Abstract:
A controller for electrically actuated engine valves operates in a switching mode to monitor back EMF during periods when the coil drive current is off. Back EMF is used to determine a position of the armature so as to control the armature current to provide for soft seating of the valve reducing valve wear.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     The present invention relates to actuators for the intake and exhaust valves of internal combustion engines, and specifically to an electronically actuable engine valve providing a signal indicating the valve position. 
     Electrically actuable valves allow improved engine control. Unlike valves actuated mechanically by cam shafts and the like, the timing on electrically actuable valves can be more freely varied during different phases of engine operation by a computer-based engine controller. 
     One type of actuator for such a valve provides a disk-shaped armature which moves back and forth between two cylindrical electromagnets. The armature is attached to the valve stem of the valve and is moved against the force of two opposing springs each positioned between the armature and an opposing core. In an unpowered condition, the armature is held in equipoise between the two cores by the opposing spring forces. 
     During operation, the armature is retained against one of the cores by a “holding” current in the retaining electromagnet. The spring between the armature and the retaining core is compressed while the other spring is stretched. 
     A change of state is effected, opening or closing the valve, by interrupting the current holding the armature in place. When this occurs, the energy stored in the compressed and stretched springs accelerates the armature off of the releasing core toward the opposing receiving core. When the armature reaches the receiving core, that core is energized with a “holding” current to retain the armature in position against its surface. 
     In a frictionless system, the armature reaches a maximum velocity at the midpoint between the two cores (assuming equal spring forces) and just reaches the receiving core assembly with zero velocity. In a physically realizable system in which friction causes some of the stored energy of the springs to be lost as heat, the armature will not reach the receiving core unless the energy lost to friction is replaced. This is accomplished by creating a “capture” current in the receiving coil which produces a magnetic force to attract the armature and pull it to the core. The capture current is necessarily initiated before the armature contacts the receiving core. Once the armature is captured by the receiving coil, the current can be reduced to a holding level sufficient to hold the armature against the core until the next transition is initiated. 
     Capture of the approaching armature requires that the capture current be of sufficient magnitude to draw the armature to the core. However, it is equally important that the speed at which the armature strikes the core be limited to prevent armature damage and/or core damage and to minimize impact noise. During valve closing, control of the capture current is necessary to limit valve-seating velocity and thereby to prevent valve and/or valve seat damage or premature valve wear and to minimize valve-seating noise. If the capturing current is turned on too soon (or is too great in magnitude), the armature may be accelerated into the core and the valve into its seat at excessive velocity. Conversely, the armature may not be captured by the receiving core and the valve may not close if the capture current is turned on too late (or is too low in magnitude). Therefore, it is important to know armature position and velocity as it approaches the receiving core to ensure that the capture current is initiated at the proper time or amount to ensure proper capturing of the approaching armature. 
     Electronic position sensors may be attached to the valve stem for this purpose. Unfortunately position sensors that are sufficiently accurate and robust enough to survive in the environment of an internal combustion engine are expensive and thus impractical. 
     BRIEF SUMMARY OF THE INVENTION 
     The present inventor has recognized that a signal providing an indication of the position of the armature with respect to the cores may be derived from a back electromagnetic force (“back EMF”) generated in the receiving coil typically when the receiving coil is energized with a small sensing current. The back EMF is dependent in magnitude on the proximity of the armature to the receiving coil and thus provides an indication of armature position that may be used for more accurate valve actuation or other purposes. 
     Specifically then, the present invention provides a controller for an electrically actuable engine valve, the valve having an actuation coil producing a magnetic field to attract a movable armature communicating with a valve. The controller includes a current control circuit receiving a valve actuation signal (such as from an engine controller) and a drive current signal to provide current to the actuation coil when the valve actuation signal is present and as a function of the value of the drive current signal. An armature detector senses a back EMF resulting from an approach of the movable armature toward the actuation coil and based on this detection, a soft seat circuit adjusts the drive current signal to the current control circuit as a function of the back EMF sensed by the armature detector. 
     Thus, it is one object of the invention to provide an electrically actuable valve that produces a position output signal such as may be used to precisely control the actuation current to the valve to reduce wear on the valve assembly. Unlike systems which detect only the time at which the armature strikes the coil, the present invention allows monitoring of the approach of the armature as is necessary for soft seating of the valve against the valve seat. 
     The current control circuit may provide a hysteretic control, outputting current to the actuation coil if the current through the actuation coil drops below a predetermined low threshold and disconnecting current from the actuation coil if the current rises above a predetermined high threshold. 
     It is thus another object of the invention to provide an efficient controller allowing monitoring back EMF. Hysteretic control operates in a switched mode to reduce power dissipation and facilitates measurement of the faint back EMF signal during periods when the hysteretic control is not outputting current. 
     The armature detector may monitor the frequency of the switching of the current control circuit in hysteretic mode. 
     Thus it is another object of the invention to provide an extremely simple measurement output of armature position. Back EMF affects the decay of current in the actuation coil during periods when the hysteretic control is off thus affecting the frequency of switching of the hysteretic control. This frequency may be readily measured. 
     Alternatively, the armature detector may directly monitor the rate of change of current in the actuation coil after the current control circuit disconnects current from the actuation coil to measure back EMF. 
     Thus it is another object of the invention to provide a measurement of back EMF that is independent from the changes in control current that may be desired during different stages of the actuator closure. 
     The soft seat circuit may be sensitive to a seating level of back EMF from the armature detector occurring upon contact of the armature and the actuation coil. The soft seating circuit may provide a capture drive current signal (producing a capture current in the actuation coil) before the seating level is detected and a holding drive current signal (providing a holding current in the actuation coil) after the seating level is detected wherein the holding current is less than the capture current. 
     Thus it is another object of the invention to provide ample capture current while significantly decreasing the power consumption of the valve during holding. 
     The soft seat circuit may also be sensitive to a capture level of back EMF from the armature detector occurring prior to contact of the armature in the actuation coil. The soft seating circuit may provide a sensing drive current signal (providing a sensing current in the actuation coil before the capture level is detected) and a capture drive current signal (providing a capture current in the actuation coil after the capture level is detected) wherein the sensing current is less than the capture current. 
     Thus it is another object of the invention to provide coil current to the actuation coil prior to the need to provide capture current so as to monitor the position of the armature as may trigger the capture current. 
     The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made to the claims herein for interpreting the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a phantom, fragmentary perspective view of a cylinder head and its valve assembly showing an electromagnet actuator suitable for use with the present invention; 
     FIG. 2 is a cross-section of the electromechanical actuator of FIG. 1 taken along lines  2 — 2  showing an armature attached to a valve stem and positioned between two electromagnet coils; 
     FIG. 3 is a block diagram of the present invention showing circuitry for driving one of the coils of FIG.  2  and for monitoring the current to that coil so as to control soft seating via a soft seat control; 
     FIG. 4 is a detailed view of the coil of FIG. 3 showing its theoretical decomposition into a back EMF voltage source, a resistance and a coil inductance; 
     FIGS.  5 ( a ) through  5 ( c ) are graphs against time of: ( a ) coil current of the coil of FIG. 3, ( b ) frequency of operation of the hysteretic supply of FIG.  3  and ( c ) distance of the armature of FIG. 2 from the attracting coil of FIG. 3; 
     FIG. 6 is a flow chart showing logic of operation of the hysteretic control of FIG. 3; 
     FIG. 7 is a flow chart showing operation of the soft seat control of FIG. 3 in providing different hold currents to the hysteretic controller; and 
     FIGS.  8 ( a ) through  8 ( c ) are graphs against time of: ( a ) an engine control input to the soft seat control of FIG. 3, ( b ) threshold voltages provided to the hysteretic controller of FIG. 3 by the soft seat controller and ( c ) back EMF events produced by the current sensor of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, an electro-magnetically actuated valve  10  suitable for use with the present invention provides a coil assembly  12  fitting around a valve stem  14 , the latter which may move freely along its axis. The valve stem  14  extends downward from the coil assembly  12  into a piston cylinder  16  where it terminates at a valve head  18 . Generally, power applied via leads  20  of the coil assembly  12  will move the valve head  18  toward or away from a valve seat  22  within the cylinder so as to provide for the intake of air and fuel or recirculated exhaust gas, or exhaust of exhaust gas. 
     Referring now to FIG. 2, the coil assembly  12  provides two toroidal coils  24  and  26  of helically wound electrical wire. The coils  24  and  26  are spaced apart coaxially along the valve stem  14  and fit within cores  28  and  30 , respectively, which provide for the concentration of magnetic flux formed when the coils  24  and  26  are energized at opposed open faces  32 . 
     Between the open faces  32  of the cores  28  and  30  is a disk-shaped armature plate  34  attached to the valve stem  14 , the surface of the armature plate  34  extending perpendicularly to the axis of the valve stem  14 . The space between the open faces  32  is sufficient so that the valve stem  14  may move by its normal range  36  before the armature plate  34  is stopped against either the open face  32  of core  28  or the open face  32  of core  30 . 
     Helical compression springs  38  extend on either side of the armature plate  34  to the cores  28  and  30 . Absent the application of current to either of coils  24  and  26 , springs  38  bias the armature plate  34  to a point approximately midway between the cores  28  and  30 . 
     Referring now to FIG. 3, power to drive each of the coils  24  or  26  is provided by a pair of solid state switches  42  and  44  activated by a coil driver circuit  40 . The configuration of the solid state switches  42  and  44  and coil driver circuit  40  is identical for the two coils  24  and  26  and therefore only one is shown for simplicity. 
     Solid state switch  42  (when on) connects a source of voltage to one lead of the coil  24  or  26 . The other lead of the coil  24  or  26  passes through a sensing resistor  46  and then to the second solid state switch  44  which (when on) provides a path to ground. The switches  42  and  44  are activated by control lines  48 . When both switches  42  and  44  are activated by control lines  48 , current flows through the associated coil  24  or  26 . Free-wheeling diodes  50 , known in the art, are attached to the leads of coil  26  and  24  to provide a current path for coil current whenever the solid state switches  44  and  42  are off. 
     The coil driver circuit  40  provides the signals on control lines  48  and includes a hysteretic controller  52 , a soft seat controller  58  and a threshold comparator  72 , each which will be described below in more detail. The hysteretic controller  52 , soft seat controller  58  and threshold comparator  72  may be implemented as discrete circuitry or by means of a microcontroller programmed as will be described. 
     In order to produce the signals on control lines  48 , the hysteretic controller  52  is provided with a positive threshold signal T +  and a negative threshold signal T −  by a soft seat controller  58 . The positive threshold signal T +  and a negative threshold signal T −  indicate generally the desired coil current as will be described. The hysteretic controller  52  also receives an enable signal  56  from a soft seat controller  58  and a feedback signal FB indicating current through the coil  24  or  26  from a current sensing amplifier  54  attached to the current sensing resistor  46 . The current sensing amplifier  54  may be a differential amplifier of conventional design. 
     Referring to FIGS. 3 and 6, a program operating the hysteretic controller  52  begins at decision block  62  immediately after an enable signal  56  is received (not shown). At decision block  62 , the hysteretic controller  52  determines whether the feedback signal FB indicating coil current has risen across the positive threshold value T + . If so, then the hysteretic controller  52  proceeds to process block  64  and solid state switch  42  (and/or solid state switch  44 ) is turned off. 
     Next, and regardless of the outcome of decision block  64  at decision block  66 , the hysteretic controller  52  checks the feedback signal FB to see if it has fallen across the minus threshold T − . If so, at process block  68 , solid state switch  42  (and/or solid state switch  44 ) is turned on. Because the solid-state switches  42  and  44  are operated either fully on or fully off, relatively little power is dissipated by the solid-state switches  42  and  44 . 
     The hysteretic controller  52  repeats the above steps as long as the enable signal  56  is present to produce in coil  24  or  26 , a sawtooth current waveform similar to that shown in FIG. 5 a.  At process block  68 , as the voltage is connected to the coil  24  or  26 , the current rises in the coil  24  or  26  (limited in rate by the inductance of the coil  24  or  26 ) until it rises past the positive threshold T + . At process block  64 , the current in coil  24  or  26  falls as the voltage is disconnected from the coil  24  or  26  (again limited in rate by the inductance of the coil  24  or  26 ) until it falls below the negative threshold T − . The separation of thresholds T +  and T −  establish a deadband in between which the current may fluctuate while the average of thresholds T +  and T −  determine the current to the coils  24  or  26 . As used herein, the terms “average current” and “current” will be used synonymously reflecting the fact that they are equivalent from the point of view of power applied to the coils  24  or  26 . 
     Referring now to FIG. 4, coils  26  and  24  are electrically equivalent to a series connected pure inductor  63 , a pure resistor  65  and perfect voltage source  67  having a voltage proportional to a back EMF from the armature plate  34 . The back EMF is caused by current induced in the armature plate  34  according to well-known principles and is of a polarity to oppose the current flowing through the coils  24  or  26 . 
     Referring now to FIG.  5 ( a ), when the hysteretic controller  52  first activates solid state switch  42  and the armature plate  34  is far from the receiving coils  24  or  26 , the back EMF is low. At this time, the current in the coils  24  or  26  rapidly increases as shown by upward slope  69  under the influence of the relatively large battery voltage. When the T +  threshold is reached, the hysteretic controller turns off switch  42  causing a slower decay in the current in the coil  24  or  26  indicated by falling slope  70 . The decay of falling slope  70  is slower than the rising slope  69  because of the relatively low resistance of the coil  26  and  24 . 
     When the current level reaches the T −  threshold, the hysteretic controller  52  again turns on switch  42  causing a second rising slope  69 ′ substantially equal to  69 . The back EMF is higher at this time because the armature plate  34  will have moved closer to the coil  24  or  26 , however, the battery voltage is so much greater that the back EMF, the slope is essentially unaffected. At the falling slope  70 ′, however, the increased back EMF will be apparent and the slope  70 ′ will fall more quickly as the back EMF fights the current in the coil  26  and  24 . 
     With subsequent cycles, the falling slope  70  becomes progressively steeper until at time t 0 , the armature strikes the core  30  or  32  of the coil which is being activated and the armature motion stops. At this point, the falling slope  70 ″ decreases abruptly as a result of the cessation of the back EMF. 
     Generally, the back EMF will be a function of movement of the armature plate  34  and the proximity of the armature plate  34  to the coil at which the back EMF is being detected. Nevertheless, despite this dual dependency, the back EMF provides a good approximation to the separation distance between the armature plate  34  and a given coil  26  as a result of the consistency in acceleration curves of the armature plate  34  in normal use. The soft seat controller  58  uses a measurement of the back EMF to adjust the current in the coil  24  or  26 . 
     Referring again to FIG. 3, the soft seat controller  58  generates the enable signal  56  from an engine control signal on control line  60  indicating that one of the valves  10  needs to be opened or closed. Generally a control signal on control line  60  for one coil  26  will be the opposite of control signal on control line  60  for the other control coil  24 . The soft seat controller  58  further generates thresholds T +  and T −  from event triggers E 0  and E 1  from the threshold comparator  72  such as reflects back EMF from the feedback current signal as will be described. 
     Referring now to FIGS. 5 a - 5   c  it will be seen that both the frequency of the feedback signal (current in the coil  24  or  26 ) as shown in FIG. 5 b,  and the slope of falling slopes  70  through  70 ″, shown in FIG. 5 c,  can be used as an indication of armature position d. A first and second frequency threshold f 0  and f 1  may be established to indicate the time t 1  when the armature plate  34  has contacted the coil and the time t 0  preceding time t 1  when the armature plate  34  is still in motion toward its respective core  28  or  30 . This former time t 0  may be used to control the initiation of the capture current so as to provide just sufficient energy to cause capture of the armature plate  34  without undue acceleration against the core face or in the valve head  18  against the valve seat  22 . 
     Referring to FIG. 3, the threshold comparator  72  may operate in a first embodiment to measure the current (FB) provided by current sensing amplifier  54  to produce two event signals E 0  and E 1  corresponding generally to t 0  and t 1  or a distance d 0  and d 1  as shown in FIG. 5 c  indicating, respectively, a distance and time at which capture current should be initiated and a distance and time at which the armature plate  34  contacts the core. These signals may be produced by a monitoring of the frequency FB or the slopes  70  as have been described above. Thus the comparator  72  may be a differentiater to provide a di/dt signal (of slopes  70 ) or a frequency counter as are well known in the art. 
     Referring now to FIGS. 7 and 8 a  through  8   c,  and FIG. 3, the soft seat controller  58  first monitors the control line  60  to determine whether actuation of the respective coil  24  or  26  should be performed as indicated by decision block  76 . The turning on of the control signal on control line  60  is shown in FIG. 8 a.    
     If the control signal is OFF, then at process block  78 , flags monitoring signal E 0  and E 1  are reset and the program returns to decision block  76 . If at decision block  76 , the control signal is ON, then the program proceeds to process block  80  to determine whether the E 0  flag has been set indicating that the E 0  event has occurred. 
     Assuming for the moment that event E 0  has not yet occurred, then the E 0  flag is not set and the program proceeds to process block  82  and a “read” current is established in the coil  24  or  26 . This is done by establishing thresholds T+ and T −  at a relatively low amount of current as indicated in time period  84 . The current level of the read current is sufficient to detect back EMF but will generally be less than the capture current. 
     If at decision block  80 , the E 0  flag is set such as will be the case in time period  86  after event E 0 , then the program proceeds to decision block  88  where it is determined whether the E 1  flag has been set or not. 
     If not as will be the case in time period  86 , then the program proceeds to process block  90  and the capture current is established by thresholds T +  and T − . These thresholds, provided to the hysteretic controller  52  produce a higher value than the read current in time period  84 . Upon the occurrence of event E 1  at decision block  88 , the program proceeds to process block  92  and in time period  94 , a holding current is established being generally lower than the capture current of time period  86 . 
     The above description has been that of a preferred embodiment of the present invention, it will occur to those that practice the art that many modifications may be made without departing from the spirit and scope of the invention. For example, a separate coil may be used to provide the read current or the detection of back EMF although at the cost of additional parts. Further, instead of adjusting the magnitude of the capture current, the soft seat controller may adjust the timing of E 0 . In order to apprise the public of the various embodiments that may fall within the scope of the invention, the following claims are made.