Abstract:
In a brushless d.c. drive which has a synchronous motor having a multiphase armature winding ( 12 ) and has a switching device ( 11 ) controlled by an electronic controller ( 16 ) and connected upstream from the armature winding ( 12 ) for commutation of the armature winding ( 12 ) and has a device for generating a fail-silent response with simple circuitry measures and without any external components, separating means ( 19 ) are provided in the armature winding ( 12 ) to respond in the event of a fault and separate the connections between the winding phases ( 13 ), preferably in the neutral point ( 20 ).

Description:
BACKGROUND INFORMATION  
         [0001]    The present invention is directed to a brushless d.c. drive according to the definition of the species of claim 1.  
           [0002]    Brushless permanent-field d.c. drives are used in motor vehicles for a variety of purposes, including electric power-assisted steering. These d.c. drives have a synchronous motor having a preferably star-connected stator winding or armature winding and a permanent-field rotor. The armature winding is connected to the direct voltage network by a converter in a bridge circuit having six semiconductor power breakers. The power inverter which causes commutation of the armature winding is controlled by an electronic controller. An example of a synchronous motor operated on a direct voltage network is described in German Patent Application 37 09 168 A1.  
           [0003]    If faults occur in the armature winding and/or in the power breakers, the d.c. drive may generate a permanent electromagnetic braking torque without a direct voltage being applied, because now the synchronous motor operates as a generator against a low-resistance load impedance. In many applications, such a braking torque has a negative effect on the functioning of the unit or system in which the d.c. drive is used. For example, in the case of electric power-assisted steering systems, the braking torque which occurs in the event of a fault necessitates a considerable steering force being applied by the driver, which is unacceptable. It is therefore known that devices can be provided on such a d.c. drive to lead to a fail-silent response of the d.c. drive in the event of a fault, i.e., the d.c. drive does not have any interfering or negative effect on the unit or system, so the latter functions as if the drive were not present.  
           [0004]    In the case of a known electric power-assisted steering system, a mechanical clutch, by way of which the output shaft of the synchronous motor acts on the steering gears, is used to produce the desired fail-silent response. In the event of a fault, the clutch is opened and thus the motor is uncoupled from the steering system.  
         ADVANTAGES OF THE INVENTION  
         [0005]    The brushless d.c. drive according to the present invention having the features of Patent claim 1 has the advantage that the desired fail-silent response of the d.c. drive is achieved without any expensive external components, such as mechanical clutches, with simple circuitry measures in the drive itself. Thus, the d.c. drive becomes more compact and requires less space, so that it can be used in a more versatile manner. The additional cost incurred for the desired response of the d.c. drive in the event of a fault is greatly reduced.  
           [0006]    Advantageous refinements of and improvements on the d.c. drive characterized in Patent claim 1 are possible through the measures characterized in the additional claims.  
           [0007]    According to a preferred embodiment of the present invention, the separating means for separating the connections between the winding phases of the armature winding can be activated by a control unit which detects a fault case.  
           [0008]    According to an advantageous embodiment of the present invention, the control unit has for this purpose measurement shunts in each connecting line between the armature winding and the switching device designed as a bridge circuit having semiconductor switches. In simultaneous blocking phases of all semiconductor switches, the electric currents flowing through the measurement shunts are measured, and in the event of a current value which differs significantly from zero in one of the measurement shunts, the control device delivers an activation signal to the separating means. Such a design of the control unit with which faults occurring in the switching device are detected has the advantage that the measurement shunts already present in the d.c. drive for measuring the current for other reasons can also be used to detect the fault case, thus further reducing the complexity of the circuitry. Faults in the armature winding itself can be detected, for example, by measuring the braking torque delivered to the output shaft of the synchronous motor, which is an advantage in the case of electric power-assisted steering systems, because sensors for measuring torques on the input and output shafts are already provided in the final control elements of the electric steering devices.  
           [0009]    According to an advantageous embodiment of the present invention, the control unit in a star connection of the armature winding has measurement shunts, each connecting a winding phase of the armature winding to the neutral point. The control unit continuously measures the amount and phase of currents flowing through the measurement shunts and adds the shunt currents as vectors. In the event of a significant deviation in the result of this addition from zero, the control unit delivers an activation signal to the separating means. With such a control unit, faults in the semiconductor switching device as well as faults in the armature winding are detected, and the separating means are activated accordingly.  
           [0010]    According to advantageous embodiments of the present invention, the separating means may be designed in such a way that they cause a reversible or irreversible separation of the connections between the winding phases of the armature winding. An irreversible separation can be brought about by way of pyrotechnic blasting charges or by fusible cutouts. For reversible separation, electric contacts controllable by electronic or mechanical means are used. In the case of armature windings in a star connection, the neutral point is separated, but in the case of armature windings in a delta connection, each winding phase must be separated from the winding terminations. 
       
    
    
     DRAWING  
       [0011]    The present invention is explained in greater detail in the following description on the basis of embodiments illustrated in the drawing, showing:  
         [0012]    [0012]FIG. 1 a circuit diagram of a brushless d.c. drive,  
         [0013]    [0013]FIG. 2 a circuit diagram of a modified armature winding for the d.c. drive in FIG. 1,  
         [0014]    [0014]FIG. 3 a circuit diagram of the armature winding of the d.c. drive in FIG. 1, having a modified control unit for controlling separating means for separating the armature winding,  
         [0015]    [0015]FIGS. 4 and 5 each show the same diagram as in FIG. 2 according to two additional embodiments. 
     
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0016]    The brushless d.c. drive illustrated in the block diagram in FIG. 1 has a synchronous motor operated by a switching device  11  for electronic commutation on a direct voltage source  10 . The synchronous motor, shown here with only its stator winding or armature winding  12 , has a stator which holds armature winding  12  in a known manner and a rotor which rotates in the stator and has permanent magnetic poles.  
         [0017]    Armature winding  12 , which is designed in three phases, has three star-connected winding phases  13  in the embodiment illustrated in FIG. 1, their terminations  1 ,  2  and  3  being connected to switching device  11  by connecting line  14 .  
         [0018]    Switching device  11 , designed as a B 6  power inverter, has six semiconductor switches  15 , preferably MOS-FETS, arranged in a bridge circuit. Connecting lines  14  leading to winding terminations  1 ,  2  and  3  are each connected to taps  4 ,  5  and  6  of a bridge branch formed by a series connection of two semiconductor switches  15 , which is in the connection of two semiconductor switches  15 . For commutation of armature winding  12 , i.e., for applying winding phases  13  to direct voltage source  10  in the correct order, semiconductor switches  15  can be controlled by an electronic controller  16 .  
         [0019]    The brushless d.c. drive has a device for forcing a fail-silent response, which ensures that in the event of a fault in the d.c. drive, possibly caused by a defective semiconductor switch  15 , for example, or by a winding termination in armature winding  12 , this does not interfere with or have a negative effect on the system working with the d.c. drive. This device includes separating means which, in the event of a fault, separate the connections between winding phases  13  and a control unit  17 , which is integrated into controller  16  and, in the event of a fault, detects the fault case on the one hand while on the other hand also activating the separating means. In the embodiment according to FIG. 1, three measurement shunts belong to control unit  17 , one being connected to each of three connecting lines  14  between switching device  11  and armature winding  12 .  
         [0020]    In time intervals during which all semiconductor switches  15  are blocked, control unit  17  measures the shunt currents flowing over measurement shunts  18 . If all semiconductor switches  15  are intact, each shunt current is zero. If control unit  17  measures a value which differs significantly from zero in one of measurement shunts  18 , it generates an activation signal which is delivered to the separating means and activates them.  
         [0021]    In the embodiment according to FIG. 1, the separating means act on neutral point  20  of armature winding  12 , causing an irreversible separation of the neutral point connection of winding phases  13  when activated. The separating means here are designed, for example, as a pyrotechnic blasting capsule  19 , such as that used in motor vehicles to deploy airbags in the event of a crash, for example. Electrically ignitable blasting capsule  19  is connected first to control unit  17  by way of a connecting line  40  and second to the negative potential of direct voltage source  10 . If one of measurement shunts  18  delivers a current value differing significantly from zero, control unit  17  generates an electric firing pulse which ignites blasting capsule  19 . The exploding blasting charge ruptures neutral point  20 , thus separating winding phases  13  from one another. In this way, the in-system d.c. drive, which is driven by the system by way of its output shaft in the event of a fault, cannot generate a braking torque because separated armature winding  12  does not allow generator operation.  
         [0022]    With control unit  17  described in conjunction with FIG. 1, only faults based on defects in semiconductor switches  15  can be detected. To also detect possible faults occurring in armature winding  12 , control unit  17  according to FIG. 3 is modified so that measurement shunts  18  present in feeder lines  14  are eliminated, and instead measurement shunts  21  are arranged between neutral point  20  and each winding phase  13 . Control unit  17  measures the amount and phase of electric currents flowing over measurement shunts  21  and adds them as vectors. In a fault-free d.c. motor, the result of this addition is always zero. If the vector sum differs significantly from zero, control unit  17  in turn generates an activation signal for the separating means, which here are also acting on neutral point  20 . In the embodiment illustrated in FIG. 3, the separating means have a fusible cutout  22  which is heated briefly on activation by control unit  17  so that it melts through and thus separates neutral point  20 . A heater coil  24  connected to direct voltage source  10  by way of a power breaker controlled by control unit  17  is used to heat fusible cutout  22 .  
         [0023]    Armature winding  12  of the synchronous motor may of course also be connected in a delta connection, for example, as illustrated in the circuit diagram in FIG. 2. Winding phases  13  here are connected to winding terminations  1 ,  2  and  3 . The separating means for separating winding phases  13  in the event of a fault are integrated into winding phases  13  and connected in series with them. In the embodiment in FIG. 2, the response of the separating means causes a reversible separation of armature winding  12 . To this end, an electric switching contact  23  which can be controlled by electronic or mechanical means is arranged between winding terminations  1 ,  2  and  3  and winding phases  13 . Electronically controllable switching contacts  23  are implemented by transistors or thyristors, for example, and mechanically controllable switching contacts  23  may be designed as electromagnetic relays, for example.  
         [0024]    In the embodiment in FIG. 4, like the embodiment according to FIG. 1, the separating means are arranged at neutral point  20  of armature winding  12 ; when activated, they cause an irreversible separation of neutral point  20 . The separating means have two switching contacts  25  which are preloaded in the direction of opening and are each held in the closed position by a holding element  26 . A switching contact  25  having a holding element  26  is arranged between neutral point  20  and the end of the winding of each of two winding phases  13 . It is not necessary to provide a third switching contact having a holding element between neutral point  20  and third winding phase  13 . A common electrically ignitable pyrotechnic blasting capsule  27  is provided for both holding elements  26  and is designed so that it is capable of destroying both holding elements  26  when deployed. As in the embodiment according to FIG. 1, blasting capsule  27  is connected by connecting line  40  to control unit  17  which applies an electric firing pulse to blasting capsule  27  in the event of a fault. With destruction of holding elements  26 , prestressed switching contacts  25  are released and they open, so that the connection of two winding phases  13  to neutral point  20  is interrupted suddenly.  
         [0025]    [0025]FIG. 4 schematically shows a structural embodiment for two switching contacts  25  which are prestressed in the direction of opening and have a holding element  26  and a common blasting capsule  27  for holding elements  26 . Each switching contact  25  has a contact plate  28  fixedly connected to an operating pin  29 . Axially displaceable operating pin  29  is loaded by a compression spring  30  which is supported on a spring plate  31  connected to operating pin  29  and on a stationary stop  32  and prestresses operating pin  29  so that contact plate  28  is lifted up from contact points  33 ,  34 . Both holding elements  26  have a common lock block  35  in which both operating pins  29  engage, each with a locking projection  36  provided on its end which faces away from contact plate  28 . When ignited, blasting capsule  27 , which is arranged inside lock block  35 , destroys lock block  35 . In assembly, switching contacts  25  are closed by pressing contact plate  28  against contact points  33 ,  34  with tensioning of compression springs  30 , so that locking projection  36  falls into lock block  35  and is held there. In the case of a fault, blasting capsule  27  is ignited by control unit  17 . This destroys lock block  35 , thus releasing operating pins  29 , and prestressed compression springs  30  lift contact plates  28  away from contact points  33 ,  34 .  
         [0026]    In the embodiment according to FIG. 5, as in the embodiment according to FIG. 2, armature winding  12  is connected in a delta connection. It is necessary here for each branch of the delta connection to be separated in the event of a fault, so that a switching contact  25  having a holding element  26  is connected to each winding phase  13  in series. In the embodiment according to FIG. 5, a separate blasting capsule  27  is provided for each holding element  26 , destroying holding element  26  when deployed, so that switching contact  25  which is prestressed in the closing direction opens automatically. It is of course also possible to use a common blasting capsule  27  to destroy all three holding elements  26 . Prestressed switching contacts  25  having holding element  26  may be designed as described in conjunction with FIG. 4. In the design of switching contacts  25  as prestressed spring tongues, separate compression springs  30  for opening switching contacts  25  may be omitted.