CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is the U.S. National Phase Application of PCT International Application No. PCT/EP2012/059594, filed May 23, 2012, which claims priority to German Patent Application No. 10 2011 076 406.2, filed May 24, 2011, the contents of such applications being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method for operating a brushless electric motor, the windings of which are controlled by an AC converter with the aid of six switches, wherein the AC converter comprises three outputs that are allocated to the windings of the electric motor and wherein a power semiconductor switch is arranged in each case between the outputs of the AC converter and the windings and wherein a detection unit for detecting defective switches, a unit for measuring the voltage at the outputs of the AC converter and a microcontroller for controlling the switches and the power semiconductor switches is provided. The present invention further relates to a device for operating a brushless electric motor. 
       BACKGROUND OF THE INVENTION 
       [0003]    AC converters for controlling brushless motors generally comprise six switches. In the event of a defect, each switch can comprise in principle two different characteristics: non-conductive, in other words in the opened switching position, the switch performs a blocking function; or conductive, in other words in the closed switching position, the switch performs a blocking function. A conductively defective switch is also colloquially known as a short circuit. It is then particularly important in applications where safety is concerned to switch the electric motor very quickly into an emergency mode or to switch said electric motor off immediately. 
         [0004]    In contrast to mechanical relays, the use of power semiconductor switches is encumbered with the disadvantage that as inductive loads are switched off, depending on the magnitude of the load current and of the inductivity of the load, it is possible to release a quantity of energy of such a magnitude that the power semiconductor switch is destroyed at the moment an inductive load is switched off and as a consequence said power semiconductor switch can no longer fulfill its role. It is necessary to avoid damage to the power semiconductor switches, especially when using power semiconductor switches for brushless motors in applications where safety is critical and where it is necessary to switch off the motor in the power circuit in order to provide a protective function. One application where safety is critical is the use of a brushless electric motor in an electromechanical steering arrangement of a motor vehicle. 
         [0005]    For applications of this type where the inductive energy in the load circuit exceeds the admissible absorption capability of a power semiconductor switch, it is known to use instead a mechanical relay that owing to its construction comprises a considerably greater absorption capability in comparison with power semiconductor switches currently available on the market. A further possibility resides in diverting the amount of energy that arises whilst switching off an inductive load into an energy sink such as by way of example a suppressor diode that relieves the load on the power semiconductor switch. 
       SUMMARY OF THE INVENTION 
       [0006]    An aspect of the present invention provides a method and a device wherein damage to the semiconductor switches is avoided whilst switching off inductive loads. 
         [0007]    This is achieved by virtue of the features of the independent claims. The AC converter is switched off following the detection of a defective switch by the microcontroller in such a manner that power is no longer introduced into the windings of the electric motor and that the power semiconductor switches are opened in succession in a previously defined motor position angle. 
         [0008]    Advantageous developments are evident in the subordinate claims. It follows from this, that it is provided in a preferred embodiment of the method in accordance with the invention that the motor position angles are allocated in each case to a winding and to the power semiconductor switch thereof and are selected in such a manner that the allocated power semiconductor switches are not damaged during the opening process. For this purpose, the motor position angles are selected so that the drain current prevailing at the power semiconductor switches is zero, almost zero or negative. 
         [0009]    It is provided that the instantaneous values of the drain currents that allow inductive loads to be switched off without damaging the power semiconductor switches is deterministically dependent upon the motor angle, which instantaneous values are prevailing in the the at the power semiconductor switches. It is significant that it is not absolutely necessary for the method to monitor the phase currents directly using measuring technology, because the information required for the purpose can be drawn from the measured motor angle. 
         [0010]    The aspect is also achieved by means of a device in which the microcontroller is designed so as to switch off the AC converter after the detection of a defective switch by the microcontroller in such a manner that power is no longer introduced into the windings of the electric motor and that the microcontroller is further designed so as to open the power semiconductor switches in succession in a previously determined motor position angle. 
         [0011]    In a particularly advantageous development of the subject matter of the invention, the microcontroller is designed so as to allocate the motor position angles in each case to a winding and the power semiconductor switches of said winding and to select said motor position angles so that the allocated power semiconductor switches are not damaged during the opening process. The microcontroller is designed so as to select the motor position angles so that the drain current that is prevailing at the power semiconductor switches is zero, almost zero or negative. 
         [0012]    It is provided that the power semiconductor switches are embodied as power MOSFET transistors. 
         [0013]    A further advantageous development of the subject matter of the invention provides that the unit for detecting defective switches and the unit for measuring the voltage at the outputs of the AC converter are integrated into the microcontroller. 
         [0014]    A motor position angle sensor is provided for determining the motor position angle in a first alternative. 
         [0015]    In a second alternative, current comparators are provided that monitor the voltage drop across the power semiconductor switches. The microcontroller determines the motor position angles with the aid of this information. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention is best understood from the following detailed description when read in connection with the accompanying drawings. Included in the following figures: 
           [0017]      FIG. 1  illustrates a schematic circuit diagram of the windings of a brushless electric motor, an AC converter as well as the power semiconductor switches as a safety switching means in a configuration as a “phase switching unit”, 
           [0018]      FIG. 2   a  illustrates a diagram having motor position angles for switching off the allocated power semiconductor switch in the case of a positive rotation direction of the electric motor, 
           [0019]      FIG. 2   b  illustrates a diagram having motor position angles for switching off the allocated power semiconductor switch in the case of a negative rotation direction of the electric motor, 
           [0020]      FIG. 3  illustrates a flow diagram of the method in accordance an aspect of with the invention, 
           [0021]      FIG. 4  illustrates a schematic circuit diagram of a second exemplary embodiment having power semiconductor switches as a safety switching means in a configuration as a “star-point switching unit”, and 
           [0022]      FIG. 5  illustrates a circuit diagram corresponding to  FIG. 1  without the motor position angle sensor. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]      FIG. 1  illustrates schematically a brushless electric motor  1 , the windings U, V, W of which are controlled by means of an AC converter  3 . For this purpose, the AC converter  3  comprises six switches  5 , wherein the upper switches  5  in  FIG. 1  are allocated to the positive supply voltage and the lower switches  5  in  FIG. 1  are allocated to the negative supply voltage. As is further evident from  FIG. 1 , voltage tapping points  17 ,  18 ,  19  are located between the upper switches  5  that are allocated to the positive supply voltage, and the lower switches  5  that are allocated to the negative supply voltage, at which voltage tapping points the voltage prevailing at the windings U, V, W is tapped and said voltage is supplied to a unit  11  that is provided for measuring the voltage. The measurement results from the unit  11  that is provided for measuring the voltage are supplied to a microcontroller  4  that on the one hand controls the switches  5  and on the other hand evaluates the information produced by the unit  11  that is provided for measuring the voltage. In addition, a detection unit  10  is provided for detecting defective switches  5 . The information produced by the detection unit  10  is likewise supplied to the microcontroller  4  for evaluation. Power semiconductor switches  2  are arranged in the phase lines that lead to the windings U, V, W. 
         [0024]    In one embodiment in practice, the switches  5  are formed by means of semiconductor switches and/or transistors or MOSFET transistors. In practice, the detection unit  10  is embodied as a bridge driver and applies a voltage to the switches  5  that are embodied as transistors and said detection unit verifies whether the switching position of the transistor changes. In practice, the unit  11  that is provided for measuring the voltage at the voltage tapping points  17 ,  18 ,  19  is embodied as a voltage divider and determines the duty cycle of a pulse-width-modulated voltage. The duty cycle corresponds to the quotient resulting from the pulse duration and the period duration. 
         [0025]    Each switch  5  can in principle comprise two different types of a defect or rather in the event of a defect, each switch can essentially be located in one of the two subsequently described states: non-conductively defective, in other words in the opened switching position, the switch performs a blocking function; or conductively defective, in other words in the closed switching position, the switch performs a blocking function. A conductively defective switch  5  is also colloquially known as a short circuit. 
         [0026]    It is particularly important in applications where safety is concerned such as in an electromechanical steering device to switch the electric motor  1  very quickly into an emergency mode or to immediately switch said electric motor off. The malfunction of the conductively defective switch  5  leads to an increased steering torque since in the event of a defect the driver of the vehicle moves the electric motor  1  in the generator mode and induces a current that immediately counteracts the steering movement at the steering wheel. This torque that counteracts the rotation direction of the driver of the vehicle is subsequently known as braking torque. A braking torque that is dependent upon the rotation speed is generated in a permanently excited synchronous machine by virtue of the short circuit in the AC converter  3 . A conductively defective switch  5  causes a short circuit to occur across the motor windings U, V, W as a result of induction. 
         [0027]    The phase connectors of the electric motor  1  are connected to the outputs of the driving AC converter  3  by way of the power semiconductor switches  2  for the purpose of using the brushless electric motor  1 . The purpose of this arrangement is to de-energize the electric motor  1  in the event of a defect in the AC converter  3  and to consequently disconnect said electric motor from the previously described braking torque. This is also described as a safety shutdown. After a defect in the AC converter  3  is detected by the microcontroller  4 , the AC converter  3  is initially switched off, in other words, the switches  5  are opened so that power is no longer introduced into the electric motor  1 . If the electric motor  1  is still in its rotating state either by means of inertia or by means of drive from outside, a current peak periodically builds up within the AC converter  3  by means of the rectification of the induction voltage of the electric motor  1  by way of parasitic diodes and a switch  5  that is assumed to be conductively defective, said current peak generating the undesired braking torque. The process of switching off the power semiconductor switches  2  in the phase lines prevent this. 
         [0028]    In order to prevent damage to the power semiconductor switches  2  as a result of an inadmissibly high inductive amount of energy from the electric motor  1 , said power semiconductor switches are switched off individually in windows of defined motor position angles (φ U , φ V , φ W , as a result any current flowing through the respective affected winding U, V, W is not damaging for the allocated power semiconductor switch  2 . The power semiconductor switches  2  are embodied as power MOSFET transistors and short-term values of the drain current I D  of zero, negative or low positive are not damaging. It is a characteristic of this method that the suitable motor position angle windows φ U , φ V , φ W  always remain constant regardless of the position of the conductively defective switch  5  in the AC converter  3  and only depend on the rotation direction of the electric motor  1 . It is thus sufficient to switch off the individual power semiconductor switches  2  in a purposeful manner by means of the detection of the admissible motor position angle windows φ U , φ V , φ W  by the microcontroller  4 . The motor position angle φ U , φ V , φ W  is detected with the aid of a motor position angle sensor  6 . It is necessary to detect the motor position angle φ U , φ V , φ W  in order to control a brushless electric motor  1 . An additional sensor is consequently not necessary in order to implement the method described here. 
         [0029]    As previously mentioned, the suitable motor position angle windows φ U , φ V , φ W  are not dependent upon the position of the conductively defective switch  5  in the AC converter  3 . The suitable motor position angles φ U , φ V , φ W  in fact remain constant. This correlation is illustrated in  FIG. 2 . 
         [0030]    In the case of a three phase BLDC motor, the motor position angle windows, in which the currents that are generated by means of the induced voltages and are caused by a conductively defective switch  5  in the AC converter  3  are zero in the individual phases, are theoretically up to 120° wide. If the motor angle is defined as φ=0° at the point where the induced voltage of the motor  1 , which is driven from the outside, of the phase U is at its maximum, the de-energized angle windows lie for the three phases in the intervals φ U =[300°; 60°], φ V =[60°; 180°] and φ W =[180°; 300°]. These values apply for idealized ratios, in which the electrical time period TPER=1/(N*number of pole pairs) that occurs by virtue of the mechanical rotational speed N of the motor remains small in comparison to the time constants of the phase impedance  T PH=L/R and therefore the currents and voltages in the motor still extend in an approximately congruent manner. In practice, however, these idealized ratios are not automatically produced. Thus, it must be taken into consideration that, depending on motor characteristic variables and the maximum rotation speed that is to be considered, the motor position angle window can become smaller than 120° and, in addition, can become dependent upon the direction of rotation, as is illustrated in  FIGS. 2   a  and  2   b.  The boundary values of the motor position angle windows are to be dimensioned for the specific application. However, this does not cause any changes in the fundamental method of switching off inductive loads within the predefined motor position angle window. 
         [0031]    The motor position angle windows φ U , φ V , φ W  for the switching off process are unfortunately not the same for each application. The rotation of the motor shaft, which rotation is produced from outside, does in fact always generate the three induced voltages with the sine-wave shaped progressions that are displaced with respect to one another by in each case 120° corresponding to a three phase system for a three phase brushless electric motor  1  and these then lead, as a consequence, to the buildup of current peaks. However, these current peaks do not extend in a congruent manner with respect to the induced voltages but rather suffer as a result of the complex impedance of the motor phases U, V, W firstly a time delay and secondly a distortion in form as a result of the external boundary condition with the short circuit in the AC converter  3 . Furthermore, the current peaks expand with the increasing motor rotation speed N and the motor position angle windows φ U , φ V , φ W  become correspondingly smaller. Finally, the process of determining the motor position angle windows φ U , φ V , φ W  is a measuring task that takes into consideration the characteristic values of the electric motor being used and also the maximum rotation speed N that is to be considered. The amount of energy that is admissible for the power semiconductor switches  2  is also a criterion: the greater this amount of energy, the greater the short-term value of the phase current that can still exist during the switching off process, which is useful when maximizing the motor position angle window φ U , φ V , φ W . 
         [0032]    In  FIGS. 2   a  and  2   b,  the idealized ratios in the case of a motor having a very small ratio of phase inductivity to phase resistance are illustrated as black continuous blocks. In these ideal conditions, the motor position angle window φ U  for the winding U amounts to from 300° to 60°, for the winding V corresponding to the motor position angle window φ V  from 60° to 180° and for the winding W motor position angle window φ W  from 180° to 300°. This includes the previously briefly described principle abbreviations of the motor position angles φ U , φ V , φ W  for motors having a high L/R ratio and high rotational speeds are illustrated here as shaded blocks. 
         [0033]      FIG. 3  illustrates a flow diagram for implementing the described method. In step S 1 , a checking process is carried out to ascertain whether the rotational speed of the electric motor  1  is sufficiently low and the induction currents corresponding to this are sufficiently low. If this is the case, in step S 2 , all power semiconductor switches  2  are opened for all windings U, V, W and the method is terminated. In the more serious case of the induction currents being intolerably low, a checking process is carried out in step S 3  to ascertain whether the motor position angle φ is located within the switching off window φ U  for the phase U. If this is the case, the power semiconductor switch  2  that is allocated to the phase U is opened in step S 4 . 
         [0034]    In step S 5 , a corresponding checking process is carried out to ascertain whether the motor position angle φ is located in the suitable motor position angle window φ V . If this is the case, the allocated power semiconductor switch  2  is opened (step S 6 ). Subsequently, the method is continued with the step S 7 : in this step a checking process is carried out to ascertain whether the motor position angle φ is located in the suitable motor position angle window φ W  and opens (step S 8 )in the case of the positive presence of the power semiconductor switch  2  that is allocated to the winding W. In step S 9 , a checking process is carried out to ascertain whether the decisions in all the steps S 3 , S 5 , S 7  were once positive and therefore in other words all the power semiconductor switches are open. As long as this is not the case, a return to step S 3  is implemented. 
         [0035]      FIG. 4  illustrates a second embodiment that comprises a topology of the power semiconductor switches  2  as a “star-point switch”: The power semiconductor switches  2  are in turn embodied as power MOSFET transistors and connect the end connectors of the motor phases U, V, W to the “star-point point”. The use of the motor position angle φ for individually switching off the power semiconductor switches  2  within the suitable motor position angle windows φ U , φ V , φ W  remains unchanged in comparison with the embodiment that is illustrated in  FIG. 1 . 
         [0036]      FIG. 5  illustrates an alternative use of the motor position angle information φ by means of monitoring the current direction in each motor phase U, V, W. In the embodiment that is illustrated in this figure, the current direction is monitored using voltage comparators  7  that monitor the power drop across the power semiconductor switches  2  and therefore provide the information regarding the current direction in each motor phase U, V, W, which information is required by the microcontroller  4  in order to switch off the power semiconductor switches  2  individually in the correct time window, in other words during a current direction that does not cause risk of destruction. 
         [0037]    Alternatively, it is possible for this purpose in systems that use a “real” current measurement for example by means of shunts the phase currents for control purposes to use this phase current information likewise for detecting the time window of the current direction that does not cause risk of destruction. 
         [0038]    The application of the described method renders possible the use of power semiconductor switches  2  as a safety characteristic in a system for operating a brushless electric motor  1 , even if the maximum possible amount of inductive energy that occurs by virtue of the magnitude of the motor inductances and currents of a system of this type exceeds the absorption capacity of the available and/or economically justifiable power semiconductor switches. It is consequently possible to avoid the use of a mechanical relay that is not desired for various reasons: due to wear characteristics, sensitivity to moisture, changing contact characteristics and a lack of reliability etc., and it is possible to use the sometimes also more preferred solution owing to its construction using power semiconductor switches without having to introduce additional switching means for external energy absorption such as suppressor diodes or devices for measuring current.
The invention relates to a method for operating a brushless electric motor, the windings of which are controlled by an inverter using six switches, wherein the inverter comprises three outputs, which are associated with the windings of the electric motor, and wherein a respective power semiconductor switch is arranged between the output of the inverter and the windings, and wherein a detection unit for detecting defective switches, a unit for measuring the voltage at the outputs of the inverter, and a microcontroller for controlling switches are provided. The invention further relates to a device operating a brushless electric motor. In order not to damage the power semiconductor switches, according to the invention of the inverter is switched off by the microcontroller after a defective switch has been detected, such that no additional power is introduced in the windings (U, V, W) of the electric motor and the power semiconductor switches are opened in a predetermined motor angle position (ph U, phv, phw). For said purpose, the motor angle positions (ph U, phv, phw) are associated with a respective winding (U, V, W) and the power semiconductor switches thereof and are selected such that the associated power semiconductor switches are not damaged during opening.