Abstract:
A current measuring module for a starter device of an internal combustion engine has a measuring device which measures a starter current of a starter motor, a soft-iron core which at least partly surrounds a conductor carrying a starter current and carries a magnetic field sensor, and control electronics controlled by the magnetic field sensor and generating a control signal for switching off the starter motor when a switch-off current of the starter motor is reached.

Description:
BACKGROUND OF THE INVENTION 
     The invention is directed to a current measuring module for a starter device of internal combustion engines with a measuring device which measures a starter current of a starter motor. 
     It is known that internal combustion engines must be started by means of a starting mechanism because they cannot start by themselves. Starter motors are usually used for this purpose. These starter motors are connected with a voltage source via a starter relay constructed as an engagement relay, as they are called, and a pinion of the starter motor is simultaneously engaged with a toothed rim of a flywheel of the internal combustion engine for cranking. In order to switch on the starter relay, it is known to control this starter relay by means of an external switch, for example, an ignition switch or starter switch of the motor vehicle. After the internal combustion engine has begun to run independently, the starter motor must be disengaged to prevent noise and wear. It is known to switch off the starter manually by releasing the ignition switch or starter switch. Solutions for turning off the starter of the internal combustion engine automatically for increased convenience in motor vehicles are known. In order to detect independent running of the internal combustion engine, an evaluation of a starter current can be carried out. The fact that the starter current changes its shape depending on the independent running of the internal combustion engine is made use of for this purpose. When the internal combustion engine reaches its independent running rotational speed, i.e., develops its own torque, the starter motor is overtaken with respect to its rotational speed, wherein the starter motor is separated from the internal combustion engine by a freewheeling clutch. From this point, the starter motor need only apply its own acceleration torque, so that the starter current drops to the idle current or no-load current. Therefore, when the starter motor reaches the no-load current, this signals the independent running of the internal combustion engine. 
     It is known from general electronic engineering that a conductor carrying current is surrounded by a magnetic field proportional to the current. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention in a current measuring module, a soft-iron core is provided which at least partly surrounds a conductor carrying the starter current and carries a magnetic field sensor, and control electronics are controlled by the magnetic field sensor and generate a control signal for switching off the starter motor when a switch-off current of the starter motor is reached. 
     The magnetic field sensor can be arranged in a coaxial ring gap between the soft-iron core and the conductor, and the current-carrying conductor can be formed by a contact bolt of a starter relay of the starter device, or by a connection contact of the starter motor. 
     When the current measuring device is designed in accordance with the present invention it offers the advantage that an evaluation of the starter current is made possible in a simple manner. A simple evaluation of the starter current is made possible without direct intervention in the starter motor by providing a soft-iron core which at least partly surrounds a conductor carrying the starter current and carries a magnetic field sensor and by providing electronics which are controlled by the magnetic field sensor and which generate a control signal for switching off the starter motor when a switch-off current, especially a no-load current, of the starter motor is reached. In particular, structural changes in the construction of the starting device as a whole are not necessary because the current measuring module according to the invention can be adapted in a simple manner to existing starter devices. Moreover, there is no need for any changes to the existing electric connection lines of the starter device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described more fully in the following with reference to embodiment examples shown in the accompanying drawings. 
     FIG. 1 shows the shape of the starter current of a starter motor; 
     FIG. 2 shows a schematic view of a magnetic field surrounding a current-carrying conductor; 
     FIGS. 3 a  to  3   c  show schematic views of a current measuring module; 
     FIGS. 4 a  to  4   c  show a possible arrangement of the current measuring module at a starter device; and 
     FIG. 5 shows another possible arrangement of the current measuring module at a starter device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the curve of a starter current I of a starter motor of an internal combustion engine over time t. When the starter motor is switched on, the starter current I climbs to a maximum value (startup current) and then passes into a ripple area  10 . The ripple of the starter current I results from the alternating compression and decompression phases of the internal combustion engine during the starting phase. When the internal combustion engine achieves independent running, the starter current I passes into the no-load current I 0 . A switch-off current lying below the ripple area  10  is designated by I A . When the current falls below the switch-off current I A , it is certain that the internal combustion engine is running independently and the starter motor can be switched off. 
     FIG. 2 shows that a conductor  12  through which current I flows generates a magnetic field B. The magnetic field B is proportional to the current I. 
     FIGS. 3 a  to  3   c  show a current measuring module  14  by means of which the starter current I is measured by detecting the magnetic field B. The current measuring module  14  is shown in a front view (FIG. 3 a ), a side view (FIG. 3 b ) and a top view (FIG. 3 c ). The current measuring module  14  comprises a sleeve-shaped soft-iron core  16 . The soft-iron core  16  has an axial through-opening  18  which is preferably round and whose diameter is greater than an electric conductor  12  (not shown in FIG. 3) guided through the soft-iron core  16 . Accordingly, a coaxial air gap remains between the electric conductor  12  and the soft-iron core  16 . A magnetic field sensor  20  which is only indicated schematically is arranged in this air gap. The magnetic field sensor  20  can be, for example, a Hall sensor or a field plate, as it is called. The function of magnetic field sensors  20  is generally known and will not be discussed further within the framework of the present description. A magnetic field sensor  20  has electric connection contacts to which a signal voltage is applied depending on a magnetic field B acting on the magnetic field sensor, wherein the signal voltage is proportional to the magnetic field B. 
     The soft-iron core  16  is arranged on a base plate  22  made from a nonmagnetic, electrically nonconducting material which is made of plastic, for example. In order to arrange the soft-iron core  16  on the base plate  22 , injection molding can be carried out around the soft-iron core  16 , for example, with a plastic forming the base plate  22 , so that the corresponding holding area  24  and a casing  26  of the soft-iron core  16  is formed simultaneously in addition to the base plate  22 . The electronics for evaluating the signal voltage supplied by the magnetic field sensor  20  can be integrated in the base plate  22  at the same time. 
     FIG. 4 a  shows a side view of a starter device  28  for an internal combustion engine, not shown, of a motor vehicle. The starter device  28  comprises a starter motor  30  and a starter relay  32  constructed as an engagement relay. The starter motor  30  is connected with a motor vehicle battery of the motor vehicle by means of the starter relay  32  on the one hand and a pinion of the starter motor is engaged with the internal combustion engine on the other hand. The starter relay  32  has a contact space  34  within which a contact bridge connects two contact bolts  36  and  38  with one another. The contact bolt  36  is connected via an electric connection line, not shown, with the positive pole of the motor vehicle battery. The contact bolt  38  is lengthened in such a way that it can receive the current measuring module  14  on the one hand and a cable lug  40  on the other hand. The cable lug  40  is connected with a connection  44  projecting out of the starter motor  30  by an electric line  42  which is preferably constructed as a stranded wire. The current measuring module  14  is slid over the contact bolt  38  with its soft-iron core  16 . The arrangement of the current measuring module  14  and the cable lug  40  on the contact bolt  38  is locked by means of a fastener  46 , for example, a threaded nut. 
     The size of the base plate  22  of the current measuring module  14  is adapted to the design factors of the starter device  28 , so that already existing installation space can be utilized for receiving the current measuring module  14  without the need for structural changes to the starter device  28 . The base plate  22  of the current measuring module  14  has a greater edge length I than an axial extension a of the soft-iron core  16  as is shown in the top view in FIG. 3 c . In this way, the cable lug  40  can be located in area b which is given by the difference between edge length I and axial extension a. 
     As a result of the discovered arrangement of the current measuring module  14 , the starter current I flows over the contact bolt  36 , the contact bridge of the starter relay  32 , the contact bolt  38 , the cable lug  40 , the stranded wire  42  and the connection  44  to the starter motor  30  when the starter motor  30  is switched on. The soft-iron core  16  is incorporated in this electric connection path in that it surrounds the contact bolt  38  in some areas. Analogous to FIG. 2, the contact bolt  38  forms the electric conductor  12  which is surrounded by a magnetic field proportional to the starter current I. Corresponding to the magnetic field B detected by the magnetic field sensor  16 , a control signal is fed via lines, not shown in detail in FIG. 4 a , to control electronics which can be integrated, for example, in the base plate  22 . This control signal is proportional to the magnetic field B which is measured by the magnetic field sensor  16  and which is in turn proportional to the starter current I. Corresponding to the switch-off limit of the starter current I discussed with reference to FIG. 1, current dropping below the switch-off current I A  is detected. When the current falls below the value I A  the control electronics  48  provide a control signal for switching off the starter motor  30 . This control signal causes switching means connecting the starter relay  32  with a control voltage to open so that the contact bridge of the starter relay  32  separates the contact bolts  36  and  38 . 
     Finally, an automatic switching off of the starter device  28 , especially the starter motor  30 , is made possible without extensive structural effort by means of a simply constructed current measuring module when the internal combustion engine achieves independent running. The construction of the starter device  28  and the mounting of the starter device  28  in motor vehicles need not be changed in order to arrange the current measuring module  14 , so that the advantages of large-series manufacture with respect to cost are retained. If need be, a contact bolt  38  whose length is only increased by the axial extension a of the soft-iron core  16  is used. Likewise, the electric connection lines to the starter device  28  in motor vehicles need not be changed. Only an additional connection line from the control electronics  48  to a switch-off device of the starter device  28  is necessary. Further, the current measuring module  14  can be retrofitted in a simple manner by means of the discovered arrangement in motor vehicles which are already in operation. Moreover, it is also readily possible, if necessary, to exchange the current measuring module  14  in a simple manner without having to disassemble the entire starter device  28 . Due to its simple and generally applicable construction, the current measuring module  14  can be used in many different types of starter device  28 , so that it is not necessary to provide or stock different current measuring modules  14 . The dimensioning of the contact bolts, especially contact bolt  38 , is essentially identical in all of the starter devices  38  in use, so that it is not necessary to adapt the current measuring module  14 , especially the through-opening  18  of the soft-iron core  16 , to different starter devices  28 . 
     FIGS. 4 b  and  4   c  show different views of the arrangement of the current measuring module  14  on the contact bolt  38 . In particular, the front view according to FIG. 4 b  shows that the base plate  22  receiving the control electronics  48  can be integrated in a free installation space between the relay cover of the starter relay  32  and the starter motor  30 . Moreover, identical parts are provided with the same reference numbers and are not discussed further. 
     FIG. 5 shows another construction variant of the arrangement of a current measuring module  14  at a starter device  28 . Parts identical to those in the preceding Figures, especially FIG. 4 a , are provided with the same reference numbers and are not further described. 
     In the construction variant shown in this case, the current measuring module  14  is arranged at a housing  50 , especially a commutator cover  52 , of the starter motor  30 . In this way, the current measuring module  14  is arranged in the vicinity of the electric connection  44  of the starter motor  30 . The electric connection  44  comprises a busbar or conductor rail  54  which projects out of the interior of a pole pipe of the starter motor  12 . The electric connection line (stranded wire)  42  is fixedly connected, e.g., welded, to this conductor rail  54  in an electrically conducting manner by its cable lug  40 . For mounting purposes, the soft-iron core  16  can be slid over the conductor rail  54  projecting out of the starter motor  30  by means of the arrangement of the current measuring module  14  shown in FIG. 5, wherein the electrically conducting connection between the conductor rail  54  and the connection line  42  is produced subsequently. 
     The soft-iron core  16  accordingly projects axially from the base plate having the control electronics  48  and surrounds the conductor rail  54 . The magnetic field sensor  20 , not shown here, is arranged between the conductor rail  54  and the soft-iron core  16 . The conductor rail  54  accordingly forms the electric conductor designated by  12  in FIG.  2 . According to the embodiment example shown in FIG. 5, the soft-iron core  16  is not annular, but extends in an oval shape from the base plate  22 . This has no effect on the detection of the magnetic field B or, therefore, on the starter current I. The base plate  22  with the control electronics  48  can be fastened to the commutator cover  52 , for example, via suitable snap, plug or screw connections.