Abstract:
A method is provided for determining magnetic characteristics of an electrically controlled solenoid. The method includes providing an electronically controlled solenoid having an armature, a stator and a coil operatively associated with the stator. The armature, stator and coil define a magnetic circuit. The armature is spaced from the stator to define an air gap between the armature and the stator. Current to the coil is ramped in a generally linear manner over a period of time to define a known current curve. A resulting rate of change of flux in the magnetic circuit is observed and recorded at certain points along the current curve. In accordance with another aspect of the invention, flux in the magnetic circuit is ramped in a generally linear manner over a period of time to define a known flux curve. A resulting current in the coil is observed and recorded at certain points along the flux curve. With the method, a magnetization curve may be developed wherein flux may be determined at any point along a curve for a given amount of current in a solenoid.

Description:
FIELD OF THE INVENTION 
     This invention relates to electronically controlled solenoids and more particularly to a method of determining magnetic characteristics of a solenoid in its finished product state. 
     BACKGROUND OF THE INVENTION 
     A conventional electromagnetic actuator for opening and closing a valve of an internal combustion engine generally includes a solenoid which, when energized, produces an electromagnetic force on an armature. The armature is biased by a return spring and the armature is coupled with a cylinder valve of the engine. The armature is held by the electromagnet in one operating position against a stator core of the actuator and, by deenergizing the electromagnet, the armature may move towards and into another operating position by the return spring. 
     Conventional high speed electronic solenoids of the fuel injector type include an armature to control movement of an injector valve. 
     In solenoids of either an electromagnetic actuator or a fuel injector type, it may be desirable, for control or other purposes, to know certain magnetic characteristics of the solenoid. For example, it may be desirable to know when the magnetic circuit approaches saturation, what current levels are expected in order to deliver a desired flux, where significant non-linearities arise along a magnetization curve, how the magnetization curve varies over the operation, life or temperature of the solenoid, or how the current varies due to demands of building flux. 
     A magnetization curve of a solenoid generally comprises two parameters: magnetic flux density and magnetomotive force. The magnetomotive force is that which tends to produce a magnetic field. In a solenoid, this force is most commonly produced by a current flowing through a coil of wire and the magnitude of the force is proportional to the current and the number of turns of the coil. For simplicity, flux and current may be the two parameters of a magnetization curve. 
     Since the purpose of a solenoid is to produce a force to perform work and force is the result of a magnetic field (flux), and the magnetic field is the result of electrical current applied to the solenoid, it would seem reasonable that if the current applied was known, the force the solenoid is producing would be known. However, the force the solenoid is producing cannot be obtained by simply measuring current. Due to the variations of magnetic characteristics of the ferromagnetic circuit, the amount of flux and therefore force generated for a given amount of current varies. 
     Conventionally, the magnetization curve has been assumed, within an error, based on data provided from the manufacturer of the raw magnetic alloy used in the magnetic circuit. Thus, the magnetization curve has classically remained an engineering design variable. 
     Accordingly, there is a need to determine magnetic characteristics of a solenoid electronically and develop a magnetization curve in a simple and real time output, and to use the magnetization curve in the electronic control of a solenoid. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to fulfill the need referred to above. In accordance with the principles of the present invention, this objective is obtained by providing a method of determining magnetic characteristics of an electrically controlled solenoid. The method includes providing an electronically controlled solenoid having an armature, a stator and a coil operatively associated with the stator. The armature, stator and coil define a magnetic circuit. The armature is spaced from the stator to define an air gap between the armature and the stator. Current to the coil is ramped in a generally linear manner over a period of time to define a known current curve. A resulting rate of change of flux in the magnetic circuit is observed and recorded at certain points along the current curve. 
     In accordance with another aspect of the invention, flux in the magnetic circuit is ramped in a generally linear manner over a period of time to define a known flux curve. A resulting current in the coil is observed and recorded at certain points along the flux curve. 
     With the above method, a magnetization curve may be developed wherein flux may be determined at any point along a curve for a given amount of current in a solenoid. 
     Other objects, features and characteristic of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a solenoid device, in particular an electromagnetic actuator, provided in accordance with the principles of the present invention, shown coupled with a gas exchange valve of an engine; 
     FIG. 2 is a block diagram of circuit structure of the solenoid device of FIG. 1; 
     FIG. 3 is a block diagram of a second embodiment of circuit structure of the solenoid device of FIG. 1; 
     FIG. 4 is a block diagram showing a flux mirror circuit of the circuit structure of FIG.  3 . 
     FIG. 5 is a graph of the invention showing ramped flux and responsive current of the solenoid device of FIG. 1 having an air gap of 200 microns; 
     FIG. 6 is a graph of the invention showing ramped flux and responsive current of the solenoid device of FIG. 1 having an air gap of 20 microns; 
     FIG. 7 is a graph of the invention showing ramped flux and responsive current of the solenoid device of FIG. 1 having an air gap of 2 mm; and 
     FIG. 8 is a graph of the invention showing flux level vs. ampere turn for air gaps of an actuator of FIG. 1 of 2 mm, 20 microns and 200 microns. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an electromagnetic actuator or solenoid is shown, generally indicated  10 , which represents a solenoid of which magnetic characteristics are determined in accordance with the principles of the present invention. Although an electromagnetic actuator for use in electronic valve timing is shown to represent a solenoid for the purposes of describing the invention, the invention is not limited to an actuator. The invention relates to any electronically controlled solenoids. 
     The electromagnetic solenoid  10  includes a first electromagnet, generally indicated at  12 , which includes a stator core  14  and a solenoid coil  16  associated with the stator core  14 . A second electromagnet, generally indicated at  18 , is disposed generally in opposing relation to the first electromagnet  12 . The second electromagnet  18  includes a stator core  20  and a solenoid coil  22  associated with the stator core  20 . The electromagnetic actuator  10  includes a ferromagnetic armature  24  which is attached to a stem  26  of a fluid exchange valve  28  through a hydraulic valve adjuster  27 . The armature  24  is disposed generally between the electromagnets  12  and  18  so as to be acted upon by the an electromagnetic force created by the electromagnets. In a deenergized state of the electromagnets  12  and  18 , the armature  24  is maintained in a position of rest generally between the two electromagnets  12  and  18  by opposing working return springs  30  and  32 . In a valve closed position (FIG.  1 ), the armature  24  engages the stator core  14  of the first electromagnet  12 . 
     Each stator core and associated coil together With the armature  24  define a magnetic circuit of the solenoid  10 . Further, as shown in FIG. 1, an air gap  34  is provided between the armature  24  and the second electromagnet  18 . It can be appreciated that an air gap is defined between the armature  24  and the upper electromagnet  12  at certain times during the oscillation of the armature  24 . The air gap  34  is the magnetic discontinuity in a ferromagnetic circuit which increases the reluctance (resistance to flux) of the circuit. 
     While keeping the armature  24  stationary, solenoid coil  22  may be energized causing a flux linkage to build up to some flux density. The resulting flux at the air gap  34  and the concentration of flux through and around the lower reluctance armature  24  results in a distorted flux field and therefore a net force on the armature  24  in the direction of decreasing reluctance (smaller gap). 
     U.S. patent application Ser. No. 09/025,986, now U.S. Pat. No. 6,176,207, entitled “Electronically Controlling the Landing of an Armature in an Electromagnetic Actuator”, the contents of which are hereby incorporated into the present specification by reference, discloses using a flux sensor to sense a rate of change of magnetic flux in an electromagnetic actuator. The rate of change of flux sensed is used as a feedback variable to control a landing velocity of an armature of the actuator. U.S. patent application Ser. No. 09/122,042, now U.S. Pat. No. 5,991,143, entitled “A Method For Controlling Velocity Of An Armature of An electromagnetic Actuator”, the contents of which are hereby incorporated into the present specification by reference, discloses feedback control based on a rate of change of magnetic flux without the need for a flux sensor. 
     The invention utilizes closed loop flux control to determine magnetic characteristic of an electronically controlled solenoid. 
     With reference to FIG. 2, a block diagram of a circuit structure according to one embodiment of the present invention is shown which incorporates closed loop feedback of magnetic flux. The circuit structure is based on controlling the armature velocity near landing by regulating a rate of change of magnetic flux in the armature/stator magnetic circuit by measuring the terminal voltage of the coil  22 . In the circuit of the FIG. 2, a terminal voltage  36  of the coil  22  is applied to a comparator  38 . A threshold level  40  is also applied to the comparator  38 . The output of the comparator  38  is “logically added” with a logic timing component  42  and is supplied to an actuator driver  44  to drive the actuator  10 . Once the actuator driver  44  is energized, the solenoid coil  22  is energized. 
     The measured coil terminal voltage  36  is compared to the threshold level  40  and the threshold level  40  is used to control a catch current supplied to the solenoid coil  22  of the actuator  10  and thus control the magnetic flux  41 . 
     Although measuring the coil terminal voltage directly is effective for controlling the landing of the armature  24  of the actuator  10 , it is preferable to not physically measure the high common mode voltage typically present at each terminal of the coil  22 . Thus, a parametrically determined mirror image of the coil terminal voltage and hence a mirror image of the rate of change of flux in the actuator&#39;s magnetic circuit may be provided by the circuit of the actuator  10  such that there is no need to physically touch the coil terminals to measure the coil terminal voltage. 
     With reference to FIG. 3, a system block diagram for controlling the solenoid or actuator  10  is shown which uses a “flux mirror” circuit. The actuator  10  is electrically connected to an “open” current amp  43  and a “close” current amp  45 . The current amps  43  and  45  are connected to a programmable current regulator or driver board  46 . The programming for controlling current is performed by a soft landing circuit board  47  which commands and regulates the desired rates of change of magnetic flux required to control the magnetic force on the armature  24  of the actuator  10 , and therefore, command the flux shape. Closed loop flux regulation is accomplished by feedback of the actuator coil voltage  48  to a flux mirror circuit  49  on the soft landing circuit board  47 . 
     FIG. 4 shows the flux mirror circuit  49  block diagram of the actuator  10 . The solenoid coil  22  of the actuator  10  is driven preferably by a PWM (switchmode) current regulator  50  which provides a pulse train to a high voltage power transistor stage (including a high operating voltage supply  52  and a power switch  54 ) which subsequently switches voltage pulses across the load of the coil  22 . It can be appreciated that power can be regulated by other means, such as, for example, a voltage regulator or amplifier. The flux mirror addition to this conventional approach consists of routing the logic level PWM signal from the current regulator  50  through a buffer  56 . The rail voltage of the buffer  56  is derived from a scaled-down replica of the system high voltage supply  52 . Finally, the scaled and buffered pulse train  58  is smoothed by a low pass filter  60  and is applied to the comparator  38 . The threshold level  40  is also applied to the comparator  38 . The output of the comparator  38  is “logically added” with a logic timing component  42  and is supplied to the driver  44  to drive the actuator  10 . Once the actuator driver  44  is energized, the solenoid coil  22  is energized. The smoothed pulse train  61  from the low pass filter  60  is compared to the threshold level  40  and the threshold level  40  is used to control a catch current supplied to the solenoid coil  22  of the actuator  10  and thus control the magnetic flux  41 . 
     The time constant of the low pass filter  60  is selected to match the rate of armature motion in the actuator  10 . The output from the low pass filter  60  is scaled-down from and mirrors the high operating voltage of the coil  22  and corresponds to the desired time rate of change of magnetic flux d(phi)/dt which is used as a feedback variable to control the landing velocity of the armature  24 . 
     In accordance with the invention, to determine magnetic characteristics of the solenoid  10 , the armature  24  was in a static condition (the air gap  34  being fixed). The number of turns of the coil  22  was fixed at 200 turns and the parameters of flux and current were the remaining controlled or observed variables. With reference to FIGS. 5-7, one approach to determine magnetic characteristics of a solenoid is ramp flux in a generally linear manner to define a known flux curve and then to observe the resulting current in coil  22  along the flux curve. FIG. 5 shows a current curve resulting from ramped flux when the air gap  34  of the actuator  10  was set at 200 microns. FIG. 6 shows a current curve resulting from ramped flux when the air gap  34  of the actuator  10  was set at 20 microns. FIG. 7 shows a current curve resulting from ramped flux when the air gap  34  of the actuator  10  was set at 2 mm. 
     Another approach to determine magnetic characteristics of a solenoid is to ramp the current of coil  22  in a generally linear manner to define a known current curve and then to observe the resulting rate of change of magnetic flux in the magnetic circuit defined by the coil  22 , stator  20  and armature  24 . The rate of change of magnetic flux may then be integrated to determine flux. 
     The rate of change of flux may be determined and controlled by using a flux sensor such as a Hall effect sensor, or by measuring the terminal voltage of the coil  22 , or by using a “flux mirror” circuit which mirrors the terminal voltage of the coil  22 , as explained above. 
     In a solenoid operated device such as the actuator  10 , the magnetomotive force is that which tends to produce a magnetic field. The magnetomotive force is most commonly produced by current flowing through a coil of wire and the magnitude of this force is proportional to the current supplied to the coil and the number of turns of the coil. Thus, a unit of magnetomotive force is the ampere turn. In that regard, the data obtained in FIGS. 5-7 is shown in a FIG. 8, which represents a magnetization curve. The curve in FIG. 8 shows the magnetic flux density (flux level) and the magnetomotive force for the three gap sizes indicated in FIGS. 5-7. 
     For simplicity of explanation, flux and current were used as the two parameters of the magnetization curve of FIG.  8 . As noted above, due to variations of the magnetic characteristics of a ferromagnetic circuit, the amount of flux and therefore, force generated for a given amount of current varies. 
     Thus, in accordance with the invention, a magnetization curve (FIG. 8) provides the flux level at any point along the curve for a given amount of current in the solenoid  10 . The invention provides a means of determining the magnetization curves of solenoids in their finished product state and, if desired, in their final installation and operating condition. The magnetic characteristics are desirable information for the electronic control of solenoids. With the magnetic characteristics known, it is then known approximately what flux density and therefore what force is generated for a given amount of input current. The magnetic characteristic determination can be made with the solenoid in operation to provide immediate controller information update. For an electromagnetic actuator, flux control implementation can use the invention for auto-calibration of the actuator to the controller, in order to correct for variations in magnetic materials, magnetic heat treatment and/or magnetic architecture. With regard to high-speed solenoids of the fuel injector type, the invention can be used for quality control of a finished fuel injector. 
     Thus, the invention provides difficult to obtain data in a simple and real time output. The specific calibration to a magnetization curve of a solenoid by its control electronics has not been considered to date because it was impractical to implement and the concept of calibration was not an option prior to the invention. 
     The foregoing preferred embodiments have been shown and described for the purposes of illustrating the structural and functional principles of the present invention, as well as illustrating the methods of employing the preferred embodiments and are subject to change without departing from such principles. Therefore, this invention includes all modifications encompassed within the spirit of the following claims.