Abstract:
An electrically assisted power steering system includes an anti-theft feature. Upon detection of an event that is indicative of a vehicle theft situation, the drive circuit for the electric assist motor is selectively actuated to excite the motor windings to either prevent or oppose movement of the steering system.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates in general to Electrically-assisted Power Steering (EPS) systems for motor vehicles and in particular to an EPS system that also provides vehicle anti-theft protection. 
     In the past, vehicles have been equipped with Hydraulically-assisted Power Steering (HPS) systems. As is well known in the prior art, conventional HPS systems include a hydraulic actuator that is connected to the vehicle steering linkage. The hydraulic actuator is controlled by a rotary control valve that is connected to the vehicle steering wheel. A pump supplies pressurized hydraulic fluid to the rotary control valve which is operable to supply a portion of the hydraulic fluid to the actuator. When the vehicle operator turns the steering wheel, the rotary control valve is rotated. Rotation of the rotary control valve causes displacement of the hydraulic actuator which, in turn applies a directed force to the steering linkage to assist the vehicle operator in turning the steerable front vehicle wheels. In a HPS system, the pump is continuously driven by a belt from the vehicle engine crankshaft. Accordingly, HPS systems impose a continuous power requirement upon the vehicle engine. 
     Recently, Electrically-assisted Power Steering (EPS) systems have been developed to replace HPS systems. An EPS systems includes an electric assist motor that, upon actuation, applies torque to the vehicle steering linkage to assist the vehicle operator in turning the front vehicle wheels. Because the electric assist motor is only actuated when the steering wheel is moved, the power requirements for generating the assist torque are intermittent instead of continuous, as with HPS systems. As a result, operating of efficiency of the vehicle is improved. Also, the electrical wiring required  by the EPS system may be easier to route within the vehicle engine compartment that the hydraulic lines needed for a conventional HPS system. 
     Referring now to  FIG. 1 , there is shown a schematic drawing of a conventional EPS system  10 . The system  10  includes a steering wheel  12  connected to an input shaft  14 . The input shaft  14  is connected through a steering torque sensor  16  to an output shaft  18 . The torque sensor  16  is operative to generate a torque requirement signal that is proportional to the torque applied to the steering wheel  12  by the vehicle operator. The torque sensor includes a torsion bar (not shown) that is connected between the input and output shafts  14  and  18 . A position sensor (also not shown) also is connected to the input and output shafts. The position sensor senses the relative rotational position between the input and output shafts  14  and  18 . Taking into account the torsional strength of the torsion bar, the sensed relative rotational position is indicative of the amount of steering torque applied to the steering wheel  12 . The torque requirement signal is generated when the operator rotates the steering wheel and will decrease as the steerable wheels respond. Additionally, the torque sensor also will generate a torque requirement signal when the steering wheel is held stationary and the steerable wheels move in response to road surface conditions. 
     The output of the torque sensor  16  is connected to a steering Electronic Control Unit (ECU)  20  that includes a microprocessor (not shown) for controlling the EPS system  10 . The microprocessor includes memory capacity, such as internal ROM and/or RAM, for storing an algorithm for controlling the operation of the EPS system  10 . A vehicle speed sensor  21  provides a vehicle speed signal to the steering ECU  20 . 
     Also shown in  FIG. 1  is a conventional mechanical lock  22  that is mounted upon the steering input shaft  14 . The lock  22  is normally actuated to prevent movement of the steering wheel as a vehicle theft deterrent. The lock  22  is electrically connected a body control module  24  that provides overall control functions over various vehicle systems, including the EPS system  10  and an engine control unit  25 . The body control module  24  is connected to a key lock  26 . In a known manner, insertion and rotation of a key in the key lock  26  causes the body control module  24  to deactuate the mechanical lock  22 , allowing rotation of the steering wheel  12 .  
     The steering output shaft  18  is connected to a pinion gear (not shown) of a rack and pinion gear set  30 . The rack and pinion gear set  30  functions to transform the rotational motion of the steering wheel  12  into linear motion of a steering rack  32 . The steering rack  32  is connected to steerable vehicle wheels  34  in a conventional manner. The linear movement of the steering rack  32  deflects the wheels  34  to the right or left. 
     The EPS system  10  includes an electric assist motor  36 . Typically, the assist motor  36  is operatively connected to the steering rack  32  through a ball nut assembly (not shown) in a conventional manner. Alternately, the assist motor  36  can be coupled to column drive systems, pinion drive systems, or other conventional steering systems. When the electric assist motor  36  is energized, the motor rotor turns, which, in turn, rotates the nut portion of the ball nut assembly. When the nut portion rotates, the balls transfer a linear force to the steering rack  32 , thereby providing an assistance torque to aid the driver in turning the wheels  34 . The direction of movement of the steering rack  32  is dependent upon the direction of rotation of the electric assist motor  36 . A motor rotor position sensor  38  is mounted upon the motor  36  and is connected to the steering ECU  20 . One of the functions of the motor rotor position sensor  38  is to provide an electrical signal indicative of the position of the motor rotor relative to the motor stator to the steering ECU  20 . For proper operation of the electric assist motor  36 , including direction of rotation and applied torque, it is necessary to know the position of the rotor relative to the stator. Optionally, the motor rotor sensor  38  also may provide additional information concerning current flow through the motor  36 . 
     A motor drive circuit  40  is connected to and actuates the electric assist motor  36 . Electric power is supplied to the drive circuit  40  through a power supply relay  42  by the vehicle power supply  44 . Both the drive circuit  40  and power supply relay  42  are connected to the steering ECU  20  which is operational to control both devices. 
     A schematic drawing of the motor drive circuit  40  is shown in  FIG. 2 . Components shown in  FIG. 2  that are similar to components shown in  FIG. 1  have the same numerical designators. As shown in  FIG. 2 , the electric assist motor  36  is a multi-phase brushless star connected permanent magnet motor. The motor  36  includes  three motor windings that are labeled R, Y and B. Each of the motor windings R, Y and B has a first end and a second end. The first ends of the motor windings are connected to the motor drive circuit  40 . Specifically, the first end of each motor winding is connected between a corresponding pair of electronic switches, which are shown as bipolar transistors in  FIG. 2  but could be other devices, such as, for example FET&#39;s. A first transistor in each pair, labeled T 1B , T 1Y  and T 1R  for the corresponding winding, is connected between each phase winding and the power supply relay  42  while a second transistor in each pair, labeled T 2B , T 2Y  and T 2R  for the corresponding winding, is connected between each winding and ground. The bases of each of the transistors are connected to the steering ECU  20 . 
     The second ends of each of the motor windings R, Y and B are connected to motor winding relay  43 . As shown in  FIG. 2 , the motor winding relay  43  has two sets of contacts each of which separates the second end of one of a pair of the motor windings B and R from the second end of the third winding Y. When the relay contacts are closed, as during normal operation of the motor  36  and motor drive circuit  40 , the second ends of all three motor windings are connected to form the star connection. The contacts in the motor winding relay  43  are opened if a fault is detected in the EPS system  10 , such as, for example, a shorted motor winding or transistor in the motor drive circuit  40 . Opening of the motor winding relay contacts isolates each of the motor windings from the other of the motor windings and from the winding that is common to the relay contacts. This prevents back EMF generated by movement of the steering column (and hence the motor rotor) from finding a low impedance path through the control circuit and thus prevents electrical braking of the steering apparatus. 
     The microprocessor in the steering ECU  20  controls the amount of steering assist provided by the motor  36  as a function of both the applied steering torque and the vehicle speed. The microprocessor is responsive to signals received from the steering torque sensor  16 , the motor rotor position sensor  38  and the vehicle speed sensor  21  to generate steering command signals. The microprocessor can also receive data from other vehicle systems, such as for example, the temperature, that it can  utilize in generating the steering command signals. The steering command signals are applied to the transistors in the motor drive circuit  40  to selectively energize the motor phase windings B, Y and R. As a result of energizing the motor phase windings, the motor rotor will rotate in a desired direction to provide an assist torque to the steerable wheels  34 . For example, switching transistors T 1B , T 1R  and T 2Y  to their conducting state will cause a current to flow through the motor windings to energize phase Y and to produce a corresponding motor torque. Similarly, switching transistors T 1Y , T 2B  T 2R  to their conducting states will result in an opposite current flowing through the motor windings to energize phase Y in the opposite direction and to produce a counter-torque. 
     As shown in  FIG. 1 , the EPS system  10  includes a mechanical lock  22  as an anti-theft device. It would be desirable to utilize the EPS system  10  to provide theft deterrence and thereby eliminate the need for a mechanical anti-theft device. An EPS system that incorporates an ant-theft feature would simplify the steering system by reducing the number of components with a corresponding reduction in cost. 
     SUMMARY OF THE INVENTION 
     This invention relates to an EPS system that also provides vehicle anti-theft protection. 
     The present invention contemplates an electrically assisted power steering system that includes an electric steering assist motor that is adapted to be connected to a vehicle steering system. The system also includes a drive circuit electrically connected to the assist motor, the drive circuit being operative to control the direction of rotation and torque generated by the assist motor. The system further includes a controller electrically connected to the drive circuit, the controller being operative, upon initialization of the system, to cause the drive circuit to control the assist motor in such a manner that movement of the vehicle steering system is inhibited until verification that a vehicle theft situation does not exist. 
     The assist motor includes a plurality of motor windings that are selectively energized as phases. The drive circuit contains pairs of electronic switches for  controlling the current flowing through each of the motor windings with a first electronic switch of each pair connected between the winding and a power supply and a second electronic switch of each pair connected between the winding and ground. Upon system initialization, the controller is operative to cause the drive circuit to short the motor windings by connecting the windings together. The windings are connected together by either the controller causing all of the first electronic switches to be in a conducting state or by the controller causing all of the second electronic switches to be in a conducting state. 
     Alternately, the controller can be operative to apply a constant current to one of the motor phases. The anti-theft protection can be implemented solely by selective operation of electronic switches that are included in the drive circuit or the electronic switches for implementing the anti-theft protection can be included in the assist motor to implement the protection. 
     During operation of the electrically assisted power steering system, the motor windings are sequentially energized in accordance with a series of sequential command signals to provide an assist torque to the vehicle steering system. The sequential command signals are generated by the controller for the drive circuit in response to motor rotor position signals and steering torque signals. Rotor position signals are generated either by the controller from monitored motor parameters or received from a rotor position sensor attached to the motor shaft. Accordingly, the invention also contemplates that controller can be operative, upon system initialization, to prevent updating of the motor rotor position signals in order to prevent movement of the vehicle steering system until verification that a vehicle theft situation does not exist. 
     The present invention also contemplates a method for theft protection of a vehicle that includes the steps of providing an electric power steering system having an assist motor that is adapted to be connected to a vehicle steering system. The electric power steering system also includes a drive circuit electrically connected to the assist motor, the drive circuit being responsive to a steering command signal to control the direction of rotation and torque generated by the assist motor. The electric  power steering system further includes a controller electrically connected to the drive circuit, the controller being responsive to steering torque signals to generate steering command signals. The electric power steering system is disabled upon actuation of the vehicle ignition system to inhibit movement of the vehicle steering system. The electric power steering system is returned to operation only upon confirmation that a vehicle theft situation does not actually exist. 
     The electric assist motor has a plurality of windings that are sequentially actuated to cause the motor rotor to rotate. The invention contemplates that the electric power steering system is disabled by shorting at least one of motor windings, passing a constant current through at least one, or more than one, of the motor windings or blocking updated motor rotor position information which is used to determine a series of steering command signals that are utilized by the motor driver circuit. 
     Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of a known conventional EPS system  10 . 
         FIG. 2  is a circuit diagram of a motor drive circuit shown in  FIG. 1 . 
         FIG. 3  is a schematic drawing of an EPS system that includes an anti-theft device in accordance with the invention. 
         FIG. 4  is a flow chart for operation of the EPS system shown in  FIG. 3 . 
         FIG. 5  is a flow chart for an alternate embodiment for operation of the EPS system shown in  FIG. 3 . 
         FIG. 6  is a flow chart for another alternate embodiment for operation of the EPS system shown in  FIG. 3 . 
         FIG. 7  is an alternate embodiment of the motor drive circuit shown in  FIG. 2  that includes the present invention.  
         FIG. 8  is another alternate embodiment of the motor drive circuit shown in  FIG. 2  that includes the present invention. 
         FIG. 9  is another alternate embodiment of the motor drive circuit shown in  FIG. 2  that includes the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, there is illustrated in  FIG. 3 , an improved EPS system  50  in accordance with the present invention. Components shown in  FIG. 3  that are similar to components shown in  FIG. 1  have the same numerical designators. The EPS system  50  includes an anti-theft feature and therefore the mechanical lock  22  shown in  FIG. 1  has been omitted. 
     The present invention contemplates a modified algorithm for operation of the microprocessor in the steering ECU  20 . The modified algorithm includes an anti-theft feature that utilizes the motor drive circuit  40  and the electric assist motor  36  shown in  FIG. 3  to “lock” the front wheels  34  in their current position as a vehicle theft deterrent. The algorithm is illustrated by the flow chart  52  shown in  FIG. 4 . In functional block  54  the vehicle ignition circuit is energized. This normally occurs when an ignition key is inserted into the vehicle ignition switch key lock; however, the circuit also can be energized when the ignition switch is bypassed during an attempted vehicle theft. In either case, electric power is supplied to both the steering ECU  20  and the body control module  24 . 
     In functional block  56 , the EPS system  50  is initialized. During initialization, the microprocessor in the ECU  20  runs a series of self tests to assure that all of the components of the EPS system  50  are functioning properly. If any of the system components fail their self test, an error code is generated and the system  50  is disabled. Additionally, an error message is displayed for the vehicle operator. Upon successful completion of the self tests, the algorithm advances to functional block  58 . 
     In functional block  58 , the modified algorithm causes the motor drive circuit  40  to be operated in a manner that will inhibit operation of the vehicle steering system. For the particular embodiment shown in  FIG. 4 , the motor winding relay  43  is closed  and all three of the second electronic switches T 2B , T 2Y  and T 2R  are switched to their conducting states. Alternately, all three of the first electronic switches T 1B , T 1Y  and T 1R  can be switched to their conducting states. As a result, all of the motor windings are shorted. Therefore, whenever one tries to steer the system by turning the steering wheel  12 , a back emf is generated in all three windings in the same direction and the assist motor  36  will act as a drag upon the steering rack  32 , making the steering wheel  12  extremely difficult to turn. 
     In the preferred embodiment, the ignition key includes an electronic chip that carries an identification code for the vehicle. When an ignition key is inserted into the ignition switch key lock  26 , the key identification code is sent to the body control module  24 , as shown in functional block  59 . Also, when the body control module  24  is initially energized, a first timer (not shown) begins running. In decision block  60 , it is determined if the key identification code is received within a first predetermined time period, T 1 , after the body control module has been energized. In the preferred embodiment, the first predetermined time period T 1  is two seconds; however, it will be appreciated that the invention also can be practiced with another time duration selected for the first time period. If a key identification code is not received with the first predetermined time period T 1 , it is an indication that a theft may be in progress and the algorithm proceeds to functional block  62  where the steering ECU  20  continues to inhibit operation of the EPS system  50  by maintaining the selected electronic switches in their conducting states. Typically, the body control unit  24  will also send signals to the engine control unit  25  to inhibit starting the vehicle engine. If a key identification code is received by the body control unit  24  within the predetermined time period, the algorithm proceeds to decision block  64 . 
     In decision block  64 , the body control module  24  compares the actual ignition key identification code to a key identification code stored within the control module. If the actual key identification code does not match the stored key identification code, the algorithm transfers to function block  62  to continue inhibiting the EPS system  50  by maintaining the selected electronic switches in their conducting states. If the key identification codes do match, it is an indication that the correct key is being used and  the body control module  24  transmits a signal to the steering ECU  20  while the algorithm transfers to decision block  66 . 
     Upon initial energization, the EPS ECU  20  starts a second timer. If the ECU  20  does not received the message from the body control module  24  indication that the correct ignition key is being used within a second predetermined time period T 2 , it is an indication the body control module  24  has been tampered with. Accordingly, in decision block  66 , it is determined whether the key identification message is received by the ECU  20  within the second predetermined time period T 2 , which in the preferred embodiment is T 1  plus 100 milliseconds. If the message is not received in decision block  66  within the second time period T 2 , the algorithm again transfers to functional block  62  to continue inhibiting the EPS system  50  by maintaining the selected electronic switches in their conducting states. If the message is received in decision block  66  within the second predetermined time period T 2 , the algorithm proceeds to functional block  70  where the EPS system  50  is returned to normal by switching all of the selected electronic switches to their non-conducting states. 
     An alternate embodiment of the algorithm is illustrated by the flow chart  72  in  FIG. 5 . Flow chart blocks shown in  FIG. 5  that are the same as blocks in  FIG. 4  have the same numerical identifiers. The primary difference between  FIGS. 4 and 5  is the method utilized to inhibit the EPS system  50 . As shown in functional block  74 , the system  50  is inhibited after initialization by energizing a selected one of the motor phases by placing three of the electronic switches into their conducting states. For example, phase B is energized when switches T 1Y , T 1R  and T 2B  are caused to be in their conducting states, allowing a current to flow through the motor windings. Thus, if phase B is energized by a 20 amp current, 10 amps will flow through T 1Y  and T 1R  while 20 amps flows through T 2B . While phase B has been used in the example, it will be appreciated that any one of the three motor phases can be selected. Additionally, the motor winding relay  43  remains closed. 
     In the preferred embodiment, a current having a sufficient magnitude to generate a counter torque in the motor windings to prevent rotation of the steering wheel  12  is applied to the windings for the selected phase. As a result, the steering  column is effectively locked. Should a thief manage to overcome the motor countertorque, the steering wheel  12  would only move in a step function manner as the steering rack  32  moves and there by forces the next motor pole pair into alignment with the same excited phase. 
     The current is maintained through the selected motor windings until the tests shown in decision blocks  60 ,  64  and  66  are completed. Failure of any of the three tests shown in decision blocks  60 ,  64  and  66  causes the algorithm to transfer to functional block  75  in which the current through the selected motor windings is maintained. Upon successful completion of all three tests, the current is removed from the selected motor windings and the EPS system  50  is returned to normal operation in functional block  70 . 
     The preferred embodiment of the algorithm is illustrated by the flow chart  76  in  FIG. 6 . As above, flow chart blocks shown in  FIG. 6  that are the same as blocks in  FIG. 4  have the same numerical identifiers. Again, the primary difference between  FIGS. 4 and 6  is the method utilized to inhibit the EPS system  50 . 
     During normal operation, the output of the torque sensor  16  is periodically sampled. As described above, the series of sampled output values from the motor rotor position sensor  38  are used by the steering ECU  20  to determine a series of steering command signals that are utilized by the motor drive circuit  40  to energize the assist motor  36 . While steering ECU  20  also uses other data, such as steering torque, vehicle speed and temperature to determine the steering command signals, the EPS system  50  can be inhibited after initialization by preventing updating of the motor rotor position or by blocking the motor rotor position signals supplied to motor drive circuit  40 . Accordingly, following system initialization in functional block  56 , the motor position updates are blocked in functional block  77 . The blocking of the signals can be implemented by any conventional method, such as setting a flag in the algorithm or opening an electronic switch (not shown) to interrupt the transmission of the signals to the ECU  20 . Once the motor position signals are blocked, the steering ECU  20  is prevented from generating new steering command signals. As a result, the steering command signals are provided to the motor drive circuit  40  in a manner that  prevents rotation of the assist motor  30 . Without updated steering command signals, the last set of motor drive circuit electronic switches to be placed in their conducting states remain in their conducting states, effectively locking the motor rotor in its current position. This not only holds the steerable wheels  34  in their present position, but also causes the motor to generate a counter-torque to oppose any torque applied to the steering wheel  12 . Additionally, current is only supplied to the EPS system  10  when the a torque is applied to the steering wheel  12 , reducing heating of the electronic components included in the EPS system  10 . 
     The motor position signals remain blocked, preventing updating of the steering command signals in functional block  78  until the three tests shown in decision blocks  60 ,  64  and  66  are completed. If any of the three tests are not successfully completed, the motor position signals remain blocked in functional block  78 . Only upon successful completion of all three tests is the motor position signals remain unblocked in functional block  79  and the EPS system  50  is returned to normal operation in functional block  70 . 
     While the preferred embodiment of the invention has illustrated and described above as a modification of the EPS system algorithm, it will be appreciated that the invention also may be practiced by a modification of the EPS motor  36  to add electronic switches. For example, the embodiment described with the flow chart shown  FIG. 4  can be implemented with the electric assist motor  80  shown in  FIG. 7 . Components shown in  FIG. 7  that are similar to components shown in  FIG. 2  have the same numerical identifiers. The assist motor  80  is a multiphase brushless star connected permanent magnet motor that has three electronic winding switches T S1 , T S2  and T S3  connected between each of the motor windings B, Y and R and ground. While bipolar transistors are shown in  FIG. 7  for the winding switches T S1 , T S2  and T S3 , it will be appreciated that other devices, such as, for example FET&#39;s, also can be utilized for the switches. The winding switches T S1 , T S2  and T S3  have bases connected to the body control module  24 . It is contemplated that the winding switches T S1 , T S2  and T S3  are located on the motor leadframe (not shown). Thus, the embodiment shown can be implemented by simply changing the assist motor and  providing an electrical connection from the bases of the winding switches to the body control module  24 . No change is necessary in the motor drive circuit  40 . 
     Following system initialization of the EPS system, the body control module  24  will cause the winding switches T S1 , T S2  and T S3  to be in their conducting state. Accordingly, the first ends of the motor windings B, Y and R are connected to ground, as described above, impeding movement of the steering rack  32  from its current position. The ground connections are maintained until all three security tests are successfully completed, at which time the winding switches T S1 , T S2  and T S3  are returned to their nonconducting state. If the security tests are not successfully completed, the winding switches T S1 , T S2  and T S3  are maintained in their conducting state, disabling the EPS system  10 . 
     An alternate embodiment  90  of the motor is shown in  FIG. 8  and provides a hardware method of implementing the embodiment shown in the flow chart in  FIG. 5 . As before, components shown in  FIG. 8  that are similar to components shown in  FIG. 2  have the same numerical identifiers. The motor embodiment  90  includes a pair of electronic switches T S4  and T S5  that are mounted upon the motor lead frame (not shown). While bipolar transistors are shown in  FIG. 8  for the winding switches T S4  and T S5 , it will be appreciated that other devices, such as, for example FET&#39;s, also can be utilized for the switches. Again, the embodiment shown can be implemented by simply changing the assist motor and providing an electrical connection from the bases of the electronic switches T S4  and T S5  to the body control module  24 . No change is necessary in the motor drive circuit  40 . 
     The first electronic switch T S4  is connected between the first end of one of the motor windings Y and the output terminal of the power supply relay  42 . The second electronic switch T S5  is connected between the first end of another one of the motor windings R and ground. While two specific motor windings, R and Y are shown, it will be appreciated that any two windings can be selected. The bases of both of the winding switches T S4  and T S5  are connected to the body control module  24 . 
     Following system initialization of the EPS system, the body control module  24  will cause the winding switches T S4  and T S5  to be in their conducting state.  Accordingly, the motor windings Y and R are excited by a maximum winding current, as described above, locking the motor rotor and thereby the steering rack  32  in its current position. The winding current is maintained until all three security teats are successfully completed, at which time the winding switches T S4  and T S5  are returned to their nonconducting state. If any of the three security tests are not successfully completed, the winding switches T S4  and T S5  are maintained in their conducting state, disabling the EPS system  10 . 
     While the preferred embodiment has been illustrated and described with a permanent magnet electric assist motor, it will be appreciated that the invention also can be practiced with other types of electric assist motors, such as, for example a Permanent Magnet Alternating Current (PMAC) motor. A particular type of PMAC motor that is utilized in EPS systems has three phases. The phases may be connected in either a Y or Delta configuration. With the Y configuration, the control circuit remains as shown in  FIG. 2 . For a delta configuration motor  94 , as illustrated in  FIG. 9 , the ends of the windings are connected to the common points between the electronic switches, as shown for the star configuration illustrated in  FIG. 2  so that the delta connection replaces the star shown at the bottom of the figure. Components shown in  FIG. 9  that are similar to components shown in the preceding drawings have the same numerical designators. Notice that the phase windings are labeled A, B and C. A winding relay  96  is included in the delta configuration shown in  FIG. 9 . 
     During operation, the motor phases are sequentially energized by the motor drive circuit to produce a desired motor torque and direction of rotation. The amount of the torque produced by a PMAC motor is a function of the amplitude of the current flowing through the phases while the direction of the torque is function of the current direction. The amplitude of the current is controlled by pulse width modulation of the applied phase voltages with the drive circuit electronic switches while the direction of the current is determined by the selection of the electronic switches. Because the motor drive circuit for a EPS system with a PMAC assist motor is the same as described above, the anti-theft feature also is included as described above.  
     By including an anti-theft function in the EPS system, the present invention eliminates the need for a mechanical steering column lock and the associated electrical busses. Accordingly, use of the invention reduces the complexity and cost of a vehicle. 
     While the preferred embodiment of the invention has been illustrated and described above with a rotor position sensor  38  mounted upon the electric assist motor  36 , it will be appreciated that the rotor position data also can be obtained without a dedicated rotor position sensor. The rotor position data can be obtained directly from monitoring motor parameters. For example, the motor back emf can be sensed and utilized to calculate the rotor position. Alternately, the signature of the motor inductance, which varies with the rotor position, and can be monitored and converted to a rotor position value. 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.