Patent Publication Number: US-11378029-B2

Title: Synchronisation method robust to engine stalling

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national phase of International Application No. PCT/EP2019/076005 filed Sep. 26, 2019 which designated the U.S. and claims priority to FR 1858886 filed Sep. 27, 2018, the entire contents of each of which are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a method for synchronizing an internal combustion engine based on the detection of the rising or falling edges of the teeth of a camshaft target, in order to determine the position of the engine. 
     The invention is particularly adapted to the implementation of a synchronization method that is effective against the stalling phases of the engine. 
     PRIOR ART 
     In order to determine the position of an internal combustion engine within the engine cycle, determining both the position of the engine crankshaft and of at least one engine camshaft is known. 
     To this end, at least two targets in the form of toothed wheels are securely mounted, respectively on the crankshaft and on a camshaft, and a respective sensor detects the edges of the teeth, respectively of each target, during the rotation of the crankshaft and of the camshaft. The detected data are subsequently processed in order to deduce the position of the engine. 
     With respect to the camshaft, it is the subject of a specific synchronization method that aims to identify each edge of the target detected by the sensor in order to deduce information therefrom that relates to the speed (engine speed in revolutions per minute) and the position of the engine, which information subsequently can be compared with the data relating to the position of the crankshaft in order to complete and/or correct said data. 
     This synchronization method is only performed by taking into account the information detected from the position of the camshaft target, i.e. without the data relating to the crankshaft, to allow the engine to operate in degraded mode if the crankshaft is faulty. 
     A conventionally implemented synchronization method involves determining, for each tooth edge of the target of the camshaft detected by the sensor, a time signature of this tooth edge, and comparing this signature with precomputed theoretical signatures of each edge of the target, through the consideration of a tolerance with respect to the value of the theoretical signature. 
     If the comparison does not result in any correspondence, the synchronization is not performed. 
     If the comparison results in a single correspondence, the synchronization is performed and the detected edge is identified as being that for which the theoretical signature corresponds to the time signature of the detected edge. 
     Finally, if the comparison results in several correspondences, the method is repeated for the following edge in order to refine the correspondence. 
     However, this type of synchronization method is not effective against all the situations experienced by the engines. 
     A first example is that of a reverse rotation of the engine, which occurs, for example, when the vehicle reverses with a gear engaged (for example, on a slope). 
     In this case, the signal measured by the sensor of the camshaft target can resemble a signal that would be measured if the vehicle advanced, and it can result in an erroneous identification of an edge of the camshaft target. 
     This is the case, for example, in  FIG. 1 a   , which at the top shows a curve of the engine speed as a function of time (which is negative in this case) and at the bottom shows the progress of the edges of the camshaft target in front of the sensor, with the crosses corresponding to edges identified during the implementation of the synchronization algorithm. The synchronization algorithm is configured to only detect a forward progression. However, in a first zone A1, about twenty consecutive false detections have been observed during the reverse rotation, and, in a second zone A2, about twenty other consecutive false detections have been observed, each time corresponding to a forward rotation, whereas in reality the engine is in reverse rotation. 
     In other words, in these zones a progression of the camshaft as a forward rotation is detected in error. 
     In this case, the information provided by the synchronization algorithm does not match the data originating from the analysis of the position of the crankshaft target, which can generate a fault in the engine computer or the undue detection of a fault in determining the position of the crankshaft. 
     In a case whereby the analysis of the position of the crankshaft also would be erroneous, the engine would operate in degraded mode only based on the signals of the camshaft. In this case, if a rotation is detected in error, an injection of fuel can be authorized and can damage the engine. 
     Another example is that of engine stalling, i.e. a phase close to engine shutdown where the engine performs multiple bounce-backs in one direction then the other before stopping. 
     The successive bounce-backs in this case can lead to, via the synchronization algorithm, the detection of edges very close to the camshaft target, and can give an impression of very high engine speed if the bounce-backs are not detected. The speed determined by the synchronization algorithm is then significantly different from the engine speed, which can be detected as compromising the safety of the vehicle and of its driver. The computer that computes the engine speed then can be considered to be defective, which can generate a breakdown involving the replacement of the engine computer. 
       FIG. 1 b    shows a case of engine speed bounce-back accompanied by false detections of the position of the crankshaft. The top of  FIG. 1 b    shows the engine speed, which, as can be seen, is alternatively negative and positive due to the bounce-back. 
     The bottom of  FIG. 1 b    shows a zone of four false detections of edges of the camshaft target. These detections occur while the engine is in a reverse rotation phase associated with the bounce-back. Once again, this false detection can generate a breakdown of the engine computer. 
     DISCLOSURE OF THE INVENTION 
     In view of the above, the aim of the invention is to at least partly overcome the disadvantages of the prior art. In particular, an aim of the invention is to propose a synchronization method that is effective against a case of engine stalling. 
     To this end, the aim of the invention is a method for synchronizing an internal combustion engine comprising:
         at least one camshaft, on which a target is mounted in the form of a toothed wheel, each tooth comprising a rising edge and a falling edge;   a position sensor for sensing the position of the camshaft, adapted to detect each rising or falling edge of a tooth of the target; and   a unit for processing data generated by the sensor;       

     the synchronization method being implemented by the processing unit and comprising, for each detected tooth edge, the implementation of the following steps:
         computing a time signature of the detected edge;   comparing the time signature of the detected edge with a set of theoretical signatures of edges of the target of the same rising or falling type as the detected edge, the comparison being implemented through a tolerance; and   generating a synchronization or synchronization fault signal as a function of the result of the comparison,       

     the synchronization method being characterized in that, when the engine speed drops below a predetermined threshold, the tolerance adopted for comparing the time signature of a detected edge with the theoretical signature of an edge of the target is reduced in relation to the tolerance adopted for the same comparison before the engine speed drops below said threshold. 
     In one embodiment, each theoretical signature is associated with a range of tolerance values defined as follows: 
     
       
         
           
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     where n is an index of the considered edge, τ th (n) is the theoretical signature of the index edge n and k is a tolerance parameter that is strictly greater than 1, 
     and the comparison of the time signature of a detected edge with a theoretical signature is implemented by determining whether the value of the time signature of the detected edge is included in the range of tolerance values associated with the theoretical signature. 
     Advantageously, the reduced tolerance is determined by a tolerance parameter k′ below the tolerance parameter k associated with the initial range of tolerance values, and preferably less than 30 to 50% of the value of the tolerance parameter k. 
     The engine speed can be determined by the processing unit based on information supplied by the detector when a synchronization is performed. 
     In one embodiment, the method further comprises, when the engine speed drops below a predetermined threshold, triggering a timer, and the range of tolerance values associated with each theoretical signature is reset to the corresponding initial range of tolerance values when the timer has elapsed and the engine speed is once again above the predetermined threshold, or when a synchronization fault signal is generated. 
     In one embodiment:
         a synchronization signal is generated if the time signature of the detected edge corresponds to the theoretical signature of a single edge of the target;   a synchronization fault signal is generated if the time signature of the detected edge does not correspond to any theoretical signature of the edges of the target with which it is compared; and   a synchronization fault signal is generated if a plurality of candidate edges corresponds to the detected edge n and, during the detection of a following edge n+1, only the theoretical signatures of the edges that follow the candidate edges that would correspond to the detected edge n are compared with the time signature of the following edge.       

     Advantageously, but optionally, the step of generating a synchronization or synchronization fault signal is also performed as a function of a preceding synchronization or synchronization fault signal transmitted by the processing unit. 
     For example, in the event of a loss of synchronization, the processing unit can be adapted to only transmit the next synchronization signal in the event of successive individual correspondences, a predetermined number N of times, between the time signatures of the following detected edges and the theoretical signatures of the edges of the target with which said time signatures of the following detected edges are compared. The number N is preferably strictly greater than 1, preferably equal to the number of edges of the target. 
     Preferably, the threshold engine speed is less than or equal to 600 revolutions per minute. 
     A further aim of the invention is a computer program product, comprising code instructions for implementing the synchronization method according to the previous description, when it is implemented by a computer adapted to implement the method described above. 
     A further aim of the invention is an internal combustion engine comprising:
         at least one camshaft, on which a target is mounted in the form of a toothed wheel, each tooth comprising a rising edge and a falling edge;   a position sensor for sensing the position of the camshaft, adapted to detect each rising or falling edge of a tooth of the target; and   a processing unit for processing signals from the detector, configured to implement the synchronization method according to the previous description.       

     The proposed synchronization method makes provision for reducing the range of tolerances associated with a theoretical signature of an edge of the camshaft target when the engine speed drops below a predetermined threshold. 
     Indeed, stalling occurs in the phase of stopping the engine from a normal operating phase, i.e. when the engine speed decreases. Reducing the range of tolerances therefore allows the risks of erroneous synchronization to be reduced during stalling. 
     Furthermore, this reduced tolerance range is advantageously implemented during a time period triggered from the moment at which the engine speed drops below the predetermined threshold, or up to a loss of synchronization, corresponding to effective stalling of the engine. Afterwards, the tolerance is reset to its initial value to allow effective resynchronization when restarting the engine. This therefore ensures that in any case the engine leaves a stalling situation or a low speed situation before resetting the tolerance to its initial value. Indeed, as the synchronization is performed by identifying edges by elimination, the edges for which the signatures are outside tolerances are eliminated and having a higher tolerance makes the synchronization more effective. In summary, a reduced tolerance allows a loss of effective synchronization, and an enhanced tolerance allows an effective synchronization (or resynchronization). 
     Finally, advantageously several identifications of edges are necessary before confirming the resynchronization to avoid an erroneous synchronization when the tolerance range is reset to its initial value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, aims and advantages of the invention will become apparent from the following description, which is purely illustrative and non-limiting, and which must be read with reference to the appended figures, in which: 
         FIG. 1 a   , already described, shows a case of an error of a synchronization algorithm of the prior art in the event of reverse rotation of the engine; 
         FIG. 1 b   , also already described, shows a case of an error of a synchronization algorithm of the prior art in the event of engine stalling; 
         FIG. 2 a    schematically shows an example of an internal combustion engine, in which the synchronization algorithm can be implemented; 
         FIG. 2 b    schematically shows an engine computer; 
         FIG. 2 c    shows an example of a camshaft target; 
         FIG. 3  schematically shows the main steps of the synchronization method according to one embodiment of the invention; 
         FIG. 4  schematically shows the implementation of the method according to one embodiment of the invention in the form of a flow chart. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
       FIG. 2 a    schematically shows an internal combustion engine M comprising a set of movable pistons  80  moving in respective cylinders  82  between a top dead centre and a bottom dead centre, the engine M also comprising a crankshaft  9  driven by the movement of the pistons in the cylinders by means of respective connecting rods  84 . 
     The crankshaft rotates, by means of a timing belt  90 , at least one camshaft  91 , the rotation of which successively causes the intake and exhaust valves  92  to open and close. 
     In one embodiment (not shown), the engine M can comprise two camshafts  91  comprising a camshaft, called intake camshaft, the rotation of which allows the intake valves to be opened and closed, and a camshaft, called exhaust camshaft, the rotation of which allows the exhaust valves to be opened and closed. 
     The crankshaft  9  comprises a toothed wheel  93  comprising a set of teeth evenly distributed over its circumference. A crankshaft angular position sensor  94  is positioned facing the toothed wheel  93  and is adapted to detect the passage of each tooth of the wheel and to deduce an angular position of the crankshaft therefrom. 
     A target in the form of a toothed wheel  1  is mounted on the camshaft  91  or on each camshaft, an example of which target is shown in  FIG. 2 c   . The target  1  comprises a set of teeth distributed over its periphery, with each tooth comprising a rising edge and a falling edge. The teeth of the target are advantageously uneven to allow the individual identification of each edge from among the set of edges of the target. 
     A sensor  2  for sensing the position of the camshaft (for example, of the Hall effect cell, magneto-resistive cell type, etc.) is positioned in front of the toothed wheel and is adapted for detecting each rising or falling edge of a tooth of the target. 
     With reference to  FIG. 2 b   , the engine M also comprises an engine computer  95  comprising a processing unit  21  comprising, for example, a processor  22  or a microcontroller and a memory  23 , the processing unit being configured to implement, on the basis of the raw signals of rising or falling edges detected by the sensor  2 , or optionally of signals preprocessed by the sensor (in the case of sensors called active sensors), a synchronization method that will be described in further detail hereafter, and for which the code instructions for its execution are stored in the memory  23 . 
     In order to implement the synchronization method, the processing unit  21  is advantageously configured to generate, based on the data from the detector, an external synchronization variable Vsyn, which can assume a value indicating a synchronization (Vsyn=Synok) and a second value indicating a synchronization fault (Vsyn=Wtsyn). The synchronization variable is set, during engine start up, to the value Wtsyn indicating a synchronization fault. 
     An external variable is understood to be a variable intended to be transmitted by the processing unit to other components or functional blocks  950  of the engine computer  95  for implementing methods requiring knowledge of the position of the camshaft, for example, the injection of fuel, the ignition, the variable distribution, etc. On the contrary, an internal variable will be subsequently called a variable that is only used in an algorithm executed by the processing unit and that is not transmitted to the other blocks of the engine computer. 
     The processing unit  21  also generates another external variable Idft representing the edge of the target that has been identified as corresponding to the edge detected by the detector. 
     The engine computer  95  advantageously comprises other processing modules  950  adapted for receiving the angular position signals of the crankshaft  9 , as well as the external variables generated by the processing unit  21 , and to deduce therefrom a state of the engine cycle at each instant and to implement control methods, for example, injection and ignition of the fuel. 
     Synchronization Method 
     With reference to  FIGS. 3 and 4 , a synchronization method will now be described that is implemented by the processing unit of the position sensor for sensing the position of a camshaft, upon each detection of a tooth edge by the detector. 
     During a first step  110 , a time signature of the edge is computed. 
       FIG. 2 c    shows an example of a camshaft target and at the top it shows the corresponding signal generated by the detector. The normal direction of rotation of the target is indicated by the arrow. In the upper part of the figure, the detection of a rising edge of the target corresponds to a falling edge of the electrical signal. 
     In one embodiment, the time signature of a detected edge is defined by:
         for the second and the third detected edge:       

     
       
         
           
             
               
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     where n is the index of a detected edge and T n  is the duration of the tooth (or of the hollow) preceding the edge n, i.e. the elapsed time between the detection of the edge n−1 and the detection of the edge n. 
     In this embodiment, the time signature can be computed from the third detected edge. 
     In an alternative embodiment, the time signature of a detected edge is defined by: 
     
       
         
           
             
               
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     In this embodiment, the time signature can only be computed from the fifth detected edge. 
     The selection between these two embodiments is set for a given engine and depends on the number of edges on the target and/or on the shape of the teeth. For example, the first method is preferably used if the target comprises a few teeth or if several teeth are identical. The second method is used for the other cases, since it is more effective in cases of acceleration and deceleration. 
     During a step  120 , the time signature of the detected edge is compared to a theoretical signature, precomputed and recorded in the memory  23 , of at least one edge of the target of the same type as the detected edge. Advantageously, during a first iteration of step  120 , the time signature of the detected edge is compared to the theoretical signatures of all the edges of the target of the same type as the detected edge. As described in further detail hereafter, during the following iterations of step  120 , this comparison can only occur for some of the edges of the target. 
     As previously indicated, the teeth of the target are advantageously uneven so that the theoretical signature of an edge can allow the edge to be identified. The theoretical signature of an edge is not necessarily unique, but identification can be possible by adding the type of edge (rising or falling) and optionally by also adding a constraint on the sequence. For example, two theoretical signatures can be found with the same value but corresponding to two different types of edges, so that a single theoretical signature does not correspond to a detected edge. 
     According to another embodiment, there can be two theoretical signatures with the same value, but followed (for the following edge, for a considered direction of rotation) by two different theoretical signatures. It is then possible to identify the edge by elimination. 
     In a first embodiment, the theoretical signature is defined by: 
                 τ     t   ⁢   h       ⁡     (   n   )       =       α   n       α     n   -   1               
where α n  is the angle between the index edge and the previous edge (some angles are shown in  FIG. 2 c    considering an edge z). The edges preceding the considered edge are not the same depending on whether the target is considered to be in forward rotation or in reverse rotation, which explains the computation of one theoretical signature for each direction of rotation.
 
     The theoretical signature of an edge of the target in reverse rotation also can be seen as the theoretical signature of the same edge of the reversed target (or seen in a mirror) in forward rotation. 
     This embodiment is retained if the time signature of an edge is computed according to the first equation indicated above: 
     
       
         
           
             
               
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     As an alternative embodiment, the theoretical signature of an edge is computed using the following equation: 
     
       
         
           
             
               
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     This alternative embodiment is implemented in the event that the time signature is only computed from the fifth detected edge as follows: 
     
       
         
           
             
               
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     Thus, a theoretical signature of the edge, as well as the type of edge, either rising or descending, is stored in the memory  23  for each edge. 
     Advantageously, in order to compare the time signature of the detected edge with the theoretical signatures of the edges of the same type of the target, a tolerance range is provided for each theoretical signature. 
     This tolerance range is defined, for each theoretical signature of an edge τ th (n) by: 
               [           τ     t   ⁢   h       ⁡     (   n   )       k     ,         τ   th     ⁡     (   n   )       ·   k       ]     ,         
where k is a tolerance factor that is strictly greater than 1, advantageously ranging between 2 and 3, for example, ranging between 2 and 2.5.
 
     The comparison of the time signature of the detected edge with a theoretical signature of an edge is performed by determining whether the time signature of the detected edge is included in the tolerance range. 
       FIG. 3  shows a step  121  for distinguishing the series of steps as a function of the number of edges of the target corresponding to the detected edge, i.e. for which the tolerance range associated with the theoretical signature contains the time signature of the edge. In  FIG. 3 , “Y” means yes and “N” means no. 
     If, on completion of step  120 , the detected edge does not correspond to any theoretical signature of an edge of the target of the same type, i.e. the time signature of the detected edge is not included in any tolerance range of the theoretical signatures of the edges of the target of the same rising or falling type, the method comprises a step  130  where the detected edge has not been identified, and the external synchronization variable assumes the value WtSyn. The method subsequently resumes at step  110  for the following detected edge. As an alternative embodiment, the method may only resume at step  110  after the detection of three or five edges, depending on the mode for computing time and theoretical signatures, so as not to retain the preceding detection times for which no edge has been identified. 
     If, on completion of step  120 , the detected edge corresponds to a single edge of the target of the same type (i.e. the time signature of the detected edge is included in the tolerance range of the theoretical signature of an edge of the same type), the method comprises a step  140  where the detected edge is identified as that for which the theoretical signature corresponds to the time signature of the edge, and the external synchronization variable assumes the first value Synok. The processing unit also returns a signal identifying the detected edge. The method subsequently resumes at step  110  for the following detected edge. In a particular embodiment, during the following iteration of step  120 , the time signature of the detected edge may only be compared with a single theoretical signature, which is that of the edge following that which was previously identified. In the absence of correspondence, the external synchronization variable assumes the value WtSyn (step  130 ). 
     If, on completion of step  120 , the detected edge corresponds to a plurality of candidate edges of the target, i.e. the time signature of the detected edge is included in the tolerance range of a plurality of theoretical signatures of edges, the external synchronization variable assumes the second value WtSyn and steps  110  and  120  are implemented again for the following edge, by only using, for the comparison of step  120 , the edges that immediately follow the candidate edges. Steps  110  and  120  can be repeated until a unique correspondence  140  has occurred, or until no correspondence  130  has occurred, in which case steps  110  and  120  are again implemented normally from the following edge. 
     Advantageously, in order to be able to make the synchronization method effective against an engine stalling phase, the implementation of step  120  of comparing the time signature of the detected edge with the theoretical signatures of the edges of the target takes into account the engine speed. Indeed, an engine stalling phase generally occurs shortly before the engine stops, and therefore generally during a reduction in the engine speed. 
     Consequently, at the same time as the synchronization method described above is implemented, the engine speed is monitored so that, if the engine speed drops below a predetermined threshold, the comparison of the time signature of an edge detected with the theoretical signatures of all the edges of the target, is advantageously implemented with a reduced tolerance range compared to the tolerance range described above in the standard case. 
     To this end, advantageously in the memory of the processing unit, each edge is associated with a tolerance range, called standard range, and a tolerance range, called reduced range, with either one being selected as a function of the development of the engine speed. 
     For the reduced tolerance range, the tolerance factor k′ is strictly less than the tolerance factor k introduced above. For example, the tolerance factor k′ is advantageously 30 to 50% less than the tolerance factor k of the standard tolerance range. 
     The engine speed threshold, below which the tolerance range is reduced, is less than the idling speed for the considered engine. Advantageously, it is less than or equal to 600 revolutions per minute. 
       FIG. 4  schematically shows the implementation of the monitoring of the engine speed  200  at the same time as the implementation of the synchronization method. In  FIG. 4 , Y means yes and N means no. 
     Advantageously, the engine speed information is obtained by the processing unit  21  during a synchronization phase, based on data relating to the position of the camshaft. Indeed, the progression speed of the edges of the camshaft allows a rotation speed, and therefore an engine speed, to be deduced therefrom. 
     A first step  210  involves determining whether the engine speed drops below the predetermined threshold. 
     If so, during a step  230 , the tolerance factor applied to the tolerance range of the theoretical signature of an edge becomes the tolerance factor k′. 
     Advantageously, a timer is also triggered during a step  220 , so that the tolerance factor remains at the reduced level (k′) until the timer has elapsed and the engine speed is again above the threshold, or until a loss of synchronization has effectively occurred (step  130 ). A step  240  of verifying these conditions is shown in  FIG. 4 . If these conditions are verified, then the tolerance factor again assumes the standard value (k) in step  250 . Otherwise, the tolerance factor is kept at the reduced level (k′). 
     The duration of the timer is advantageously determined during a preliminary calibration step (not shown), so as to exceed the average duration of a stalling phase from the moment at which the engine speed drops below the predetermined threshold. 
     This timer allows a reduced tolerance state to be maintained throughout the entire stalling period to avoid incorrect synchronization during this period. 
     With further reference to  FIG. 3 , in one embodiment, once a loss of synchronization has occurred (i.e. when the variable Vsyn has transitioned from the value SynOk to WtSyn), the recovery of the synchronization is only performed when a sufficient number of consecutive edges has been identified (i.e. that a single correspondence  140  has been found). 
     To this end, a counter cpt is installed, for example, at an initial value N, and, during the implementation of the synchronization method on the following edges, in the event that on completion of this step  120  of comparing between the time signature of the detected edge and the theoretical signatures of the edges of the target, a single edge of the target corresponds to the detected edge ( 140 ), the change of value of the external synchronization variable Vsyn depends on the value of the counter. 
     If the counter has a non-zero value, then it is decremented during a step  320 , but the external synchronization variable retains the synchronization fault value WtSyn. 
     It only again assumes the synchronization value Synok (step  140 ) when the value of the counter becomes zero, i.e. only when a plurality of edges has been successively detected. The counter is reset (not shown) when the external synchronization variable assumes the value Synok or when no edge is identified (step  130 ). 
     The initial value N of the counter is greater than or equal to 1, preferably strictly greater than 1, for example, equal to the number of edges of the target. This counter is used to validate that the engine has effectively exited a stalling phase, before confirming the synchronization. 
     As an alternative embodiment, the counter cpt can be set to 0 and be incremented until it reaches the maximum value N leading to the recovery of the synchronization.