Abstract:
A method for controlling valve actuation comprises the steps of communicating at least one dithering signal to preload the valve. An event signal is then communicated. The dithering signal is altered based on the cycle time for the valve. The preferred application is for camless engine valve control.

Description:
This application claims priority to U.S. Provisional Patent Application Serial No. 60/257,305 filed on Dec. 20, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method and algorithm for controlling the amount of “dithering” or preloading of a cyclic valve system. 
     Valve systems are known which are cyclically opened and closed. In many of these systems the speed of the opening and closing cycle may vary as the device associated with the valve system also changes in speed. One challenge is to associate the control sending the signal to open and close the valve in a proper timed fashion with the actual operation of the valve. Most valves have some “lag” time associated with their operation. 
     One recent application of such valves is to camless engine technology. A camless engine employs solenoid actuators to control hydraulic fluid flow and operate the valves of the engine rather than an engine cam. In the typical engine, a cam mechanically rocks open and close the valves of a combustion chamber of an engine. Thus, timing is a simple method of cam position. For a camless engine, however, the valves are actuated by electromagnetic plungers tied to an engine control unit. Camless engines may be more fuel efficient than engines using cams, however, timing is not as easily achieved. 
     Correct engine timing is as important to a camless engine as it is to a cam engine. However, there are challenges to the design of a camless engine not present for conventional engines. Specifically, the solenoids and hydraulic valves introduce a certain amount of lag. This lag may arise from the additional time required to charge the solenoid valves as well as to actuate the hydraulic valves of the system. At lower speeds this lag is particularly pronounced and presents challenges. 
     One proposed way to cure this lag has been to dither the electromagnetic plunger to prime the system for correct engine timing. For example, a series of short electrical signals may be sent to the electromagnetic plunger actuating the plunger and thereby allowing hydraulic fluid flow to begin actuating the hydraulic valve. After the electromagnetic plunger and the hydraulic valve are primed, then the engine control unit sends an “event signal,” either fully opening or closing the valve, for the desired duty cycle of the event. 
     Such a system would have a significant drawback. As a consequence of sending numerous priming signals prior to sending an event signal, large amounts of energy are consumed by the electrical system. Thus, employing the strategy may not be practical. 
     A need, therefore, exists for an engine control strategy that consumes less energy than existing techniques. 
     SUMMARY OF THE INVENTION 
     In general, the present invention utilizes a system which “dithers” or preloads a cyclically operating valve system in such a fashion that the amount of dithering changes as the speed of operation of the valve system changes. Generally, as the cycle time for the valve is reduced the amount of necessary preloading to reduce lag time is also reduced. Thus, as the cycle time decreases the amount of dithering decreases. 
     One preferred application of the present invention offers an inexpensive and economical engine management strategy. Rather than employing the same dithering scheme throughout the entire range of engine speeds, the invention employs a dithering scheme that varies with engine speed. Accordingly, a dithering signal is communicated to an actuator of the camless engine. The signal activates the actuator for a portion of an event time period less than the event time period. An event signal is sent to the actuator that lasts the entire event time period, activating the actuator for this period. The engine speed is sensed. Then, based upon this sensed speed, the dithering signal may be altered. For example, in some cases plural dithering signals may be sent prior to the event signal. In such a system the number of dithering signals could decrease as the speed of the camless engine increases. Additionally, the length of each dithering signal may decrease as the speed of the camless engine increases. This vary the dithering signal based on engine speed thereby results in significantly less consumption of energy during operation of the vehicle engine. 
     Such a strategy may be employed for the air intake and exhaust valves of a combustion chamber of the engine. In addition, the technique may be employed for a fuel injector system. 
     Moreover, the signal strategy may only alter the dithering signals over selected ranges of engine speeds. The dithering signals may also vary from each other. Thus, the strategy permits flexibility in the dithering of the camless engine. The technique employs existing hardware as well to avoid expense. The technique may use existing engine control units and a crank position sensor. This hardware is already in use with current camless engines. 
     The disclosed system utilizes a solenoid which is driven to open and close and allow the flow of hydraulic fluid to the actual valve. It is a solenoid valve which is “dithered” to begin the flow of hydraulic fluid and preload the hydraulic valve. 
     However, it should be understood that the basic concept can be expanded to any type of valve system which may benefit from the “preloading” as mentioned above. Generally, any valve which has a cyclic operation time which varies across the range of the system associated with the valve system may benefit from this invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
     FIG. 1 illustrates a schematic diagram of the engine control unit, sensor, engine, actuator, and hydraulic valve. 
     FIG. 1A shows an example solenoid and hydraulic valve. 
     FIG. 2 illustrates a dithering strategy. 
     FIG. 3 illustrates two different types of dithering signals that maybe employed as part of a dithering strategy. 
     FIGS. 4 and 5 illustrate the inventive dithering strategy, highlighting the alteration of the dithering signal as engine speed changes. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a schematic view of a system employing the inventive method. Shown are engine  10 , crank position sensor  14 , engine control unit  18 , actuator  22 , and valves  26 . As known, engine control unit  18  communicates with and manages the actuation of valves  26  through actuators  22 , typically solenoid electromagnetic plungers. Valves  26  may each represent an air intake or exhaust valve for camless engine  10 . A signal for an event, such as the opening or closing of the valve, is sent to actuator  22  by engine control unit  18 . Actuator  22  then actuates hydraulic valve  26 . However, a time lag exists between the sending of the event signal and the actual movement of hydraulic valve  26 . This delay may stem from the time required to charge actuator  22  to fire as well as the time required to actuate hydraulic valve  26 . As shown, there are plural valves  26  (typically there will be one for each engine cylinder) which are actuated cyclically. 
     As shown in FIG. 1A, the solenoid  22  moves between a retracted and extended position to control the flow of hydraulic fluid through passages  100  under the control of two coils  99 . As can be appreciated from the schematic of FIG. 1A, the passage  100  communicates with the hydraulic valve  26 , driving the hydraulic valve between its extreme open and close positions. As can be also appreciated, by preloading the hydraulic valve, the “lag” time between the decision to fire the valve and the actual movement of the valve can be greatly reduced. This one valve arrangement is shown only as one example, and other valve arrangements will come within the scope of this invention. 
     As shown in FIG. 2, one way to address this time lag is to prime actuator  22  and hydraulic valve  26  to receive the event signal. Accordingly, dithering signal A and dithering signal B may be sent from engine control unit  18  to actuator  22  in advance of event signal  30 . These signals have sufficient pulse current to actuate or fire the electromagnetic plunger of actuator  22 , which itself is coupled to hydraulic valve  26 . Following these two dithering signals, event signal  30 , driving the solenoid to allow fluid flow and cause hydraulic valve  26  either to open or close, is sent. Dithering signal A and B have duty cycles, T 1  and T 2  respectively, which are less than the duty cycle, TE, of event signal  30 . 
     As shown in FIG. 3, duty cycle of dithering signal A may be less than duty cycle of dithering signal B. Moreover, each dithering signal, A or B, may comprise two components, an actuation current and a hold current. Dithering signal A may have actuation current  42 , which on average is enough current to actuate actuator  22  and accordingly hydraulic valve  26 . Dithering signal A may also have hold current  46 , which is sufficient current to continue actuation of actuator  22  but is less than current required to fire actuator  22 . Also, dithering signal B may have actuation current  50 , which is also sufficient to fire actuator  22  and hydraulic valve  26 . It may also have hold current  54 , which is sufficient to continue actuation of actuator  22 . 
     Notably, dithering signal A and dithering signal B may differ in the length of time that hold current is maintained. As shown in FIG. 3, hold current  46  is maintained for time period T 4  while dithering signal B has hold current  54 , which is maintained for time period T 5 , a time period less than time period T 4 . One of ordinary skill in the art can appreciate other ways to alter dithering signals to best suit the engine timing for a particular engine. 
     Also shown in FIG. 3 are variations in actuation current  42  and hold current  46 . Variation  58  and variation  62  may comprise signal noise and should be sufficiently small to avoid undesired actuation of actuator  22  and hydraulic valve  26 . Such noise may also be present on event signal  30 . 
     FIGS. 4 and 5 illustrate a power saving technique in the use of dithering signals. FIG. 4 illustrates a signal pattern from the engine control unit to actuator  22  for a particular range of engine speeds as sensed by crank position sensor  14 . For open coil  60 , event signal  30  is shown preceded by five dithering signals and followed by four dithering signals. Depending on the engine and management strategy, dithering signals may be of different types determined by trial and error or testing to maximize responsiveness. For close coil  64 , event signal  32 , signal for closing coil, is shown preceded by four dithering signals and followed by five dithering signals. Of course, these signals are example, and other numbers may be determined to be preferred for certain applications. 
     FIG. 5 illustrates a signal pattern for the same engine for a higher range of engine speeds than represented by FIG.  4 . As shown, the number of dithering signals decreases as the engine speed increases. This is so because there is less need to dither as engine speed increases. The amount of “down time” between actuation decreases. Thus, event signal  30  may be preceded by three dithering signals and followed by two. Event signal  32  may be preceded by two dithering signals and followed by three. Of course, these signals are examples and other numbers maybe determined to be preferred for certain applications. 
     The reduction of dithering signals may follow an inverse linear pattern. That is, as engine speed increases, the number of dithers may decrease linearly. Alternatively, as engine speed decreases, the numbers of dithers may increase linearly. Moreover, the duration, duty cycle, of dithering signals may also decrease with engine speed and increase as engine speed slows. Both the number of dithers and their duration may vary for particular ranges of engine speed as well. For example, for one particular engine speed range, say 0 to 2000 rpm&#39;s, there may be x number of dithers prior to the sending of an event signal and y number of dithers set after. When the engine increases speed to a higher range, say from 2000 rpm&#39;s to 4000 rpm&#39;s, the total number of dithering signals may be reduced. 
     As shown in FIGS. 4 and 5, the dithering strategy employed is for a two coil actuator. The actuator has an open coil that when powered opens or moves the electromagnetic plunger in one direction while another coil, a close coil, moves the electromagnetic plunger in the opposite direction when powered. Such a system maybe used for a fuel injector system. Also as known, for a combustion chamber valve, such as an air intake valve or an air exhaust valve, there may be only an open coil with the work of the closed coil replaced by a closing spring. 
     Again, the most preferred application of the present invention is to the disclosed camless engine, or other camless engine valve arrangements. However, the basic idea can be extended to any cyclically operated valve wherein preloading of the valve is desirable, and more important particularly at slower cycle times. 
     The aforementioned description is exemplary rather that limiting. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed. However, one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. Hence, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For this reason the following claims should be studied to determine the true scope and content of this invention.