Patent Publication Number: US-2007097589-A1

Title: Method of preadjusting an electromagnetic actuator

Description:
The invention relates to a method of preadjusting an electromagnetic actuator.  
     BACKGROUND OF THE INVENTION  
      In the automotive field, electromagnetic valve actuators are known that comprise an actuator member acting on the valve that is mounted to move between two extreme positions on either side of an equilibrium point defined by return means acting on the actuator member.  
      The actuator also includes an electromagnetic member comprising an armature connected to the actuator member and coils which are disposed to attract and hold the armature against abutments, generally defined by respective magnetic cores, serving to define the extreme positions of the actuator member.  
      The dynamic behavior of the actuator depends on the position of the equilibrium point. The actuator includes adjustment means enabling the equilibrium point to be moved. The equilibrium point is adjusted using a dynamic method of fine adjustment during which the actuator is caused to operate. However implementing that method of fine adjustment assumes that the equilibrium point is already placed initially within an acceptable range of positions that enable the actuator to start and to run.  
      For this purpose, a method is known of manually preadjusting the actuator by moving the equilibrium point of the actuator member to a predetermined position by measuring the distance of the armature relative to the abutments by means of spacers inserted between the abutments and the armature. That method is difficult to apply on an industrial scale.  
      U.S. Pat. No. 5,804,962 discloses a method of adjusting the equilibrium point of the actuator member of a two-coil actuator, which method consists in measuring the induction of each coil while the armature is in contact with the corresponding coil. One of the measured values, or the difference between the two measured values is then compared with a predetermined target value. That comparison makes it possible to deduce an offset to be applied to the position of the equilibrium point of the actuator member.  
      In order to be implemented, that method thus requires the target value to be predetermined, either by modeling, or by taking measurements on a batch of already-adjusted actuators. In addition, the same target value is used for all of the actuators to be adjusted, whereas the position of the equilibrium point of the actuator member is specific to each actuator.  
     OBJECT OF THE INVENTION  
      A particular object of the present invention is to mitigate that drawback.  
     BRIEF SUMMARY OF THE INVENTION  
      To this end, the preadjustment method of the invention comprises the steps of:  
      for a parameter representative of different positions of the actuator member, determining two characteristic values corresponding to the extreme positions of the actuator member;  
      determining a target value from the two characteristics values; and  
      moving the equilibrium point so as to bring it into a preadjustment position in which the representative parameter has a preadjustment value that is substantially equal to the target value.  
      Thus, the target value is no longer predetermined, but is deduced directly from the measured characteristic values. The target value as deduced in this way is thus specific to each actuator.  
      The use of a suitable representative parameter makes it easy to determine the preadjustment position that will enable the actuator to start and operate.  
      In a particular implementation of the method of the invention, the target value is equal to a mean of the characteristic values.  
      In an advantageous implementation of the method of the invention, the equilibrium point is moved while measuring the representative parameter.  
      This operation makes it possible to verify that the equilibrium point is properly positioned while measurement is taking place.  
      In a first particular implementation of the method of the invention, in order to determine at least one of the characteristic values of the representative parameter, the actuator member is brought into the corresponding extreme position, and the representative parameter is measured while the actuator member is in said position.  
      In another particular implementation of the method of the invention, in order to determine at least one of the characteristic values of the representative parameter, maximum and minimum theoretical values are determined that can be taken by the representative parameter given manufacturing and assembly tolerances, and a mean of the theoretical values is taken as the characteristic value.  
      In a preferred implementation of the method of the invention, the parameter is an electrical signal coming from a sensor for sensing the position of the actuator member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other characteristics and advantages of the invention appear in the light of the following description of particular and non-limiting implementations of the invention given with reference to the accompanying figures, in which:  
       FIG. 1  is a section view of an electromagnetic actuator;  
       FIG. 2  is a diagrammatic graph showing a first implementation of the adjustment method of the invention; and  
       FIG. 3  is a graph analogous to  FIG. 2  showing a second implementation of the method of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      With reference to  FIG. 1 , and in conventional manner, an electromagnetic actuator  10  is mounted on a cylinder head  4  of an engine in order to actuate a valve  1 . The stem  3  of the valve  1  is mounted to slide in a bearing  5  in the cylinder head  4 .  
      The actuator  10  comprises a pusher  12  that acts on the stem  3  of the valve. The end of the stem  3  of the valve  1  and the end of the pusher  12  are urged towards each other by two opposing springs  7  and  15  acting respectively on the pusher  12  and on the valve stem  3 . The springs  7  and  15  define an equilibrium point, corresponding to the valve  1  being in a half-open position.  
      The pusher  12  is secured to an armature  13  mounted to move between two coils  14 . 1  and  14 . 2 . The stroke of the pusher  12  is limited between a top extreme position defined by the armature  13  coming into abutment against the core of the coil  14 . 1 , and a bottom extreme position defined by the armature  13  coming into abutment against the core of the coil  14 . 2 , the two extreme positions corresponding substantially to the valve  1  being in an open position and being in a closed position.  
      In operation, the pusher  12  is moved from one extreme position to the other by the combined action of the springs  7  and  15 , and of the coils  14 . 1  and  14 . 2  attracting the armature  13  in alternation. In order to measure the positions of the pusher  12 , the actuator is fitted with a position sensor  21 , e.g. Hall effect sensor comprising permanent magnets carried by the pusher  12  and a detector carried by the housing  11  of the actuator.  
      The spring  15  bears against a seat  16  occupying a position relative to the housing  11  that is adjustable by screw means, under the control of a thumbwheel (not shown). By moving the seat  16 , it is possible to move the equilibrium point of the pusher  12 .  
      With reference to  FIG. 2 , in which the positions of the armature  13  are identified on the position axis on the right-hand side, the armature  13  can take any position in a range  50  defined by the top extreme position  51  and the bottom extreme position  52 .  
      Because of manufacture and assembly tolerances that apply to the component elements of the actuator and of the engine, and also because of mounting tolerances concerned with mounting the actuator on the engine, the position of the equilibrium point of the armature  13  after the actuator has been mounted on the engine lies somewhere in a range  53 . The amplitude of this range can be very large (up to half the range  50 ) because of the wide dispersion in the preparation of the springs  7  and  15 . Under these circumstances, prior to implementing the method of the invention, the equilibrium point is an initial position  54  in which the armature  13  is too far away from the coil  14 . 2  to enable it to be attracted thereto, thus preventing any transition of the armature from one extreme position to the other. The actuator cannot operate.  
      The method of the invention seeks to bring the equilibrium point into a range  65  of acceptable positions that enable the actuator to start and to operate, thus making it possible subsequently to implement a dynamic fine adjustment method that is itself known.  
      To do this, the method of the invention comprises a step of determining a “top” characteristic value  55  for the voltage across the terminals of the position sensor  21 , with this top characteristic value corresponding to the top extreme position  51 . A “bottom” characteristic value  56  for the voltage across the terminals of the position sensor  21  is likewise determined, corresponding to the bottom extreme position  52 .  
      The terms used for qualifying the characteristic values serve to associate each of them with the corresponding extreme position, however it should be understood that these terms do not necessarily relate to the real relationship between these characteristic values. In particular, the voltage having the top characteristic value could be less than the voltage having the bottom characteristic value.  
      In  FIG. 2 , the top characteristic value  55  and the bottom characteristic value  56  are marked on a voltage axis on the left-hand side of the figure, and placed in correspondence with the extreme positions associated with the position axis on the right-hand side of the figure.  
      The method of the invention consists in moving the equilibrium point so as to place it in a preadjustment position  57  that lies within the operating range  65  and that corresponds to a voltage value on the sensor  21  that is substantially equal to the mean of the top and bottom characteristic values  55  and  56 .  
      In order to determine the characteristic values  55  and  56  for the voltage across the terminals of the position sensor  21 , several techniques are possible within the method of the invention.  
      In a first technique, the characteristic values  55  and  56  are obtained by calculation, estimating a theoretical minimum value  59 . 1  and a theoretical maximum value  59 . 2  that can be taken by each of the characteristic values. The theoretical maximum and minimum values define a range of uncertainty  59  that results from manufacturing tolerances on all of the elements that might have an influence on the associated characteristic value. The characteristic value is then estimated by taking the mean of the corresponding theoretical values  59 . 1  and  59 . 2 .  
      The preadjustment voltage  57  as obtained in this way is associated with an uncertainty range  60  that is substantially equivalent to the uncertainty range  59 .  
      Although in the example shown the uncertainty range  60  corresponds to an uncertainty range in the preadjustment position  57  that lies within the range  65  of acceptable positions, it can be advantageous to approach the ideal equilibrium position  66  that lies substantially halfway between the extreme positions  51  and  52 , so as to be certain that the actuator will start.  
      To reduce the uncertainty, and in a second technique as shown in  FIG. 3 , the characteristic values  55  and  56  are obtained by bringing the armature  13  into abutment against the corresponding coil at the extreme position in question, and measuring the voltage across the terminals of the position sensor  21  when the armature  13  is in abutment.  
      This technique serves to reduce the uncertainty on the characteristic values  55  and  56  very considerably, since the uncertainty is then restricted to the measurement accuracy of the sensor  21 , giving rise to an uncertainly range  61  that is small in comparison with the amplitude of the range  65  of acceptable positions.  
      In a first variant of this technique, the characteristic values  55  and  56  are measured in the workshop where the actuator is assembled, prior to the actuator being mounted on the engine.  
      The bottom characteristic value  56  is obtained by measuring the voltage across the terminals of the position sensor  21  while the armature  13  is naturally in abutment against the coil  14 . 2  since the pusher  12  is subjected to the action of the spring  15  only.  
      For the top characteristic value  55 , advantage is taken of the fact that the actuator is generally delivered with a spacer interposed between the armature  13  and the coil  14 . 2  enabling the pusher  12  to be held in a retracted position.  
      To put the spacer into place in the assembly workshop, the corresponding coil  14 . 1  of the actuator is powered so as to attract the armature  13  into abutment in the top extreme position  51 . The voltage is then measured across the terminals of the sensor  21  while the armature  13  is held in abutment in the top extreme position  51 .  
      In a second variant of this technique, the characteristic values are measured once the actuator is mounted on the engine.  
      For the top characteristic value  55 , advantage is taken of the fact that it is necessary to remove the blocking spacer after the actuator has been mounted on the engine. To do this, the coil  14 . 1  of the actuator is powered so as to attract the armature  13  into abutment in the top extreme position  51 . The voltage across the terminals of the sensor  21  is then measured while the armature  13  is held in abutment in the top extreme position  51 .  
      For the bottom characteristic value  56 , care is initially taken to move the equilibrium point so that it is closer to the bottom extreme position  52  than to the top extreme position  51 . Thus, when the spacer is removed and the armature  13  is released by the coil  14 . 1 , the armature  13  is propelled by the spring  15  towards the coil  14 . 2  and touches it. The voltage across the terminals of the position sensor  21  is then measured at the moment when the armature  13  touches the coil  14 . 2 . This measurement can be taken on the fly. It is also possible to hold the armature temporarily against the coil  14 . 2  in order to take the measurement. The prior offset of the equilibrium point guarantees that the armature  13  does indeed touch the coil  14 . 2 .  
      Numerous variants of the method of the invention are possible, and in particular:  
      a first implementation in which the two characteristic values are calculated;  
      a second implementation in which the top characteristic value  55  is measured in the assembly workshop before the actuator is mounted on the engine, while the bottom characteristic value  56  is calculated;  
      a third implementation in which the top characteristic value  55  is measured after the actuator has been mounted on the engine, while the bottom characteristic value  56  is calculated;  
      a fourth implementation in which both characteristic values  55  and  56  are measured in the assembly workshop prior to mounting the actuator on the engine; and  
      a fifth implementation in which both characteristic values  55  and  56  are measured after the actuator has been mounted on the engine.  
      In order to perform the step of preadjusting the armature, several variants are possible.  
      In a preferred implementation, the voltage across the sensor is measured while preadjustment is taking place and the equilibrium point is moved until the desired equilibrium point has been reached.  
      In an open-loop variant, the characteristic values and their average are determined, and then an initial voltage value is measured corresponding to the initial position of the equilibrium point. This voltage is measured after the actuator has been mounted on the engine, and after the spacer has been withdrawn and the pusher  12  has stabilized at the equilibrium point.  
      A position offset is then determined that corresponds to the difference between the initial value and the desired value. If the position sensor  21  is linear, then the position offset is proportional to the difference. The equilibrium point of the pusher  12  is then moved by turning the adjustment thumbwheel by an amount that corresponds to the desired offset.  
      The invention is not limited to the particular implementations described above, but on the contrary extends to cover any variant coming within the ambit of the invention as defined by the claims.  
      In particular, although in the implementations illustrated the sensor delivering the representative parameter is permanently mounted on the actuator, the sensor could be mounted on the actuator in detachable manner for the purpose of measuring the positions of the actuator member while performing the method of the invention.  
      Although the actuator member as described herein is constituted by a pusher, the invention also applies to an actuator in which the actuator member is a lever.  
      Although it is stated that when determining characteristic values by calculation each of the characteristic values is obtained as a mean of potential theoretical values of the representative parameter for the associated extreme position, it is also possible to combine the maximum and minimum theoretical values defining the range of uncertainty in which the theoretical values lie, e.g. by using a weighted mean with coefficients that take account of a statistical distribution for the theoretical values in the range of uncertainty.