the first core 28, the plunger 22 and an air gap formed between the first core 28 and the plunger 22, the magnetic circuit surrounding the first electromagnetic coil 24. Accordingly, when a current is supplied to the first electromagnetic coil 24, an electromagnetic force is generated in the plunger 22 so that the plunger 22 is moved toward the first core 28. Additionally, a magnetic circuit is formed by the second core 30, the plunger 22 and an air gap formed between the second core 30 and the plunger 22, the magnetic circuit surrounding the second electromagnetic coil 26. Accordingly, when a current is supplied to the second electromagnetic coil 26, an electromagnetic force is generated in the plunger 22 so that the plunger 22 is moved toward the second core 30. Accordingly, if a current is supplied to the first electromagnetic coil 24 and the second electromagnetic coil 26, alternately, the plunger 22 can be reciprocally moved between the first core 28 and the second core 30. That is, the valve body 12 can be moved between the open position and the closed position. FIG.3 is a graph showing a relationship between a displacement of the valve body 12 and the electromagnetic force generated in the plunger 22 and a relationship between the displacement of the valve body 12 and a pressing force exerted on the plunger 22 by the springs 34 and 36. In FIG.3, each of a plurality of oblique dotted lines represents a relationship between a displacement of the valve body 12 from the closed position to the half-open position and a pressing force exerted on the plunger 22 by the springs 34 and 36 with respect to various strokes. Zero position in the horizontal axis corresponds to the closed position of the valve body 12. As shown in FIG.3, the pressing force exerted on the plunger by the springs 34 and 36 is in reverse proportion to the travel of the valve body 12. In FIG.3, each of a plurality of solid line curves represents a relationship between travel of the valve body 12 and an electromagnetic force generated in the plunger 22 when a constant current is supplied to the first electromagnetic coil 24 with respect to various values of the constant current. As shown in the figure, the electromagnetic force generated in the plunger 22 sharply increases as the valve body 12 approaches the end of its stroke, that is, the closed position. Accordingly, in the electromagnetic actuator 10, if an intensity of the current supplied to the first electromagnetic coil 24 is constant, an increase in the electromagnetic force generated in the plunger 22 is much greater than an increase in the pressing force exerted on the plunger 22 as the valve body 12 approaches the closed position. In this condition, the valve body 12 will collide with the valve seat 16 at a high speed. This may generate noise due to the collision and reduces service life of the valve body 12 as previously mentioned. Although Fig.3 shows the characteristic of the actuator 10 when the valve body 12 moves between the half-open position and the closed position, a similar characteristics may be observed when the valve body 12 moves between the half-open position and the open position. On the other hand, if the current supplied to the first electromagnetic coil 24 and the second magnetic coil 26 is sharply decreased when the plunger 22 moves to a position close to the closed position or the open position, the valve body can be gently moved to the closed position or the open position. However, since the first electromagnetic coil 24 and the second electromagnetic coil 26 have an inductance, if the current flowing to the coils is simply interrupted by a switching operation, an undesired electromotive force is generated in the coils. The electromotive force will generates a high-voltage which may require the driving circuit to have a high dielectric withstanding property. Additionally, as shown in FIG.3, the electromagnetic force exerted on the plunger 22 becomes smaller as the current supplied to the first electromagnetic coil 24 is decreased. However, when the valve body 12 is extremely close to the closed position, a large electromagnetic force is exerted on the plunger even if the current supplied to the first electromagnetic coil is small. Accordingly, in order to effectively decrease the electromagnetic force, the current supplied to the electromagnetic coil 24 must be rapidly and greatly decreased before the valve body 12 reaches the end of its stroke. The driving circuit shown in FIG.1 is provided so as to rapidly and greatly decrease the current supplied to the first electromagnetic coil 24 and the second electromagnetic coil 26. The current generated due to the electromotive force in the coils is consumed by a resistor. As a result, the valve body 12 is gently seated on the valve seat 16. Thus, generation of noise is prevented and a long service life of the valve body 12 can be obtained. The driving circuit shown in FIG.1 is provided for the first electromagnetic coil 24. A similar driving circuit may be provided for the second electromagnetic coil 26. A description will be given of only the driving circuit for the first electromagnetic circuit 24, for the sake of convenience. In FIG.1, an inflow terminal 24a of the first electromagnetic coil 24 is connected to a terminal 40a of a power source 40. An outflow terminal 24b of the first electromagnetic coil 24 is connected to a collector terminal 42c of a main switching element 42. The main switching element 42 comprises an NPN-type transistor. An emitter terminal 42e of the main switching element 42 is connected to a ground terminal of the power source 40. Accordingly, when the main switching element 42 is turned on, a power source voltage Vb is applied between the inflow terminal 24a and the outflow terminal 24b of the first electromagnetic coil 24. When the main switching element 42 is turned off, the power source voltage Vb is not supplied to the first electromagnetic coil 24. Additionally, the electromagnetic coil 24 is connected to a flywheel circuit which comprises a secondary switching element 44, a resistor 46 and a flywheel diode 47. The secondary switching element 44 comprises an NPN-type transistor. The resistor 46 is connected in parallel to the secondary switching element 44. The outflow terminal 24b is connected to an anode terminal 47a of the flywheel diode 47. A cathode terminal 47b of the flywheel diode 47 is connected to a collector terminal 44c of the secondary switching element 44 and an end of the resistor 46. The inflow terminal 24a of the first electromagnetic coil 24 is connected to an emitter terminal 44e of the secondary switching element 44 and the other end of the resistor 46. Accordingly, if an electromotive force is generated in the first electromagnetic coil 24 when the current flowing in the first electromagnetic coil 24 is decreased, the voltage at the outflow terminal 24b becomes higher than the voltage at the inflow terminal 24a. In this case, a current flows in through the flywheel circuit in a direction from the outflow terminal 24b to the inflow terminal 24a. A base terminal of the main switching element 42 is connected to an output terminal of a main switching element driving circuit 48. Additionally, an input terminal of the main switching element driving circuit 48 is connected to an engine control unit (ECU) 50. The engine control unit 50 calculates a value of the current to be supplied to the first electromagnetic coil 24. The engine control unit 50 supplies a current designating signal Ic, which corresponds to the calculated value of the current to be supplied to the first electromagnetic coil 24, to the main switching element driving circuit 48. The main switching element driving circuit 48 comprises a triangular waveform generating circuit and comparing circuit. The triangular waveform generating circuit generates a triangular wave having a predetermined frequency. The comparing circuit compares the current designating signal Ic with the triangular waveform. The main switching element driving circuit 48 outputs a PWM pulse signal having a duty ratio corresponding to a value of the current designating signal Ic. That is, the PWM pulse signal having a duty ratio corresponding to the value of the current designating signal output from the engine control unit 50 is supplied to the base terminal 42b of the main switching element 42. Accordingly, the turn on period of the main switching element 42 is increased as the value of the current designating signal Ic is increased. When the main switching element 42 is turned on, the power source voltage Vb is supplied between the inflow terminal 24a and the outflow terminal 24b of the first electromagnetic coil 24. In this case, a current determined by an impedance of the first electromagnetic coil 24 and the power source voltage Vb flows through the first electromagnetic coil 24. On the other hand, when the main switching element 42 is turned off, an electromotive force is generated in the first electromagnetic coil 24 so that a current flows in a direction opposite to the current which has been flowing in the first electromagnetic coil 24. Thus, a current flows through the first electromagnetic coil 24, a value of the current being determined by a magnitude of the electromotive force and the resistance of the flywheel circuit. In this case, an average value of the current flowing through the first electromagnetic coil 24 is influenced by the resistance of the flywheel circuit in addition to the duty ratio of the PWM pulse signal. That is, as the duty ratio of the PWM pulse signal becomes greater or the resistance of the fly wheel circuit becomes smaller, a larger current flows through the first electromagnetic coil 24. Accordingly, it is preferable to set a smaller resistance of the flywheel circuit to drive the flywheel circuit 10 so as to exert an electromagnetic force on the plunger 22. On the other hand, if the resistance of the flywheel circuit is increased, the current flowing in the first electromagnetic coil 24 immediately after the main switching element 42 is turned off is rapidly decreased. Accordingly, it is preferable to set a larger resistance of the flywheel circuit to drive the flywheel circuit 10 so as to rapidly decrease the electromagnetic force generated by the first electromagnetic coil 24. In the driving circuit shown in FIG.1, an output terminal of a secondary switching element driving circuit 52 is connected to the base terminal 44b of the secondary switching element 44. Additionally, an input terminal of the secondary switching element driving circuit 52 is connected to the engine control unit 50 so that a secondary switching element control signal Vc is input to the input terminal of the secondary switching element driving circuit 52. The secondary switching element control signal Vc falls at a predetermined crank angle of the associated engine at which crank angle the valve body 12 is close to the closed position. The secondary switching element driving circuit 52 binarize the signal Vc and outputs to the secondary switching element 44. Accordingly, a high-level output is supplied to the base terminal 44b of the secondary switching element 44 until the valve body 12 moves to the position close to the closed position. Thereafter, a low-level output is supplied to the base terminal 44b of the secondary switching element 44. Accordingly, in the driving circuit of the present embodiment, the resistance of the flywheel circuit is maintained to be a small value until the valve body 12 moves close to the closed position, since the secondary switching element 44 is turned on. On the other hand, after the valve body has moved close to the closed position, the resistance of the flywheel circuit is increased since the secondary switching element is turned off. This satisfies the above-mentioned preferable condition of the resistance of the flywheel circuit. A description will now be given, with reference to FIG.4, of an operation of the valve driving apparatus according to the present embodiment. It should be noted that characteristic curves indicated by dotted lines in FIG.4-(A), 4-(B) an 4-(C) are obtained when the secondary switching element 44 is turned on, that is, when the resistance of the flywheel circuit is maintained to be a small value. FIG.4-(A) indicates a lift (hereinafter referred to as a valve lift) of the valve body 12. FIG.4-(B) indicates the current designating signal for the first electromagnetic coil 24 and an actual current flowing in the first electromagnetic coil 24. FIG.4-(C) indicates the current designating signal for the second electromagnetic coil 26 and an actual current flowing in the first electromagnetic coil 26. FIG.4-(D) indicates the control signal for the secondary switching element 44 connected to the first electromagnetic coil 24. FIG.4-(E) indicates the control signal for the secondary switching element 44 connected to the second electromagnetic coil 26. As shown in FIG.4-(A), the valve body is maintained at the closed position before time t0. Since a small electromagnetic force is enough to maintain the valve 12 at the closed position, a relatively low-level current designating signal is supplied to the first electromagnetic coil 24 as shown in FIG.4-(B). In this state, an actual current is substantially equal to the current designating signal. As mentioned above, the resistance of the flywheel circuit is preferably set to be as small as possible. Thus, the high-level signal is supplied to the secondary switching element 44 of the first electromagnetic coil 24 as shown in FIG.4-(D). Accordingly, in the present embodiment, the valve body 12 is maintained at the closed position with a small power consumption. It should be noted that the high-level control signal is supplied also to the secondary switching element 44 connected to the second electromagnetic coil 26 at time t0 as shown in FIG.4-(E) so as to prepare for the activation of the second electromagnetic coil 26. At time t0, the crank angle of the associated engine reaches the crank angle at which the opening operation of the valve member 12 should be started. Thus, as shown in FIG.4-(B), the current designating signal is changed to zero so as to release the valve body 12 from the closed position. At the same time, as shown in FIG.4-(D), the control signal supplied to the secondary switching element 44 connected to the first electromagnetic coil 24 is switched from the high-level to the low-level. Accordingly, as shown in FIG.4(B), the actual current flowing in the first electromagnetic coil 24 is decreased faster than the actual current when the secondary switching element 44 is always turned on (as indicated by the dotted curved line in the figure). As a result, the electromagnetic force which moves the plunger 22 toward the closed position is rapidly decreased after time t0. Thus, as shown in FIG.4-(A), the valve body 12 starts to move toward the open position with a rapid response after time t0. As mentioned above, when the valve body 12 initiates the movement, the electromagnetic force in the opposite direction is rapidly decreased. Thus, the valve body 12 can be moved by a smaller force as compared to the state in which the electromagnetic force in the opposite direction is not decreased. This results in less power consumption when the movement of the valve body is initiated. As shown in FIG.4-(C), after time t0, the current designating signal to the second electromagnetic coil 26 is raised at a predetermined rate at time t1. After that, until time t3, the current designating signal having a predetermined profile is supplied to the second electromagnetic coil 26. A current is not supplied to the second electromagnetic coil 26 during the period from t0 to t1 since an electromagnetic force sufficient to attract the plunger 22 toward the open position is not effectively generated during that period. The profile of the current designating signal during the period from t1 to t2 is determined by considering a fluctuation of the actual current with respect to the current designating signal and a characteristic of the movement of the valve body 12 corresponding to the actual current. Accordingly, when the current designating signal for the second electromagnetic signal 26 is controlled as mentioned above, the valve body 12 is smoothly moved from the closed position to a position close to the open position as shown in FIG.4-(A). When the valve body 12 has moved close to the open position at time t2, the current designating signal is rapidly decreased as shown in FIG.(C). At the same time, the control signal supplied to the secondary switching element 44 connected to the second electromagnetic coil 26 is switched from the high-level to the low-level. Thus, as shown in FIG.4-(C), the actual current flowing in the second electromagnetic coil 26 is decreased faster than the actual current when the secondary switching element 44 is always turned on. As a result, the electromagnetic force which moves the plunger 22 toward the open position is rapidly decreased after time t2. Thus, as shown in FIG.4-(A), the valve body 12 gently moves to the open position. After that, as shown in FIG.4-(C), the current designating signal to the second electromagnetic coil 26 is gradually increased to the level sufficient for maintaining the valve body at the open position. At the same time, as shown in FIG.4-(E), the current designating signal to the secondary switching element 44 connected to the second electromagnetic coil 26 is switched from the low-level to the high-level. Thus, the valve body 12, which has been gently moved to the open position is maintained at the open position as shown in FIG.4-(A). It should be noted that the control signal supplied to the secondary switching element 44 connected to the first electromagnetic coil 24 is also switched from the low-level to the high-level at time t3 as shown in FIG.4-(D) so as to prepare for the activation of the first electromagnetic coil 24. Thereafter, in the same manner, the current designating signals and the control signals are appropriately controlled. Thus, the valve body 12 can be positively and smoothly moved from the open position to the closed position. According to the present embodiment, the valve body 12 does not strongly collide with the valve seat 16. Additionally, the plunger 22 does not strongly collide with the first core 28 and the second core 30. Thus, an improvement in low-noise operation of the engine is achieved. Additionally, the service life of the valve body 12 and the plunger 22 is also improved. If an attempt is made to simply prevent a strong collision of the valve body 12, a damping mechanism may be provided to absorb the energy of movement of the valve body 12 at positions close to the closed position or the open position. However, in such a construction, a part of the energy applied to the valve body 12 is consumed by the damper mechanism. This is not in accordance with energy conservation, and construction of the valve driving apparatus may become complex. In the valve driving apparatus according to the present embodiment, most of the energy applied to the valve body 12 by the first and second electromagnetic coils 24 and 26 is used for moving and maintaining the valve body 12. Thus, the valve driving apparatus according to the present embodiment achieves an ideal energy conservation while achieving the above-mentioned improvements. FIG.5 is a circuit diagram of a driving circuit according to a second embodiment of the present invention. In FIG.5, parts that are the same as the parts shown in FIG.1 are given the same reference numerals, and descriptions thereof will be omitted. The driving circuit shown in FIG.5 comprises a current detecting circuit 54 which detects an actual current Im flowing through the first electromagnetic coil 24. A subtracting circuit calculates a difference (Ic-Im) between the actual current Im and the current designating signal Ic. The difference (Ic-Im) is supplied to a main switching element driving circuit 58. The main switching element driving circuit 58 outputs to the main switching element 42 the PWM pulse signal having a duty ratio controlled by the difference (Ic-Im). According to the second embodiment, the duty ratio of the PWM pulse signal output from the main switching element driving circuit 58 is controlled so that the actual current Im becomes equal to the current designating signal Ic. In this case, the actual current Im flowing through the first electromagnetic coil 24 is adjusted by a feedback control. Thus, a stable current can be obtained in a wide operational range against fluctuation in the power source voltage Vb and the characteristics of the circuit. FIG.6 is a circuit diagram of a driving circuit according to a third embodiment of the present invention. In FIG.6, parts that are the same as the parts shown in FIGS.1 and 5 are given the same reference numerals, and descriptions thereof will be omitted. In the driving circuit shown in FIG.6, an engine control unit 60 supplies the current designating signal Ic to the subtracting circuit 56. The subtracting circuit 56 calculated the difference (Ic-Im) between the current designating signal and the actual current Im detected by the current detecting circuit 54. The calculated value (Ic-Im) is supplied to the main switching element driving circuit 58 and a determining circuit 62. The determining circuit 62 comprises, as shown in FIG.7, a triangular wave oscillating circuit 62a and a comparing circuit 62b. The triangular wave oscillating circuit 62a generates triangular wave signal having a minimum value of -V2 and a maximum value of -V1. The comparing circuit 62b compares the value (Ic-Im) (hereinafter referred to as a value A) supplied by the subtracting circuit 56 with a value B of the triangular wave signal. If A-B≥0, the determining circuit 62 supplies the high-level control signal Vc to the secondary switching element driving circuit 52. If A-B<0, the determining circuit supplies the low-level control signal Vc to the secondary switching element driving circuit 52. Accordingly, as the value A (Ic-Im) is increased, that is, as the actual current Im is smaller than the current designating signal Ic, the PWM pulse signal having a greater duty ratio is supplied to the secondary switching element 44 from the secondary switching element driving circuit 52. On the contrary, as the value A (Ic-Im) is decreased, that is, as the actual current Im is greater than the current designating signal Ic, the PWM pulse signal having a smaller duty ratio is supplied to the secondary switching element 44 from the secondary switching element driving circuit 52. In particular, if Ic-Im≥-V1 , the PWM pulse signal having a duty ratio of 100% is supplied to the secondary switching element 44. On the contrary, if IC-Im<-V2 the PWM pulse signal having a duty ratio of 0% is supplied to the secondary switching element 44. That is, the secondary switching element 44 is subjected to a duty ratio control so that a turn-on period of the secondary switching element 44 per unit period of time becomes longer as the actual current Im must be increased to equalize the actual current Im to the current designating signal Ic. The turn-on period of the secondary switching element 44 per unit period of time becomes shorter as the actual current Im must be decreased. In this case, the resistance of the flywheel circuit becomes smaller as the actual current Im must be increased. The resistance becomes greater as the actual current Im must be decreased. As shown in FIG.4-(B) and 4-(C), the current designating signal Ic rises at time t1 in FIG.4-(C), at which movement of the valve body 12 from the open position or the closed position must be initiated. The current designating signal rapidly falls when the valve body 12 moves close to the other end (time t2 in FIG.4-(c)). In this condition, the actual current Im flowing in the first electromagnetic coil 24 and the second electromagnetic coil 26 is delayed from the current designating signal due to the impedance of the first electromagnetic coil 24 and the second electromagnetic coil 26. Accordingly, the current designating signal Ic is greater than the actual current Im when the current designating signal Ic is increasing, and the current designating signal Ic is less than the actual current Im when the current designating signal Ic is decreasing. Accordingly, in the above-mentioned condition, the value (Ic-Im) is always equal to or greater than -V1 ( Ic-Im≥-V1) when the valve body 12 is moving but not yet reached a position close to either one of the ends of its stroke. The value (Ic-Im) is less than -V2 immediately after the valve body 12 reached the position close to either one of the ends of its stoke. Thereafter, a condition -V2≤Ic-Im<V1 is temporary established during a process in which the valve body 12 is set to a stable state. In the present embodiment, when the condition Ic-Im≥-V1 is established, the secondary switching element 44 is maintained to be turned on since the secondary switching element 44 is driven by the PWM pulse signal having the duty ratio of 100%. When the condition Ic-Im<-V2 is established, the secondary switching element 44 is maintained to be turned off since the secondary switching element 44 is driven by the PWM pulse signal having the duty ratio of 0%. Additionally, when the condition -V2≤Ic-Im<-V1 is established, the secondary switching element 44 is driven by the PWM pulse signal having an appropriate duty ratio corresponding to the value (Ic-Im). Accordingly, the resistance of the flywheel circuit provided in the present embodiment is maintained to be at the minimum value after the valve body 12 is started to move and until the valve body 12 is moved close to the other end of its stroke. The resistance becomes the maximum value immediately after the valve body 12 has reached a position close to the other end of the stroke. Thereafter, the resistance is gradually decreased to the minimum value as the valve body 12 further approaches the end of the stroke. In the above-mentioned condition, the resistance of the flywheel circuit is maintained to be the minimum value when the valve body 12 is moving toward an end of the stroke and the when the valve body 12 is maintained at either one of the ends of the stroke. Thus, a desired current can be efficiently supplied to the first electromagnetic coil 24 or the second electromagnetic coil 26. Additionally, the resistance of the flywheel circuit is increased to the maximum value when the valve body 12 has reached the position close to one of the ends of the stroke. Thus, the current flowing through the first electromagnetic coil 24 or the second electromagnetic coil 26 is rapidly decreased. This prevents a strong collision of the valve body 12 with the valve seat 16 or the strong collision of the plunger 22 with the first core 28 or the second core 30. In the present embodiment, the control signal Vc is not needed to be supplied from the engine control unit 60 to the secondary switching element driving circuit 52. Additionally, in the present embodiment, since the resistance of the flywheel circuit is changed, a fine adjustment of the current flowing to the first electromagnetic coil 24 or the second electromagnetic coil 26 can be achieved. This results in a fine control of a characteristic of movement of the valve body 12. In the driving circuit shown in FIG.6, the driving pattern of the secondary switching element 44 is determined so as to correspond to the characteristic of the electromagnetic actuator 10. However, the driving pattern is not limited to the above-mentioned pattern. That is, in the driving circuit according to the present embodiment, the current supplied to the first electromagnetic coil 24 or the second electromagnetic coil 26 can be changed without changing a value of a resistor which may be provided in the flywheel circuit. This means that the driving circuit of the present embodiment can be used with various actuators requiring different current characteristics without a change in hardware construction. It should be noted that, in the driving circuits shown in FIGS.1, 5 and 6, the main switching element 42 and the main switching element driving circuit 48 or 58 together constitute current controlling means. The resistor 46, the secondary switching element 44 and the secondary switching element driving circuit 52 constitute a variable resistor circuit. In the above-mentioned embodiments, although a transistor is used as the main switching element 42, another switching element such as a high speed relay switch may be used. Additionally, although the current supplied to the first electromagnetic coil 24 or the second electromagnetic coil 26 is adjusted by controlling the main switching element 42 by a PWM control method, the current may be adjusted by using another control method such as a method using a linear region of the main switching element 42. The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention as defined in the appended claims. An electromagnetic force generated in a valve driving apparatus is rapidly decreased when a valve body 12 moves to a position close to the end of its stroke so that a shock generated when the valve body 12 reaches the end of the stroke is reduced. The valve body 12 is movable between opposite ends of its stroke so as to open and close a valve provided in an internal combustion engine. An electromagnetic coil 24, 26 generates an electromagnetic force exerted on the valve body 12 . A current supplied to the electromagnetic coil 24, 26 is controlled in accordance with an operational condition of the internal combustion engine. The current flowing in the electromagnetic coil 24, 26 is rapidly decreased when the valve body 12 approaches the end of its stroke.

Claims: A valve driving apparatus for driving a valve provided in an internal combustion engine, said valve having a valve body (12) movable between a first position and a second position so as to open and close said valve, said valve driving apparatus comprising an electromagnetic coil (24, 26) generating an electromagnetic force exerted on said valve body (12), and current controlling means for controlling a current supplied to said electromagnetic coil (24, 26) in accordance with an operational condition of said internal combustion engine; characterized in that current decreasing means for decreasing said current when said valve body (12) approaches one of said first position and said second position, and said current decreasing means comprises: a flywheel circuit returning a current flowing in said electromagnetic coil (24, 26); and a variable resistor circuit (44, 52) increasing resistance of said flywheel circuit when said valve body (12) approaches at least one of said first position and said second position. The valve driving apparatus as claimed in claim 1, characterized in that said current controlling means comprises: a main switching element (42) varying the current provided to said electromagnetic coil (24, 26); and a main switching element driving circuit (48) driving said main switching element (42) by supplying a first control signal to said main switching element (42), said first control signal being generated in accordance with a current designating signal supplied by an engine control unit (50) provided in said internal combustion engine, said main switching element (42) varying the current supplied to said electromagnetic coil (24, 26) in accordance with said first control signal supplied by said main switching element driving circuit (48). The valve driving apparatus as claimed in claim 2, characterized in that said first control signal comprises a pulsed voltage signal and said main switching element (48) varies the current supplied to said electromagnetic coil (24, 26) by pulse width modulation. The valve driving apparatus as claimed in claim 2, characterized in that current detecting means (54) is further provided for detecting the current provided to said electromagnetic coil (24, 26) so as to generate a current detection signal, said variable resistor circuit (44, 52) varying the resistance of said flywheel circuit in accordance with a difference between a value of said current designating signal supplied by said engine control unit (50) and a value of said current detection signal supplied by said current detecting means. The valve driving apparatus as claimed in claim 4, characterized in that said variable resistor circuit comprises a secondary switching element (44) and a secondary switching element driving circuit (52), said secondary switching element (44) substantially varying the resistance of said variable resistor circuits, said secondary switching element driving circuit (52) driving said secondary switching element (44). The valve driving apparatus as claimed in claim 5, characterized in that a determining circuit (62) is further provided for determining one of the difference being greater than a first predetermined value and the difference being smaller than a second predetermined value which is less than one of said first predetermined value and the difference between said first predetermined value and said second predetermined value, said determining circuit (62) outputting a second control signal to said secondary switching element driving circuit (52), a value of said second control signal being varied in accordance with a determination of said determining circuit (62). The valve driving apparatus as claimed in claim 6, characterized in that said determining circuit (62) comprises a triangular waveform oscillating circuit (62a) generating a triangular waveform signal having a maximum value corresponding to said first predetermined value and a minimum value corresponding to said second predetermined value, the determination of said determining circuit (62) being performed by comparing said difference with a value of said triangular waveform signal.

Inventors: IIDA TATUO, IZUO TAKASHI, MAEDA IWAO, YANAI AKIHIRO, IIDA, TATUO, IZUO, TAKASHI, MAEDA, IWAO, YANAI, AKIHIRO, IIDA, TATUO, C/O TOYOTA JIDOSHA K.K., IZUO, TAKASHI, C/O TOYOTA JIDOSHA K.K., MAEDA, IWAO, C/O TOYOTA JIDOSHA K.K., YANAI, AKIHIRO, C/O TOYOTA JIDOSHA K.K.
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1996
Main classification: F01L9
IPC codes: H01F7, F02D13, F02D41, F01L9