Method and apparatus for controlling an electric assist motor

Claims

Having described the invention, the following is claimed:

1. An apparatus for controlling an electric motor comprising:

a first switching device operatively coupled between one terminal of said electric motor and a first terminal of a source of electrical energy;

a second switching device operatively coupled between a second terminal of said electric motor and a second terminal of said source of electrical energy; and

control means for controlling energization of said electric motor, said control means including means for periodically switching both said first and second switching devices between ON and OFF conditions during a motor commutation period, the switching period of said first and second switching devices being less than the motor commutation period and each said first and second switching devices having more than one ON condition during the motor commutation period and only a portion of each of the ON conditions of the first and second switching devices overlap during the switching period, electrical current flows through said electric motor during each overlap portion of the ON conditions and power dissipation switching losses are shared by both said first and second switching devices, the amount of the ON condition overlap controlling current in said electric motor, said control means including means for controlling said ON condition overlap.

2. The apparatus of claim 1 further including a first fly-back diode connected between said second motor terminal and said first terminal of said source of electrical energy and a second fly-back diode connected between said first motor terminal and said second terminal of said source of electrical energy.

3. The apparatus of claim 2 further including a first current sensing resistor connected between said second flyback diode and said second terminal of said source of electrical energy and a second current sensing resistor connected between said second switching device and said second terminal of said source of electrical energy.

4. An apparatus of claim 2 wherein said control means includes means for pulse-width-modulating each of said first and second switching devices with the period of each pulse-width-modulation being less thin the motor commutation period.

5. The apparatus of claim 4 wherein said control means includes ramp generating means for providing a periodic ramp signal having a period less than the motor commutation period, means for comparing said ramp signal against first and second error reference signals, means for sensing current through said electric motor and providing a signal indicative thereof, means for controlling the value of said first and second error reference signals in response to said sensed current, said means for pulse-width-modulating each of said first and second switching devices being response to said comparison.

6. The apparatus of claim 5 wherein each said first and second switching devices are periodically in an OFF state in response to said comparison during a PWM cycle and the time period of the OFF states during a PWM cycle is equally shared between said first and second switching devices.

7. The apparatus of claim 1 wherein said control means includes means for periodically switching said first and second switching devices between ON and OFF conditions so that the ON condition of the first and second switching devices does not overlap.

8. The apparatus of claim 1 further including current monitoring means for monitoring current through said motor and means for providing an error signal if excess current is sensed.

9. An apparatus for controlling a multiphase electric motor comprising:

a first switching device operatively coupled between one terminal of a phase of said electric motor and a first terminal of a source of electrical energy;

a second switching device operatively coupled between a second terminal of said phase of said electric motor and a second terminal of said source of electrical energy; and

control means for controlling said electric motor including means for periodically switching said first and second switching devices between ON and OFF conditions so that no portion of the ON conditions of the first and second switching devices overlap and so that each of said first and second switching devices have more than one ON condition during a commutation period of said phase of said electric motor.

10. The apparatus of claim 9 further including a first fly-back diode connected between said second motor terminal and said first terminal of said source of electrical energy and a second fly-back diode connected between said first motor terminal and said second terminal of said source of electrical energy.

11. The apparatus of claim 10 wherein said control means includes means for pulse-width-modulating each of said first and second switching devices, the period of each pulse-width-modulation being less than the motor commutation period.

12. The apparatus of claim 11 wherein said control means includes ramp generating means for providing a periodic ramp signal having a period less than the motor commutation period, means for comparing said ramp signal against first and second error reference signals, means for sensing current through said electric motor and providing a signal indicative thereof, means for controlling the value of said first and second error reference signals in response to said sensed current, said means for pulse-width-modulating each of said first and second switching devices being response to said comparison.

13. The apparatus of claim 12 wherein each said first and second switching devices are periodically in an ON state in response to said comparison during a PWM cycle and the time period of the ON states during a PWM cycle is equally shared between said first and second switching devices.

14. An apparatus for controlling an electric assist steering system having an electric assist motor operatively connected to a steerable member, energization of said motor providing steering assist to said steering member, said apparatus comprising:

torque sensing means for sensing applied steering torque and providing a torque signal indicative thereof;

a first switching device operatively coupled between one terminal of said electric assist motor and a first terminal of a source of electrical energy;

a second switching device operatively coupled between a second terminal of said electric assist motor and a second terminal of said source of electrical energy; and

control means for controlling energization of said electric assist motor, said control means including means for periodically switching said first and second switching devices between ON and OFF conditions during a motor commutation period, the switching period of said first and second switching devices being less than the motor commutation period and each said first and second switching devices having more than one ON condition during the motor commutation period and only a portion of each of the ON conditions of the first and second switching devices overlap during the switching period, electrical current flows through said electric assist motor during each overlap portion of the ON conditions and power dissipation switching losses are shared by the first and second switching devices, the amount of the ON condition overlap controlling current in said electric assist motor, said control means including means for controlling said ON condition overlap in response to said sensed applied steering torque.

15. The apparatus of claim 14 wherein said control means includes means for pulse-width-modulating both said first and second switching devices between said ON and OFF conditions with the period of each pulse-width-modulation being less than the motor commutation period.

16. The apparatus of claim 14 wherein said electric assist motor is a variable reluctance motor.

17. The apparatus of claim 14 further including a first fly-back diode connected between said second motor terminal and said first terminal of said source of electrical energy and a second fly-back diode connected between said first motor terminal and said second terminal of said source of electrical energy.

18. The apparatus of claim 17 wherein said control means includes means for pulse-width-modulating each of said first and second switching devices with the period of each pulse-width-modulation being less than the motor commutation period.

19. The apparatus of claim 18 wherein said control means includes ramp generating means for providing a periodic ramp signal having a period less than the motor commutation period, means for comparing said ramp signal against first and second error reference signals, means for sensing current through said electric assist motor and providing a signal indicative thereof, means for controlling the value of said first and second error reference signals in response to said sensed current, said means for pulse-width-modulating each of said first and second switching devices being response to said comparison.

20. The apparatus of claim 19 wherein said control means further includes means for comparing said first and second error reference signal against first and second absolute reference values and means for disabling said electric assist system if either said first and second error reference signal exceed their associated first and second absolute reference values.

21. The apparatus of claim 19 where said control means further includes means for comparing said sensed current against a current threshold value and means for disabling said electric assist system if said sensed current exceeds said current threshold value.

22. A method for controlling an electric motor comprising the steps of:

providing a first switching device operatively coupled between one terminal of said electric motor and a first terminal of a source of electrical energy;

providing a second switching device operatively coupled between a second terminal of said electric motor and a second terminal of said source of electrical energy; and

controlling energization of said electric motor by:

(a) periodically switching said first and second switching devices between ON and OFF conditions during a motor commutation period so that the switching period of said first and second switching devices is less than the motor commutation period and each said first and second switching devices having more than one ON condition during the motor commutation period and only a portion of each of the ON conditions of the first and second switching devices overlap, electrical current flows through said electric motor during the overlap portion of the ON conditions, the amount of the overlap controlling current in said electric motor, and

(b) controlling said overlap.

23. The method of claim 22 further including the step of providing a first current sensing resistor connected between said second flyback diode and said second terminal of said source of electrical energy and a second current sensing resistor connected between said second switching device and said second terminal of said source of electrical energy.

24. The method of claim 22 further including the step of providing a first fly-back diode connected between said second motor terminal and said first terminal of said source of electrical energy and providing a second fly-back diode connected between said first motor terminal and said second terminal of said source of electrical energy.

25. The method of claim 24 wherein said step of switching includes pulse-width-modulating each of said first and second switching devices with the period of each pulse-width-modulation being less than the motor commutation period.

26. The method of claim 25 wherein said step of controlling energization further includes providing a periodic ramp signal having a period less than the motor commutation period, comparing said ramp signal against first and second error reference signals, sensing current through said electric motor and providing a signal indicative thereof, controlling the value of said first and second error reference signals in response to said sensed current, and wherein said step of pulse-width-modulating includes pulse-width-modulating each of said first and second switching devices in response to said comparison.

27. The method of claim 26 wherein each said first and second switching devices are periodically in an OFF state in response to said comparison during a PWM cycle and the time period of the OFF states during a PWM cycle is equally shared between said first and second switching devices.

28. The method of claim 22 further comprising the step of controlling said electric motor by periodically switching said first and second switching devices between ON and OFF conditions so that the ON condition of each said first and second switching devices does not overlap.

29. The method of claim 22 further comprising the steps of sensing motor current and providing an error signal if excess current is sensed.

30. A method for controlling a multiphase electric motor comprising the steps of:

providing a first switching device operatively coupled between one terminal of a phase of said electric motor and a first terminal of a source of electrical energy;

providing a second switching device operatively coupled between a second terminal of said phase of electric motor and a second terminal of said source of electrical energy; and

controlling said electric motor by periodically switching said first and second switching devices between ON and OFF conditions so that no portion of the ON conditions of each said first and second switching devices overlap and so that each of said first and second switching devices have more than one ON condition during a commutation period of said phase of said electric motor.

31. The method of claim 30 further including the step of providing a first fly-back diode connected between said second motor terminal and said first terminal of said source of electrical energy and providing a second fly-back diode connected between said first motor terminal and said second terminal of said source of electrical energy.

32. The method of claim 31 wherein said step of switching includes pulse-width-modulating each of said first and second switching devices with the period of each pulse-width-modulation being less than the motor commutation period.

33. The method of claim 32 wherein said step of controlling energization further includes providing a periodic ramp signal having a period substantially less than the motor commutation period, comparing said ramp signal against first and second error reference signals, sensing current through said electric motor and providing a signal indicative thereof, controlling the value of said first and second error reference signals in response to said sensed current, and wherein said step of pulse-width-modulating includes pulse-width-modulating each of said first and second switching devices in response to said comparison.

34. The method of claim 33 wherein each said first and second switching devices are periodically in an ON state in response to said comparison during a PWM cycle and the time period of the ON states during a PWM cycle is equally shared between said first and second switching devices.

35. A method controlling an electric assist steering system having an electric assist motor operatively connected to a steerable member, energization of said motor providing steering assist to said steering member, said method comprising the steps of:

sensing applied steering torque and providing a torque signal indicative thereof;

providing a first switching device operatively coupled between one terminal of said electric assist motor and a first terminal of a source of electrical energy;

providing a second switching device operatively coupled between a second terminal of said electric assist motor and a second terminal of said source of electrical energy; and

controlling energization of said electric assist motor, by:

(a) switching said first and second switching devices between ON and OFF conditions during a motor commutation period, the switching period of said first and second switching devices being less than the motor commutation period and each of said first and second switching devices having more than one ON condition during the motor commutation period and only a portion of each of the ON conditions of the first and second switching devices overlap during the switching period, electrical current flows through said electric assist motor during each overlap portion of the ON conditions and power dissipation switching losses are shared by the first and second switching devices, the amount of the ON condition overlap controlling current in said electric assist motor, and

(b) controlling said ON condition overlap in response to said sensed applied steering torque.

36. The method of claim 35 wherein said step of switching includes pulse-width-modulating both said first and second switching devices between said ON and OFF conditions with the period of each pulse-width-modulation being less than the motor commutation period.

37. The method of claim 35 further including the steps of providing a first fly-back diode connected between said second motor terminal and said first terminal of said source of electrical energy and providing a second fly-back diode connected between said first motor terminal and said second terminal of said source of electrical energy.

38. The method of claim 35 further including the steps of providing a periodic ramp signal with the period of each pulse-width-modulation being less than the motor commutation period, comparing said ramp signal against first and second error reference signals, sensing current through said electric assist motor and providing a signal indicative thereof, controlling the value of said first and second error reference signals in response to said sensed current, and wherein said step of pulse-width-modulating includes pulse-width-modulating each of said first and second switching devices in response to said comparison.

39. The method of claim 38 further including the steps comparing said first and second error reference signal against first and second absolute reference values and disabling said electric assist motor if either said first and second error reference signal exceed their associated first and second absolute reference values.

40. The apparatus of claim 38 further including the steps of comparing said sensed current against a current threshold value and disabling said electric assist motor if said sensed current exceeds said current threshold value.

TECHNICAL FIELD

The present invention is directed to an electric assist steering system and is particularly directed to a method and apparatus for controlling energization of an electric assist motor in a steering system.

BACKGROUND OF THE INVENTION

There are many known power assist steering systems for automotive vehicles. Some provide steering assist by using hydraulic power and others by using electric power.

Electric assist steering systems include an electric motor drivably connected to the steerable vehicle wheels. When energized, the electric motor assists the steering movement of the steerable wheels. The electric assist motor is controlled in response to steering torque applied to the steering wheel.

Known electric assist steering systems typically include a D.C. permanent magnet electric assist motor electrically energized through an H-bridge drive circuit. One such drive arrangement is disclosed in U.S. Pat. No. 4,660,671 to Behr et al., an assigned to TRW Inc. In this arrangement, the H-bridge includes four field-effect-transistors ("FET's") connected in the "H" pattern. The electric assist motor is energized by turning one of the FET's continuously ON and pulse-width-modulating ("PWM") the diagonally opposed FET of the H-bridge. Current is controlled by varying the duty cycle of the PWM signal. The direction of current through the motor controls the direction in which the motor will rotate and, in turn, controls the steering direction.

It is desirable to use a variable reluctance motor in an electric assist steering system because of its small size, low friction, and its high torque-to-inertia ratio. The direction of rotation of a variable reluctance motor is controlled by controlling the sequence in which the stator windings are energized. Torque is controlled by controlling the amount of current through the stator windings. Accurate current control is necessary to produce low ripple torque, thereby reducing vibration felt at the steering wheel. One control arrangement for a variable reluctance electric assist motor is disclosed in U.S. Pat. No. 5,257,828 to Miller et al., and assigned to TRW Inc., and which is hereby fully incorporated herein by reference. The '828 patent controls current in the assist motor by pulse-width-modulating a series connected solid state switch.

As with systems using DC electric motors, fly-back current in systems using a variable reluctance motor is typically controlled through fly-back diodes. The fly-back diodes provide a current path for the fly-back current that results from the collapsing magnetic field of a recently de-energized stator coil.

Also of concern in an electric assist steering system is the operating temperature of the switching devices used to control the current through the electric assist motor. The above-mentioned Behr et al. '671 patent discloses a temperature sensing arrangement that folds back the PWM control signal if the temperature of the switching FET's exceeds a predetermined value. Also, a current sense arrangement senses current through the motor and folds back current if the sensed current exceeds a predetermined current threshold value.

A further concern of an electric assist steering system is the operational integrity of the system. The '671 patent also teaches several self-diagnostic features of the system. These diagnostic features include a torque signal absolute limit test, a summed torque signal test, an excessive PWM test, and a direction test.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for controlling an electric assist steering system that includes an improved current sensing ability, reduces the number of components needed in the drive control circuit, and improves thermal management of switching devices used to control current through the electric assist motor.

In accordance with the present invention, an apparatus is provided for controlling an electric motor. The apparatus includes a first switching device operatively coupled between one terminal of the electric motor and a first terminal of a source of electrical energy, and a second switching device operatively coupled between a second terminal of the electric motor and a second terminal of the source of electrical energy. The apparatus further includes control means for controlling energization of the electric motor. The control means includes means for periodically switching the first and second switching devices between ON and OFF conditions so that the ON condition of each the first and second switching devices is less than one-hundred percent of a period and a portion of the ON condition of the first and second switching devices overlap. Electrical current flows through the electric motor during an overlap portion of ON conditions. The amount of the overlap controls current in the electric motor. The control means includes means for controlling the overlap.

In accordance with another aspect of the present invention, an apparatus is provided for controlling an electric assist steering system having an electric assist motor operatively connected to a steerable member. Energization of the motor provides steering assist to the steering member. The apparatus further includes torque sensing means for sensing applied steering torque and providing a torque signal indicative thereof. The apparatus further includes a first switching device operatively coupled between one terminal of the electric assist motor and a first terminal of a source of electrical energy and a second switching device operatively coupled between a second terminal of the electric assist motor and a second terminal of the source of electrical energy. Control means are provided for controlling energization of the electric assist motor. The control means includes means for periodically switching the first and second switching devices between ON and OFF conditions so that the ON condition of each the first and second switching devices is less than one-hundred percent of a period and a portion of the ON condition of the first and second switching devices overlap. Electrical current flows through the electric assist motor during an overlap portion of ON conditions of the switching devices. The amount of the overlap controls current in the electric assist motor. The control means includes means for controlling the overlap in response to the sensed applied steering torque.

Preferably, the electric assist motor is a variable reluctance motor. The control means preferably includes means for pulse-width-modulating both the first and second switching devices to the ON and OFF conditions. A first fly-back diode is connected between the second motor terminal and the first terminal of the source of electrical energy and a second fly-back diode is connected between the first motor terminal and the second terminal of the source of electrical energy. The control means includes ramp generating means for providing a ramp signal, means for comparing the ramp signal against first and second error reference signals, means for sensing current through the electric assist motor and providing a signal indicative thereof, and means for controlling the value of the first and second error reference signals in response to the sensed current. The means for pulse-width-modulating each of the first and second switching devices is response to the comparison. The control means further includes means for comparing the first and second error reference signals against first and second absolute reference values, respectively, and means for disabling the electric assist system if either the first and second error reference signal exceed their associated first and second absolute reference value.

In accordance with another aspect of the present invention, a method of controlling an electric motor comprises the steps of providing a first switching device operative coupled between one terminal of the electric motor and a first terminal of a source of electrical energy, and providing a second switching device operative coupled between a second terminal of the electric motor and a second terminal of the source of electrical energy. Energization of the electric motor is controlled by (a) periodically switching the first and second switching devices between ON and OFF conditions so that the ON condition of each the first and second switching devices is less than one-hundred percent of a period and a portion of the ON condition of the first and second switching devices overlap, electrical current flowing through the electric motor during an overlap portion of ON conditions of the switching devices, the amount of the overlap of the ON conditions controlling current in the electric motor, and (b) controlling said overlap.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from reading the following detailed description with reference to the accompanying drawings, in which:

FIG. 1 is a schematic block diagram of an electric assist steering system made in accordance with the present invention;

FIG. 2 is a cross-sectional view of the variable reluctance electric assist motor shown in FIG. 1;

FIG. 3 is a schematic illustration of the power switch module shown in FIG. 1;

FIG. 4 is a schematic block diagram of a portion of the system of FIG. 1 shown in further detail;

FIG. 5 is a schematic block diagram of the error amplifier shown in FIG. 4;

FIG. 6 is a schematic block diagram of a portion of the motor drive controller shown in FIG. 4;

FIG. 7 is a graphical representation of a PWM cycle during an energization portion of a cycle;

FIG. 8 is a graphical illustration of a stator winding current profile during an energization cycle;

FIG. 9 is a graphical representation of a PWM cycle during a non-energizing portion of a cycle;

FIG. 10 is a schematic block diagram similar to FIG. 4 of a portion of the controller shown in FIG. 1 in greater detail;

FIG. 11 is a schematic block diagram of the fault detector circuit shown in FIG. 10;

FIG. 12 is a schematic block diagram of an alternate embodiment of a portion of the system of FIG. 1; and

FIG. 13 is a schematic diagram of the ramp circuit shown in FIGS. 4 and 10.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1, an electric assist steering system 10 includes a steering wheel 12 operatively connected to a pinion gear 14. Specifically, the vehicle steering wheel 12 is connected to an input shaft 16 and the pinion gear 14 is connected to a pinion shaft 17. The input shaft 16 is operatively coupled to the pinion shaft 17 through a torsion bar 18. The torsion bar 18 twists in response to applied steering torque thereby permitting relative rotation between the input shaft 16 and the pinion shaft 17. The amount of relative rotation is functionally related to the torsion bar strength and the amount of applied steering torque. Stops, not shown, operative between the input shaft 16 and the pinion shaft 17 limit the amount of such relative rotation between the input and pinion shafts in a manner well known in the art.

The pinion gear 14 has helical teeth which are meshingly engaged with straight cut teeth on a rack or linear steering member 20. The pinion gear in combination with the straight cut gear teeth on the rack member form a rack and pinion gear set. The rack 20 is steerably coupled to the vehicle's steerable wheels 22, 24 with steering linkage in a known manner. When the steering wheel 12 is turned, the rack and pinion gear set converts the rotary motion of the steering wheel into linear motion of the rack. When the rack moves linearly, the steerable wheels 22, 24 pivot about their associated steering axis and the vehicle is steered.

A shaft position sensor 25 is operatively connected across the input shaft 16 and the pinion shaft 17 and provides an electrical signal having a value indicative of the relative rotational position between the input shaft 16 and the output shaft 17. The shaft position sensor 25 in combination with the torsion bar 18 form a torque sensor 27. The output of the torque sensor 27 is indicative of the applied steering torque to the vehicle steering wheel 12 by the vehicle operator.

The output of the torque sensor 27 is connected to a controller 76. The controller 76 processes the torque signal provided by the torque sensor 27, and determines, in accordance with any one of the many methods known in the art, a torque command and direction value therefrom. Preferably, the torque command and direction signal is determined in accordance with the process described in the above-incorporated U.S. Pat. No. 5,257,828 to Miller et al. The torque command and direction value represents the amount and direction of torque to be generated by an assist motor 26.

An electric assist, variable reluctance motor 26 is drivingly connected to the rack 20 preferably through a ball-nut drive arrangement. When the motor 26 is energized, it provides steering assist to aid in the rotation of the vehicle steering wheel 12 by the vehicle operator and, in turn, steering of the steerable wheels 22, 24. A variable reluctance motor is desirable for use in an electric assist steering system because of its small size, low friction, and its high torque-to-inertia ratio.

Referring to FIG. 2, the variable reluctance motor 26, in accordance with a preferred embodiment of the present invention, includes a stator 28 with eight stator poles 30 and a rotor 32 with six rotor poles 34. Each stator pole 30 has an associated stator coil (not shown). The stator poles are arranged so as to be energized in pairs designated Aa, Bb, Cc, and Dd thereby resulting in four stator pole pairs and six rotor poles 34. The motor 26 is mounted in a motor housing 36 so that the stator 28 is fixed relative to the housing 36.

The principle of operation of a variable reluctance motor is well known in the art. Basically, the stator poles are energized in pairs. Specifically, electric current is provided to the stator coils associated with a given pair of stator poles. The rotor moves so as to minimize the reluctance between the energized stator poles and the rotor poles. Minimum reluctance occurs when a pair of rotor poles are aligned with the energized stator poles. Once minimum reluctance is achieved, i.e., when the rotor poles align with the energized stator poles, those energized stator poles are de-energized and an adjacent pair of stator poles are energized. The direction of motor rotation is controlled by controlling the sequence in which the stator poles are energized. The torque produced by the motor is controlled by controlling the amount of current through the energized stator coils.

Referring to FIG. 1, a rotor position sensor 38 is operatively connected between the motor rotor 32 and the motor stator 28 or housing 36. The stator 28 and motor housing 36 are relatively stationary. The function of the rotor position sensor 38 is to provide an electrical signal indicative of the position of the rotor 32 relative to the motor stator 28. For control of the operation of the variable reluctance motor 26, including direction of rotation and motor torque, it is necessary to know the position of the rotor 32 relative to the stator 28. One arrangement for sensing rotor position in a variable reluctance motor in an electric assist steering system is fully disclosed in the above-incorporated U.S. Pat. No. 5,257,828 to Miller et al.

A controller 76 is operatively connected to the torque sensor 27, to the motor position sensor 38, and to a power switch module 40. The power switch module 40 is operatively connected between the vehicle battery and the electric assist motor 26. The controller 76 controls the energization sequence of the stator pole pairs Aa, Bb, Cc and Dd (FIG. 2) to control motor direction in response to the direction of applied steering torque and motor position. The controller 76 controls the current in the stator windings in response to the amount of applied steering torque.

Referring to FIG. 3, the power switch module 40 includes a normally open electromagnetic relay circuit 42 having a terminal 43 of a relay coil 52 electrically connected to the positive supply of a vehicle's battery through the vehicle's ignition switch. The anode of a Zener diode 54 is connected to coil 52 at terminal 43. The cathode of Zener diode 54 is connected to the cathode of a diode 56. When coil 52 is de-energized, the collapsing magnetic field creates a reverse polarity inductive current. The Zener breakdown voltage of Zener diode 54 is selected to provide a current path for the reverse polarity current due to the collapsing magnetic field. This permits the relay 42 to switch open faster. The anode of diode 56 is connec