This invention relates generally to electronic valve actuator (EVA) control systems and methods more particularly to EVA control systems and methods for reducing valve impact.
As is known in the art, one common approach to electronically control the valve actuation of an internal combustion engine is to have two electromagnets toggle an armature coupled to the valve between an open position and a closed position. More particularly, referring to FIG. 2A, when a first, here upper, one of the electromagnets is activated, the armature is attracted to the activated electromagnet thereby driving the valve to its closed position. Also, as the armature is attracted to the activated electromagnet, a first spring, in contact with the upper end of the armature is compressed. When the first electromagnet is deactivated, the first compressed spring releases it stored energy and drives the armature downward thereby driving the valve towards it open position. As the armature approaches the second, lower electromagnet, the second electromagnet is activated driving the valve to its full open position. It is noted that a second, lower spring becomes compressed during the process, i.e., during capture of the armature by the activation of the second electromagnet). After being fully open for the desired period of time, the second electromagnet is deactivated, and the lower spring releases its stored energy and thereby drives the armature towards its upper position, the first electromagnet is activated and the process repeats. Thus, the two electromagnets toggle the armature couples to the valve between an open or closed position where it is held, while the pair of springs is used to force the valve to move (oscillate) to the other state (FIG. 2B).
One problem with the approach described above is that, it suffers from large impacts at several different locations due to the motion of the armature and valve. These impacts may be excessively loud and may lead to actuator failure. One technique suggested to control the position trajectory of the armature during xe2x80x9ccapturexe2x80x9d is described in a paper entitled xe2x80x9cValve Position Tracking For Soft Landing of Electromechanical Camless Valvetrainxe2x80x9d, by Wolfgang Hoffman and Anna G. Stefanopoulu, published in the 3rd IFAC Workshop Advanced Automotive Control Preprints Volume I, Karisruhe, Germany Mar. 28-30, 2001. The technique described therein includes the use of a feedback controller having an observer used to stabilize the system at an equilibrium point close to the armature capture point. The observer provides estimates of the magnetic flux produced by the coil and the velocity of the armature. Of particular interest to this invention are the impacts which occur between the armature and valve stem during the release of the armature.
More particularly, the armature is in contact with the valve and its motion forces the valve to open or close. The contact velocity between the armature and valve stem during release of the armature needs to be reduced to an acceptable level (below 0.4 m/s for the engine RPM range of 700 to 6000) to avoid excessive noise and wear. The contact velocity has to be maintained within this range robustly, i.e., despite varying ambient conditions and changes in engine speed, load, temperature and power supply voltage that can occur in the course of normal engine operation. In addition, excessive power consumption is to be avoided to maximize fuel economy and avoid over-heating of the actuator coils.
In accordance with the present invention a system is provided for controlling a valve of an internal combustion engine. The system includes an electromagnet actuator having a coil and an armature magnetically coupled to the coil. The armature is coupled to the valve to stroke the valve between an open and closed position in response to a drive signal fed to the coil. The system produces an error signal as a function of a difference between a predetermined desired position time history (i.e., position trajectory), yd, for the armature for each stroke of the armature and the actual position trajectory of the armature, y, during such stroke. The error signal is used to produce a feedforward command signal to a feedfoward controller for use in providing the drive signal to the coil during a subsequent stroke. The response of the feedforward controller to the error signal in providing the drive signal is an inverse function the relationship between a change in armature position in response to a change in the drive signal.
In one embodiment, the feedfoward controller may be represented as:
xe2x80x83Ucontroller=yr[n,k+1]+[{(did/dtxe2x88x92iddy/dt)/(kb+yd)}/(2ka)/(kb+yd)]+rid
where:
Ucontroller is the control signal fed to the coil of the electromagnetic;
yr[k+1] is the feedforward command signal for the subsequent stroke;
yr[k] is the produced feedforward command signal;
yd is the desired position trajectory; and
r is the electrical resistance of the of the electromagnetic coil
ka and kb are constants determined by the magnetic properties of the electromagnetic coil
id is the theoretical current that would cause the armature to track yd and is given by:       i    d    =                                                        k              s                        ⁡                          (                              l                -                                  y                  d                                            )                                +                      k            pre                                    k          a                      ⁢          (                        k          b                +                  y          d                    )      
where:
ks is the stiffness of a spring used to initiate the motion of the armature in response to removal of the drive signal;
kpre is the preload of the spring in the actuator spring used to initiate the motion of the armature in response to removal of the drive signal;
l is one-half the total travel of the armature.
That is, representing y as being equal to a function, fnc of (ucontroller), i.e., fnc(ucontroller), the feedfoward controller 400 may be represented as a function which is fncxe2x88x921 (i.e., the inverse of the function fnc). Or, to put it another way, the feedforward control is fncxe2x88x921, where: fncxe2x88x921 is the inverse of the function relating the position of the armature to drive signal to the armature.
In one embodiment, the feedforward controller is used to modify the drive signal only during a second phase of the valve stroke; in the first, or initial, release phase, the release valve is controlled by the predetermined drive signal. During release of the armature this predetermined drive signal is used to ensure that the current in the coil being presently used to hold the armature is sufficiently reduced as to initiate motion of the armature.
In accordance with another feature of the invention, a method is provided for controlling a valve of an internal combustion engine system having an electromagnet with an armature magnetically coupled to a coil of such electromagnet. The armature is coupled to the valve. The armature strokes the valve between an open and closed position in response to a drive signal fed to the coil. The method includes providing an open-loop, pre-set control signal to the coil, such signal being representative of a desired position trajectory for the armature for each stroke of the armature. The pre-set control signal is maintained at a pre-set level during an initial phase. Subsequent to the initial phase, the feedforward control system is used to generate the drive signal to the coil to drive the armature such that the armature and valve collide at low contact velocities. A cycle-to-cycle (i.e., stroke-to-stroke) adjustment is made of the feedforward command signal to adjust for better tracking of the desired position trajectory; this adjustment mechanism is referred to as an iterative learning control (ILC) mechanism.
The desired position time history (i.e., trajectory) is designed to provide a desired low impact velocity between the armature and valve stem. In general, the desired position trajectory cannot be followed exactly due to the system dynamics or unknown disturbances. To account for all of these difficulties, the ILC modifies the commanded feedforward command signal so that the actual armature position trajectory follows the desired position trajectory during release of the armature.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
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