Abstract:
Sensing magnetic flux in an electromechanical actuator having dual axially opposed stators and dual springs, a control system modifies the rise of flux in such a manner to produce a substantially zero velocity landing of an armature against the stators. The flux is sensing by means of a sensor such as a hall effect sensor that is mounted each stator. The control system controls the flux rise when the armature is within the landing zone of the armature against the stator to match the spring rate of axially opposed springs in the actuator.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/067,872, filed Dec. 8, 1997. 
    
    
     FIELD OF INVENTION 
     This invention relates to high speed, high force electromechanical actuators as may be found in actuators such as are used in electronic control of the opening and closing of engine valves in an internal combustion engine. More particularly a system for controlling the landing speed of the armature against the stator. 
     BACKGROUND OF INVENTION 
     In U.S. Pat. No. 4,515,343, there is taught a contact damper at one end of the travel of the armature to provide dampening as the armature approaches the pole piece. In other systems, the power to the actuator is applied to move the armature across the gap and when the armature is close to the stator, a magnetic force from the stator coil is removed to slow down the armature and hope for a “soft”, near zero velocity, landing. Just before the landing, the stator coil is then re-energized to pull the armature into a landing. The actuator has at least two opposing springs which sequentially release their potential energy to move the armature from one stator pole to the other. The stator coils, i.e. the receiving coil when energized, adds enough force to the stored up and released spring force to move and seat the armature. 
     The purpose of the actuator is to open and close an engine valve of an internal combustion engine. 
     The problem is to devise a control algorithm that provides enough extra energy from the stator coils to always complete the armature travel during a stroke but at the same time produce a “soft” (near zero velocity) landing of the armature against a stator to prevent excessive impact wear on the armature and stator and to reduce the amount of noise produced by such impact. 
     SUMMARY OF INVENTION 
     An electronic control system for controlling the movement of an armature in an electromechanical actuator, has dual coils, one at each end of the travel of an armature. The armature is mounted intermediate the ends of a shaft having an engine valve coupled through a hydraulic valve adjuster at one end and a shaft extension means axially extending from the armature at the other end. Dual spring means are coupled to the armature shaft to store up potential energy, which when released provides kinetic energy along with the magnetic energy of one of the coils to pull the armature across the gap between the pair of axially aligned coils. Each of the stators are coupled to one or more flux sensors. The flux sensors sense the rise of magnetic flux in the receiving coil and supplies this information to an electronic circuit. Timing means controls the application of power to both coils to turn off one coil to launch the armature and to briefly turn on the second or receiving coil to pull the armature and then after a time period to return on the receiving coil to catch the armature. The turning on of the receiving coil to generate “catch current” is controlled from system timing and the flux sensor for sensing the build-up of magnetic flux, hence magnetic force in the armature. Once the armature seats on the receiving stator, the catch current is changed to a hold current holding the armature until the next operation of the valve. By controlling the build up of the flux, the armature has a soft landing on the stator face. 
     In the preferred embodiment, a hall sensor has been positioned in or on the stators to measure the flux and flux change. The important characteristic of the sensor is that it accurately measures the flux being generated by an electrical field or the flux being generated in response to the movement of the armature. Such sensors can be mounted in or on the stators, in or on the armature, coupled to the armature or valve stem or any other location that is magnetically responsive to the movement of the armature and or its shaft. 
    
    
     DESCRIPTION OF DRAWINGS 
     In the drawings: 
     FIG. 1 is a voltage waveform as applied to the actuator in an open loop control mode; 
     FIG. 2 is a waveform of flux generation in an actuator of FIG. 6 in the system operation as described for FIG. 1; 
     FIG. 3 is a block diagram of an operating system according to the present invention to achieve zero velocity landing of the armature; 
     FIG. 4 is a graphic representation of a prior art actuation of an actuator in a nominal open loop control: 
     FIG. 5 is a graphic representation of the actuation of an actuator according to the present invention; 
     FIG. 6 is a sectional view of an actuator in the open position just prior to the application of the voltage of FIG. 1; and 
     FIG. 7 is a sectional view of an actuator in the closed position after the armature has traveled across the gap. 
    
    
     DETAILED DESCRIPTION 
     Referring to the Figs. by the characters of reference, there is illustrated in FIG. 1 a system voltage timing wave form  10 . For the purposes of explaining the operation of the system, the bottom stator  12  coil  13  of FIGS. 6 and 7 will be identified as the valve open or bottom coil and the axially opposed coil, or the upper stator  14  coil  15  will be identified as the valve closed or receiving coil or upper coil. When the bottom coil  13  is energized, the armature  16  is seated against the stator bottom  12  holding the valve  18  open. Conversely, when the upper coil  15  is energized, the armature  16  is seated against the upper stator  14  holding the valve  18  closed. As will be seen, there are also times when one of the coils  13 ,  15  is energized and the armature  16  is moving across the gap between stators  12 ,  14 . 
     An example of such an armature and actuator is found in copending patent applications “Armature for Electromagnetic valve Actuator” filed Dec. 9, 1997 having Ser. No. 60/067,984 and “Electromagnetic Valve Actuator” filed Dec. 9, 1997 having Ser. No. 60/069,144 both of which are incorporated herein by reference. 
     In normal operation to go to valve  18  closed as shown in FIG. 7 from valve open as shown in FIG. 6, full voltage is applied to the upper coil  15  at the beginning of an armature stroke at time T, to get the armature  16  moving. Simultaneously power is removed from the bottom coil  13  to release the armature  16  from the bottom stator  12 . Once the armature  16  is moving, the voltage at the upper coil  15  is removed at time T 1  to permit the armature  16  to travel as a spring-mass system under simple harmonic motion until it is near closing. The oppositely opposed bias springs  20 ,  22  function to store potential energy when compressed and deliver kinetic energy when the armature  16  is released. Then at time T 2 , full coil voltage is applied to the upper or receiving coil  15  to initiate the catch current phase. Finally at time T 3 , the coil voltage on the receiving coil  15  is reduced to a value sufficient to provide the hold current for holding the armature  16  against the upper stator  14  and against an opposing compressed spring  20 . This is a bang-bang type of optimal control. 
     The of the spring means  22 , as illustrated in FIGS. 6 and 7, functions as the normal valve spring that, absent the electromagnetic actuators, would normally hold the valve  18  closed. The second spring means  20  is another spring which is positioned at the end of the shaft means  24  axially extending from the armature  16  which is positioned to open the valve  18 . The springs  20 ,  22  are balanced and in their normal position, neither of the stator coils  13 ,  15  being energized, the armature  16  would be balanced between the stators and the valve  18  is partially opened. 
     FIG. 2 is a simplified flux wave form  24  for the system of FIG. 1 without the present invention. When the initial voltage pulse  26  is applied to the coils  13  or  15 , the flux begins to build up until T 1 . At time T 1 , the voltage is removed and as the armature  16  is moving across the gap, there is only a slight amount of flux increase. At time T 2  the voltage is reapplied to the coil, the flux increases rapidly and at T 3  the voltage is then reduced to provide holding current. In theory, values can be calculated for time T 1 , T 2  and T 3  to achieve the desirable soft landing of the armature  16  against the stator  14 . In practice, however, this is almost never achievable because the system is constantly being perturbed by real world variable parameters such as damping, temperature, deflections, tolerance stack up, vibration, engine gas loads, etc., to name a few. 
     Laboratory tests with very careful adjustment of the catch current, i.e. the current resulting from the voltage applied at T 2 , in conjunction with viscous oil damping has yielded single stroke soft landings. However, a more typical performance is where the landing is quite harsh at a velocity of approximately 1.0 meters per second. A velocity value of 0.7 meters per second appears to be the practical limit achievable in open loop actuator control mode to insure completion of every armature stroke. The required velocity value of the armature  16  calculated to provide “quiet” actuator operation is less than 0.04 meters per second at 600 engine rpm and less than 0.4 meters per second at 6000 engine rpm. 
     From this it is clearly understood that some form of feedback algorithm is required to increase the robustness of the armature control. Conventional approaches to the feedback problem have proved effective in simulation, but as hereinbefore noted, have failed in the real world. The common point of failure for these techniques has been the inability to process the required feedback equations in the allotted time and to sense the required stated variables with sufficient accuracy and resolution. For example, analysis testing has shown that a position sensor must resolve the eight millimeter armature stroke distance to an error of less than ten microns (0.125%) in the presence of high engine vibration and electrical noise. At the present this is not feasible in a serial production design. Consider that a typical engine has four valves per cylinder, such an electronic valve timing system would then require four actuators, similar to that illustrated in FIG. 6, for each cylinder. Multiply this by the number of cylinders times the number of engines and the magnitude of the problem is seen. 
     FIG. 3 is a block diagram of an operating system according to the present invention to achieve zero velocity landing of the armature  16 . Again for the purposes of description, the armature  16  is moving from the bottom coil  13  and stator  12  to the upper coil  15  and stator  14  or the valve  18  is going from open to close. This system is based on controlling the armature velocity near landing by regulating the rate of change of magnetic flux in the armature/stator core magnetic circuit. The flux is sensed by means of a sensor  28 . There are many types of sensors such as a Hall sensor, GMR sensor, eddy current sensors, and even employing the non-energized stator coil of the actuator to sense the time derivative of the flux. In the preferred embodiment a Hall sensor  28  was used. 
     Refer to FIGS. 6 and 7 there is illustrated one location of the Hall sensor  28  and that is in each stator core  12 ,  14 . Another location of the sensor may well be on the armature  16  itself. The selection of using a flux sensor has the following advantages; 
     (1) a flux sensor is extremely sensitive in response (inverse square law) to the armature motion in the region near the landing and 
     (2) its signal voltage is monotonically increasing with increasing displacement of the armature (i.e. as the armature approaches its landing). 
     The theoretical wave form  24  is illustrated in FIG.  2  and in FIGS. 4 and 5, the wave shape labeled “F” is copied from a trace on an oscilloscope. FIG. 4 is very similar to FIG.  2  and FIG. 5 illustrates the desired wave shapes as a result of the invention. 
     The system of FIG. 3 has a flux sensor  28 , an amplifier  30  and a differentiator  32  feeding one leg  33  of a comparator  34 . The other leg  36  of the comparator  34  is a threshold level device  38 . The output of the comparator  34  is “logically anded” with a logic timing component  40  and is supplied to the drive circuit  42  of the actuators  44 . Once the actuator drivers are energized, the actuator coil is energized. 
     The flux sensor  28  has its output waveform amplified and differentiated. The flux sensor wave shape is illustrated in FIG. 5 as waveform “F”. The threshold level is used to control the flux between T 2  and T 3  as illustrated in FIG.  5 . This is a closed loop control and the velocity waveform, labeled “V”, illustrates a landing velocity near zero at or near T 3 . The key feature in FIG. 5 is that the highly nonlinear characteristic of the flux buildup, which also represents the force on the armature  16 , is forced to build linearly in the region near the impact. The buildup of the flux in this region  45 , between T 2  and T 3 , is set by the catch current becomes an “inclined line” electronically equivalent to the spring rate of the spring  20 . Thus, as the armature  16  is coming into a landing, the flux is low reducing the magnetic force from the receiving stator  14  and coil  15  causing the velocity of the armature  16  to approach zero. At T 3 , the flux is no long inhibited and the armature  16  is held against the stator  14 . 
     Referring to FIGS. 4 and 5 the wave shape labeled “A” illustrates the movement of the armature  16  from one position, the sending position, to the other position, the receiving position, across the gap. The wave shape labeled “I” is the current build up in the coil  15  wound on the stator  14  that the armature  16  is approaching which in our example is the upper coil  15 . This shows the change in current from T 2  when the current is applied to T 3  when the hold current is applied. The characteristic dip  46  in current when the armature  16  seats is illustrated. 
     The final value of flux, which is the force on the armature, is now set, at T 3  by the hold current to just exceed the opposing spring force, the upper spring  20 . This will allow a rapid release of the armature  16  at the beginning of the next stroke, to open the valve  18 . In addition the hold current is defined by the minimum power required to control the actuator. 
     Referring to FIG. 3, the logic timing  40  is the system control timing wave forms  10  that are indicated in FIG.  1 . It is a system parameter that defines the time that the armature  16  moves across the gap is between T 1  and T 2 . At T 2 , the armature  16  is approaching the desired landing zone for a zero-velocity landing. At T 3  the flux is allowed to build up normally. 
     FIGS. 6 and 7 illustrate the actuator having the normal valve spring  22  operating on the valve stem  24 , the opposing valve spring  20  at the end of the valve stem  24  mechanism opposite the valve, the upper and bottom stator  12 ,  14  and stator coils  13 ,  15 , and the armature  16  which is connected to the valve stem  24  through a hydraulic valve adjuster  48 . 
     It is to be understood, while the description heretofore has been limited to just one of the coils, it also applies to the other coil and the travel of the armature in the opposite direction. A key feature is that this invention removes the highly non-linear characteristic of the flux buildup, and hence the force on the armature, and forces the flux to build linearly in the region near impact which is illustrated in FIG.  5 . The final value of flux is set by the hold current to just exceed the opposing spring force. The result allows a rapid release of the armature at the beginning of the next stroke and the minimum power required to control the actuator. 
     There has thus been described and defined an electronic control system for the movement of an armature from a sending position to a receiving position and controlling the landing speed of the armature against the stator to a near zero velocity thereby minimizing the impact force.