Abstract:
A brushless d.c. drive including a synchronous motor, including a multiphase armature winding, a switching device controlled by an electronic controller and connected upstream from the armature winding for commutation of the armature winding, and a device for generating a fail-silent response with simple circuitry measures and without any external components, and which includes a separating apparatus, arrangement or structure in the armature winding to respond in the event of a fault and separate the connections between the winding phases, including at the neutral point.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to a brushless d.c. drive. 
     BACKGROUND INFORMATION 
     Brushless permanent-field d.c. drives may be used in motor vehicles for a variety of purposes, including electric power-assisted steering. These d.c. drives have a synchronous motor, which may have a 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 discussed in German Published Patent Application No. 37 09 168. 
     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, if the synchronous motor operates as a generator against a low-resistance load impedance. In at least some applications, such a braking torque may have 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 may necessitate a considerable steering force being applied by the driver, which may be unacceptable. Accordingly, 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. 
     In an electric power-assisted steering system, a mechanical clutch, by way of which the output shaft of the synchronous motor acts on the steering gears, may be used to produce the desired fail-silent response. In the event of a fault, the clutch is opened to uncouple the motor from the steering system. 
     SUMMARY OF THE INVENTION 
     The exemplary brushless d.c. drive according to the present invention may have 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 may be greatly reduced. 
     According to an exemplary embodiment of the present invention, the separating apparatus, arrangement or structure for separating the connections between the winding phases of the armature winding may be activated by a control unit which detects a fault case. 
     According to another exemplary 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 apparatus, arrangement or structure. Such a design of the control unit with which faults occurring in the switching device are detected may have 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. This may be an advantage in the case of electric power-assisted steering systems, since sensors for measuring torques on the input and output shafts are already provided in the final control elements of the electric steering devices. 
     According to yet another exemplary embodiment of the present invention, the control unit in a star connection of the armature winding includes 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 may be detected, and the separating apparatus, arrangement or structure is activated accordingly. 
     According to other exemplary embodiments of the present invention, the separating apparatus, arrangement or structure may 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 an electronic or mechanical apparatus, arrangement or structure 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a circuit diagram of a brushless d.c. drive. 
     FIG. 2 shows a circuit diagram of a modified armature winding of the d.c. drive in FIG.  1 . 
     FIG. 3 shows a circuit diagram of the armature winding of the d.c. drive in FIG. 1, having a modified control unit for controlling separating apparatus, arrangement or structure for separating the armature winding. 
     FIG. 4 shows another exemplary embodiment of the circuit diagram of FIG.  2 . 
     FIG. 5 shows another exemplary embodiment of the circuit diagram of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     The brushless d.c. drive illustrated in FIG. 1 includes 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  and a rotor which rotates in the stator and has permanent magnetic poles. 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 . 
     Switching device  11 , designed as a B6 power inverter, has six semiconductor switches  15 , which may be 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 . 
     The brushless d.c. drive has a device for forcing a fail-safe silent response, which should ensure that in the event of a fault in the d.c. drive, possibly caused by a defective semiconductor switch  15  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 apparatus, arrangement or structure 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 and activates the separating means. In the exemplary embodiment according to FIG. 1, three measurement shunts  18  belong to control unit  17 , one being connected to each of three connecting lines  14  between switching device  11  and armature winding  12 . 
     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 apparatus, arrangement or structure and activates it. 
     In the embodiment according to FIG. 1, the separating apparatus, arrangement or structure acts 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 apparatus, arrangement or structure may be, for example, a pyrotechnic blasting capsule  19 , such as that used in motor vehicles to deploy airbags in the event of a crash. 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. 
     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 apparatus, arrangement or structure, which also act on neutral point  20 . In the exemplary embodiment illustrated in FIG. 3, the separating apparatus, arrangement or structure includes 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 . 
     Armature winding  12  of the synchronous motor may also be connected in a delta connection, for example, as illustrated in the circuit diagram in FIG.  2 . Winding phases  13  are connected to winding terminations  1 ,  2  and  3 . The separating apparatus, arrangement or structure for separating winding phases  13  in the event of a fault is integrated into winding phases  13  and connected in series with them. In the exemplary embodiment in FIG. 2, the response of the separating apparatus, arrangement or structure causes a reversible separation of armature winding  12 . To achieve the reversible separation, an electric switching contact  23 , which may be controlled by an electronic or mechanical apparatus, arrangement or structure, 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 electromagnetic relays, for example. 
     In the exemplary embodiment in FIG. 4, like the exemplary embodiment according to FIG. 1, the separating apparatus, arrangement or structure are arranged at neutral point  20  of armature winding  12 . When activated, the separating apparatus, arrangement or structure causes an irreversible separation of neutral point  20 . The separating apparatus, arrangement or structure includes 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 capable of destroying both holding elements  26  when deployed. As in the exemplary embodiment according to FIG. 1, blasting capsule  27  is connected by connecting line  15 S  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 opened, so that the connection of two winding phases  13  to neutral point  20  is interrupted suddenly. 
     FIG. 4 illustrates 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 in place. 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 . 
     In the exemplary embodiment according to FIG. 5, as in the exemplary embodiment according to FIG. 2, armature winding is connected in a delta connection. It is necessary in this exemplary embodiment 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 exemplary 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. A common blasting capsule  27  may also be used to destroy all three holding elements  26 . Prestressed switching contacts  25  having holding element  26  may be as described in conjunction with FIG.  4 . In the switching contacts  25  as prestressed spring tongues, separate compression springs  30  for opening switching contacts  25  may be omitted.