Abstract:
A protective device for a power-assisted steering system, such a power-assisted steering system, and a method for protecting a power-assisted steering system are described. Provided in the described power-assisted steering system is a circuit that closes a separation in the neutral point of an electric motor when the rotational speed of the electric motor exceeds a prespecified value.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a protective device for a power-assisted steering system, such a power-assisted steering system, and a method for protecting a power-assisted steering system. 
   With power-assisted steering systems there can be problems when the wheels move at high speeds. Thus for instance in a vehicle workshop the steering is frequently moved by rotating the wheels of the motor vehicle while the power-assisted steering is turned off. When enough force is exerted, the steering can be accelerated such that a very high steering speed is attained. This is far greater than the steering speed that occurs during normal operation. The parts of the steering system that are operationally connected to the wheels thus obtain high rotational speeds. 
   If the steering system stops at the steering stop at a high steering speed, all components are sharply braked. Due to the great delay in the rack and the electric motor drivingly connected thereto there can be very high torques and forces on the transmission and electric motor. These torques and forces can be so great that the components are damaged. 
   The higher the speed with which the stop is reached, the higher the risk of damage. Although the high rotational speeds can be avoided during operation using suitable control of the motor in that active intervention is used to prevent high rotational speeds, this is not possible when it is turned off. When it is turned off damage is probable if no other measures are taken. 
   It should furthermore be kept in mind that in steering systems with electrical neutral point separation using field effect transistors and a synchronous motor, at a high rotational speed the motor can generate so much voltage that the field effect transistors of the neutral point go into avalanche mode. Then a majority of the energy added by the mechanic is converted to heat in the neutral point field effect transistors. This can lead to the field effect transistors being thermally destroyed. 
   A protective device for an electrical steering system is known from publication DE 100 13 133 A1. In it, the electric motor for the power-assisted steering system is connected to a brake circuit that applies an electrical load to the motor if the rotational speed exceeds a prespecified value. 
   It is disadvantageous in the protective device described in the publication that the protective device functions only with direct current motors. However, as a rule permanently excited synchronous motors are used in electric steering systems with greater power. For safety reasons it is provided that the neutral point is separated when turned off. 
   Completing the circuit of the output stage, as is suggested in the publication, does not have any effect in a relay separation because the separated neutral point does not provide any current and thus does not permit braking of the motor. 
   During normal operations this device must be turned off because otherwise the system does not function (for instance cut-out relay). This cut-out device can itself be faulty, which reduces the availability of the steering system. 
   It is therefore the object to suggest a protective device that can also be used with a synchronous motor with neutral point separation. 
   SUMMARY OF THE INVENTION 
   The inventive protective device for a power-assisted steering system of a motor vehicle, the power-assisted steering system having a control device, an electric motor with neutral point separation, and a transmission, includes a circuit that is connected to the electric motor and closes the neutral point separation when the rotational speed of the synchronous motor exceeds a prespecified value. 
   The motor is thus connected to the inventive protective device with a circuit that closes the neutral point separation when the rotational speed of the motor exceeds a prespecified value. Care can be taken that the prespecified rotational speed value is at least high enough that the driver can safely steer the vehicle even if the electronics are turned off, e.g. if there is a fault. In the embodiment, the rotational speed value is high enough that the driver can still turn the steering wheel at least 90°/s. This is particularly important when additional FETs of a power output stage are to be closed. 
   In the embodiment, a closed circuit is created for the voltage generated by the motor so that the neutral point is bridged. The circuit can be formed e.g. in that an electrical connection is created between at least one motor terminal and the neutral point. 
   If the motor voltage generated is smoothed, it can act as a reference signal for the rotational speed. 
   The generated voltage can be used directly for closing the field effect transistors. 
   Alternatively, the generated voltage, which can be smoothed, can be used for switching a second voltage, such as e.g. a battery voltage. 
   In the embodiment, phases of the synchronous motor have high ohmic connections to sides of the neutral point compared to the coils. This can occur by mutual connection using resistors upstream of the neutral point separation, mutual connection using resistors between phase and neutral point, connecting to the ground using resistors upstream of the neutral point separation, or connecting to the ground using a resistor in the neutral point. However, the use of additional resistors is not absolutely necessary due to the parasitic diodes or body diodes of the field effect transistors. 
   A power-assisted steering system with a synchronous motor that is provided to support steering torque, wherein a separation is to be made in the neutral point of the synchronous motor, is characterized in that a protective device, in particular a protective device of the type described in the foregoing, is provided that has a circuit that closes the separation when the rotational speed of the synchronous motor exceeds a prespecified value. 
   In the power-assisted steering system described, a protective device having the aforesaid features is employed in embodiments. 
   The inventive method protects a power-assisted steering system that includes a synchronous motor, wherein separation is to be made in the neutral point of the synchronous motor, and is characterized in that a circuit of a protective device, in particular a protective device of the type described in the foregoing, closes the separation when the rotational speed of the synchronous motor exceeds a prespecified value. 
   In the embodiment of the method, a generated motor voltage is smoothed on the sides of the power output stage. At least one phase is used. However, two phases or all phases can be used. The neutral point is closed when the smoothed voltage is at a specific, prespecifiable level of the smoothed voltage and represents a measure for the rotational speed or acts as a reference signal for the rotational speed. Normally the smoothed voltage is used directly for this, or the smoothed voltage is used in order to switch the vehicle voltage for closing the neutral point. 
   With the described method the motor is thus braked after or while closing the neutral point if the generated voltage is high enough that the voltage can drive current via the power output stage against the voltage applied there. This is the battery voltage when a battery is attached. Otherwise this is the voltage from the input capacitors, which is limited e.g. by a power zener diode between plus and ground. It should also be noted that during this procedure it is not necessary to actively turn off the protective circuit during normal operations. 
   In another embodiment of the inventive method, field effect transistors, in particular the so-called low-side field effect transistors, are also short-circuited. This functions regardless of whether the battery is attached or not. However, in normal operations this circuit must be turned off or interrupted. However, for reasons of safety this procedure should not be used unless a great deal of care has been taken. 
   Thus with the method there is effective protection for a power-assisted steering system with a synchronous motor. Therefore it is possible to prevent the field effect transistors from going into avalanche mode. So it is assured that the circuit used works passively and thus the microcontroller remains non-functioning. In the embodiment, the circuit is configured such that there is no need to turn it off during normal operations. 
   The invention makes it possible to limit the rotational speed of the motor to a specific prespecified value. This value should be low enough that no damage occurs when the final stop is reached and the field effect transistors in the neutral point separation do not first go into avalanche mode. On the other hand, the rotational speed must be high enough that if there is an error in the steering system the driver can still steer with sufficient steering speed, e.g. &gt;90°/s. 
   Additional advantages and embodiments of the invention result from the description and the enclosed drawings. 
   It is understood that the aforesaid features and the features to be explained in the following can be used not only in the specified combination but also in other combinations or alone without departing from the framework of the present invention. 
   The invention is depicted schematically in the drawings using exemplary embodiments and is described in detail with reference to the drawings. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of one embodiment of the inventive protective device; 
       FIG. 2  depicts an alternative embodiment for a segment from  FIG. 1 ; 
       FIG. 3  depicts another alternative to  FIG. 2 ; 
       FIG. 4  depicts yet another alternative to  FIG. 2 ; 
       FIG. 5  depicts another alternative to  FIG. 2  having an alternative circuit for closing the neutral point using battery voltage; 
       FIG. 6  depicts another alternative to  FIG. 2  in which the current path for the generating motor is via the protective circuit; and 
       FIG. 7  depicts an alternative to  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  uses a circuit diagram to depict one embodiment of the inventive protective device, labeled  10  overall. A power output stage  12  that encompasses six field effect transistors, a synchronous motor  14  that is excited by permanent magnets and that is equipped with three phases  13 , and a circuit  16  for closing a neutral point separation  18  that encompasses three field effect transistors can be seen. 
   Resistors  20  for a relatively high ohmic connection of the motor phases  13  of the synchronous motor  14  via the neutral point are provided for each of the field effect transistors of the neutral point separation  18 . 
   Furthermore, three diodes  22  for rectifying and an RC unit for smoothing a generated motor voltage are provided on the side of the power output stage  12 . 
   The circuit  16  closes the neutral point separation  18  as soon as the rotational speed of the synchronous motor  14  exceeds a certain value. In addition, the field effect transistors of the power output stage  12  close. The voltage generated by the synchronous motor results in the closed circuit. 
   The illustration furthermore depicts a terminal  24  with which during normal operation, in which the microprocessor works, the neutral point is closed or opened. 
     FIG. 2  depicts the synchronous motor  14  with the neutral point separation  18 , three resistors  30  being provided for a relatively high ohmic connection of the motor phases  13  against one another as an alternative to resistors for relatively high-ohmic connection of the motor phases. 
     FIG. 3  depicts another alternative for resistors for relatively high ohmic connection of the motor phases, one resistor  32  being provided for a relatively high ohmic connection of the motor phases in the neutral point to the ground. However, this only makes sense if the battery voltage is switched for closing the neutral point. 
     FIG. 4  again depicts the synchronous motor  14  with the neutral point separation  18 , a diode  40  being provided in this embodiment for connecting the motor phases in the neutral point to the ground  42 . In this case, as well, this only makes sense if the battery voltage is switched for closing the neutral point. 
     FIG. 5  depicts how a battery voltage U 13  Bat is used instead of directly using the smoothed voltage. In addition, relays  50  are provided for the neutral point separation instead of the field effect transistors and controlled by a relay control circuit  52 . Alternatively the voltage of only one motor phase can be used. 
     FIG. 6  depicts a modified embodiment of the protective circuit  16  from  FIG. 1 , labeled with reference number  60  overall. In this embodiment no additional components are provided in the area of the motor  62  and the neutral point field effect transistors  64 . The body diode of the neutral point FETs is used for closing the circuit of the generated motor voltage. This is possible in that the protective circuit  60  is switched between motor output and neutral point. 
   Only one motor phase is used in the embodiment depicted. Alternatively, the two other phases can also be connected by diode to the circuit  60 . 
   In addition, the terminal  66  for closing and opening the neutral point can be seen in the depiction. 
     FIG. 7  depicts two additional variants  70  and  72  of the protective circuit. As for the protective circuit  60  in  FIG. 6 , no additional components are provided in the area of the motor or the neutral point FETs in the circuits  70  and  72 . The body diode of the neutral point FETs is used for closing the circuit of the generated motor voltage. This is possible because the protective circuits  70  and  72  are each switched between motor output and neutral point. 
   The protective circuit  70  uses a comparator  74  with a push-pull output. The protective circuit  72  uses a comparator with an open collector output. 
   Only one motor phase is used in each of the protective circuits  70  and  72 . Alternatively, the two other phases can also be connected by diode to the circuit  70  or  72 .