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Description  |
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MICROFICHE APPENDIX
This patent includes a Microfiche Appendix that includes nineteen frames on a single sheet of microfiche.
TECHNICAL FIELD
The present invention is directed to an electric assist steering system and is particularly directed an electric assist steering system having an improved motor current controller that provides a controllable bandwidth consistent to control
objectives such as constant bandwidth substantially independent of motor operating conditions, reduced motor acoustic noise, increased torque output at speeds, and reduced torque ripple.
BACKGROUND OF THE INVENTION
Electric assist steering systems are well known in the art. Electric power assist steering systems that utilize a rack and pinion gear set provide power assist by using an electric motor to either (i) apply rotary force to a steering shaft
connected to a pinion gear, or (ii) apply linear force to a steering member having the rack teeth thereon. The electric motor in such systems is typically controlled in response to (i) a driver's applied torque to the vehicle steering wheel, and (ii)
sensed vehicle speed.
U.S. Pat. No. 4,415,054 to Drutchas (now U.S. Reissue Pat. No. 32,222,), assigned to TRW Inc., utilizes a D.C. electric assist motor driven through an "H-bridge" arrangement. The assist motor includes a rotor encircling a steering member.
The steering member has a first portion with a thread convolution and a second portion with straight cut rack teeth. Rotation of the electric assist motor rotor causes linear movement of the steering member through a ball-nut drivably connected to the
thread convolution portion of the steering member. A torque sensing device is coupled to the steering wheel for sensing driver applied torque to the steering wheel. The torque sensing device uses a magnetic Hall-effect sensor that senses relative
rotation between the input and output steering shafts across a torsion bar. An electronic control unit ("ECU") monitors the signal from the torque sensing device. A vehicle speed sensor provides a signal to the ECU indicative of the vehicle speed. The
ECU controls current through the electric assist motor and, in turn, steering assist in response to both the sensed vehicle speed and the sensed applied steering torque. The ECU decreases steering assist as vehicle speed increases. This is commonly
referred to in the art as speed proportional steering.
U.S. Pat. No. 5,257,828 to Miller et al., and assigned to TRW Inc., discloses an electric assist steering system having yaw rate control. This system uses a variable reluctance ("VR") motor to apply steering assist to the rack member. The
torque demand signal is modified as a function of a steering rate feedback signal so as to provide damping.
U.S. Pat. No. 5,504,403 to McLaughlin, and assigned to TRW Inc., discloses a method and apparatus for controlling an electric assist steering system using an adaptive blending torque filter. The adaptive blending torque filter processes the
applied steering torque signal and maintains a selectable system bandwidth during system operation. This arrangement provides a steering system having a bandwidth that is substantially independent of vehicle speed and applied steering torque.
Ideally, the electric motor of an electric assist steering system will have a bandwidth much greater than that of the electric steering system so that the response of the electric motor does not negatively impact the stability of the steering
system. A variable reluctance motor is such a high bandwidth motor. A constant bandwidth motor is desirable so as to achieve control not only over the low frequency steering operation, but also over the higher frequency acoustic noise so that the motor
is quiet. However, the uncontrolled bandwidth of a VR motor varies and is a function of the motor current i, the rotor position .theta. relative to the stator, the motor resistance, and motor temperature t. It is desirable to maintain a consistent
system bandwidth independent of such motor operating conditions. The controller must compensate for this varying bandwidth to achieve a constant bandwidth. VR motors have acoustically sensitive structural modes in which the motor's stator housing
("shell") experience movement in a radial direction, and at particular drive frequencies, the motor shell will resonate. Unfortunately, this resonance can occur in the human audible range. The motor will, in effect, act as a "speaker" producing an
undesirable motor buzz. The motor can further exhibit a "microphone" effect as a result of shell acceleration resulting in current oscillations in the motor coils inducing further noise out of the motor. It is, therefore, desirable to reduce such
acoustic noise and, in turn, torque ripple.
SUMMARY OF THE INVENTION
In accordance with the present invention, a motor controller is provided having a variable gain. The gain is controlled as a function of the motor's rotor position and motor current. The gain is controlled so as to provide a consistent current
bandwidth substantially independent of rotor position and motor current. A filter is provided in a control loop to filter from a current command signal frequencies that could result in motor shell resonance. The system, in accordance with the present
invention provides (i) consistent operating bandwidth, (ii) reduced acoustic noise, (iii) a fast response time, (iv) reduced torque ripple, and (v) increased torque output at speeds. In accordance with one aspect of the present invention, a motor
controller includes means for summing a received motor current command signal with a motor current feedback signal and for providing an error current command signal having a value functionally related to the difference between the motor current command
signal and the motor current feedback signal. Notch filter means filters the error current command signal and provides a filtered current command signal. The notch filter is adapted to notch out frequencies from the error current command signal about
the resonant frequency of a motor. A drive circuit energizes the motor in response to the filtered current command signal. A motor current sensor for sensing motor current and providing the motor current feedback signal.
In accordance with another aspect of the present invention an electric assist steering system includes a torque sensor for sensing applied steering torque to a vehicle steering wheel and for providing a signal having a value functionally related
to the applied steering torque. A motor drivably connected to a steering member of a vehicle for, when energized, providing steering assist. A motor controller is operatively connected to the torque sensor for providing a motor current command signal
having a value functionally related the value of the applied steering torque signal. The system further includes means for summing the motor current command signal with a motor current feedback signal and provides an error current command signal having
a value functionally related to the difference between the motor current command signal and the motor current feedback signal. The system further includes notch filter means for filtering the error current command signal and for providing a filtered
current command signal. The notch filter is adapted to notch out frequencies from the error current command signal about the resonant frequency of the motor. A drive circuit energizes the motor in response to the filtered current command signal. A
motor current sensor senses motor current and provides the motor current feedback signal to the means for summing. In accordance with another aspect of the present invention, a method for controlling a motor includes the steps of summing a motor current
command signal with a motor current feedback signal and providing an error current command signal having a value functionally related to the difference between the motor current command signal and the motor current feedback signal. The method further
includes the steps of notch filtering the error current command signal and providing a filtered current command signal. The step of notch filtering notches out frequencies from the error current command signal about the resonant frequency of a motor The
motor is energized in response to the filtered current command signal. The method further includes the step of sensing motor current and providing the motor current feedback signal.
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 a reading of the following detailed description with reference to the accompanying drawings, in
which:
FIG. 1 is a schematic block diagram illustrating a power assist steering system in accordance with the present invention;
FIG. 2 is a schematic block diagram of a portion of the system of FIG. 1 showing the adaptive blending torque filter in greater detail;
FIG. 3 is a schematic block diagram of a portion of the system of FIG. 1 showing the digital motor current controller in greater detail;
FIG. 4 is a schematic circuit diagram of a portion of the drive circuit and power switches shown in FIG. 1;
FIG. 5 is block diagram showing a portion of the closed loop control function of the present invention;
FIG. 6 is a three dimensional graphical representation of an inductance map of a VR motor;
FIG. 7 is a three dimensional graphical representation of a proportional gain map as a function of sensed current and rotor angle.
FIG. 8 is a Bode plot of an open loop transfer function of a steering system at different current values with a motor offset at 0.degree.;
FIG. 9 is an illustration of a frequency response of motor shell acceleration to motor current command for the open loop system of FIG. 8;
FIG. 10 is a Bode plot of an open loop transfer function of a steering system at different current values with a motor offset at 30.degree.;
FIG. 11 is an illustration of a frequency response of motor shell acceleration to motor current command for the open loop system of FIG. 10;
FIG. 12 is a Bode plot of a closed loop transfer function of a steering system not having the gain scheduler of the present invention at different current values with a motor offset at 0.degree.;
FIG. 13 is an illustration of a frequency response of motor shell acceleration to motor current command for the closed loop system of FIG. 12;
FIG. 14 is a Bode plot of a closed loop transfer function of a steering system having the gain scheduler of the present invention at different current values with a motor offset at 0.degree.;
FIG. 15 is an illustration of a frequency response motor shell acceleration to motor current command for of the closed loop system of FIG. 14;
FIG. 16 is a gain plot of a Bode plot of a typical notch filter;
FIG. 17 is a phase plot of the Bode plot for the notch filter of FIG. 16;
FIG. 18 is a root locus plot of a notch filter in a complete current control system;
FIG. 19 is a graphical illustration of a gain comparison of the notch consistent bandwidth controller of the present invention;
FIG. 20 is a graphical illustration of a phase comparison of the notch consistent bandwidth controller of the present invention; and
FIG. 21 is a graphical illustration of motor shell acceleration with and without the notch consistent bandwidth controller of the present invention .
DESCRIPTION OF PREFERRED EMBODIMENT
Referring to FIG. 1, a power 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
an output shaft 18. The input shaft 16 is operatively coupled to the output shaft 18 through a torsion bar 20.
The torsion bar 20 twists in response to applied steering torque thereby permitting relative rotation between the input shaft 16 and the output shaft 18. Stops, not shown, limit the amount of such relative rotation between the input and output
shafts in a manner known in the art. The torsion bar 20 has a spring constant referred to herein as K.sub.t. In accordance with a preferred embodiment, the spring constant K.sub.t =20 in-lb/deg. The amount of relative rotation between the input shaft
16 and the output shaft 18 in response to applied steering torque is functionally related to the spring constant of the torsion bar.
As is well known in the art, the pinion gear 14 has helical teeth, not shown, which are meshingly engaged with straight cut teeth, not shown, on a rack or linear steering member 22. The pinion gear 14 in combination with the straight cut gear
teeth on the rack member 22 form a rack and pinion gear set. The rack is steerably coupled to the vehicle's steerable wheels 24, 26 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 12 into linear motion of the rack 22. When the rack moves linearly, the steerable wheels 24, 26 pivot about their associated steering axes and the vehicle is steered.
An electric assist motor 28 is drivingly connected to the rack 22 through a ball-nut, drive arrangement. When the electric motor 28 is energized, it provides power assist steering by aiding in the linear drive of the rack so as to aid in the
rotation of the vehicle steering wheel 12 by the vehicle operator.
In accordance with a preferred embodiment of the present invention, the electric assist motor 28 is a variable reluctance ("VR") motor. A VR 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. The VR motor 28, in accordance with a preferred embodiment of the present invention, is a four phase motor having eight stator poles and six rotor poles. The stator poles are arranged so as to be
energized in pairs thereby forming the four phases of the motor.
The principles of operation of a VR motor are well known in the art and, therefore, are not described herein in detail. Basically, the stator poles are energized in pairs. The rotor moves so as to minimize the magnetic reluctance between the
energized stator poles and the closest pair of rotor poles. Minimum reluctance occurs when a pair of rotor poles is 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 coils are de-energized and, assuming further motor movement is desired, an adjacent pair of stator coils (depending on the desired motor direction) are energizer.
In many DC motors, controlling the direction of current flow through the motor windings controls direction of motor rotation. In a VR motor, current is passed through the stator coils in only one direction independent of the desired direction of
motor operation. The direction of motor rotation is controlled by the sequence in which the stator coils are energized. For example, for the motor to move in one direction, phase Aa is energized followed by Bb. If it is desirable to move the motor in
the opposite direction, the energization of phase Aa would be followed by the energization of phase Dd.
Controlling the current through the stator coils controls the torque produced by the motor. When the assist steering motor is energized, the rotor turns which, in turn, rotates the nut portion of the ball-nut drive arrangement. When the nut
rotates, the balls transfer a linear force to the rack. The direction of rack movement and, in turn, the direction of steering movement of the steerable vehicle wheels, is dependent upon the direction of rotation of the motor.
A motor rotor position sensor 30 is operatively connected to the motor rotor and to the motor housing. The function of the rotor position sensor 30 is to provide an electric signal indicative of the position of the
motor rotor relative to the motor stator. As is well known in the art, operation of a VR motor requires this position information. While any known rotor position sensor may be used with the present invention, a rotor position sensor of the
type disclosed in U.S. Pat. No. 5,625,239 to Persson et al., and assigned to TRW Inc., is preferred.
A steering shaft position sensor 40 is operatively connected across the steering input shaft 16 and the steering output shaft 18 and provides an electric signal having a value indicative of the relative rotational position or relative angular
orientation between the input shaft 16 and the output shaft 18. The position sensor 40 in combination with the torsion bar 20 form a torque sensor 44 that provides and electric signal having a value indicative of the applied steering torque. The
steering wheel 12 is rotated by the driver during a steering maneuver through an angle .theta..sub.HW. The relative angle between the input shaft 16 and the output shaft 18 as a result of applied input torque is referred to herein as .theta..sub.P.
Taking the spring constant K.sub.t of the torsion bar 20 into account, the electric signal from the sensor 40 is also indicative of the applied steering torque referred to herein as .tau..sub.s.
The output of the torque sensor 44 is connected to a torque signal processing circuit 50. The processing circuit 50 monitors the applied steering torque angle .theta..sub.p and, "knowing" the spring constant K.sub.t of the torsion bar 20,
provides an electric signal indicative of the applied steering torque .tau..sub.s.
The torque sensor signal is passed through a filtering circuit 52. Preferably, the filter 52 is an adaptive blending torque filter of the type disclosed in U.S. Pat. No. 5,504,403 to McLaughlin, and assigned to TRW Inc. The adaptive blending
torque filter 52 receives a vehicle speed signal from a vehicle speed sensor 56. The adaptive blending torque filter 52 is adapted to have a non-linear characteristic at torque frequencies less than a blending frequency and a linear characteristic at
torque frequencies greater than the blending frequency. The blending filter 52 establishes the blending frequency at a value functionally related to the vehicle speed. It is contemplated that other torque signal filter arrangements may be used with the
present invention. The purpose of the adaptive blending torque filter is to maintain a selectable system bandwidth during system operation and thereby, prevent steering sluggishness as vehicle speed increases.
Referring to FIG. 2, the blending filter 52 includes both a low pass filter 70 and a high pass filter 71, both connected to the output of the torque signal processor 50. The filters 70, 71 are designed such that summation of the two filters is
identically one for all frequencies. The low pass filter 70 allows all of the signal .tau..sub.s with frequency .tau..sub.sl content below a predetermined blending frequency W.sub.b to pass through while rejecting all high frequency data. The high pass
filter 71 allows all of the signal .tau..sub.s with frequency .tau..sub.sh content above some blending frequency W.sub.b to pass through while rejecting all low frequency data. The value of the blending filter frequency W.sub.b is a function of vehicle
speed and is determined by the blending filter determination circuit 68 connected to the output of the speed sensor 56. The determination of W.sub.b may be accomplished using a look-up table in a microcomputer having predetermine stored values or by
calculation in accordance with a desired control function.
The low pass torque sensor output is connected to an assist curve circuit 69, which is preferably a look-up table. The vehicle speed sensor 56 is also operatively connected to the assist curve circuit. As is well known in the art, the amount of
power assist desired for a vehicle steering system decreases as vehicle speed increases. Therefore, to maintain a proper or desirable feel for steering maneuvers, it is desirable to decrease the amount of steering power assist as the vehicle speed
increases. This is referred to in the art as speed proportional steering.
The assist torque .tau..sub.assist values are determined from stored values in a look-up table representing a plurality of assist curves of torque-in values verses torque-out values. Since torque assist varies as a function of vehicle speed,
these curves range from values required during dry surface parking to those needed at high vehicle speeds. Generally, the value of the output from the assist curve circuit 69 is referred to as .tau..sub.assist. The actual values for control are
determined from interpolation of the predetermined values stored in the look-up table if needed. Preferably, dual assist curves with interpolation are used as described in U.S. Pat. No. 5,568,389 to McLaughlin et al. and assigned to TRW Inc.
The high passed torque sensor signal .tau..sub.sh from a high pass filter is multiplied 72 by a predetermined gain value S.sub.c1 that is a function of the vehicle speed. The determination of S.sub.c1 may be accomplished using a look-up table in
a microcomputer or may be accomplished using an actual calculation in accordance with a desired control function. Modification of the high frequency assist gain value S.sub.c1 allows the bandwidth of the steering system to be modified.
The assist curve value .tau..sub.assist output from 69 and the determined high frequency assist gain value from 72 are summed in a summing function 79. This summed value output of the summing circuit 79 is referred to as .tau..sub.ba and is
connected to an adaptive torque filter circuit 80.
The adaptive torque filter circuit 80 filters the input blended assist torque signal .tau..sub.ba. The filter is adaptive in that its poles and zeros are allowed to change as the vehicle speed changes so as to provide an optimal control system.
This filtering and results in a filtered torque signal .tau..sub.m is referred to herein as the torque demand signal. The torque demand signal .tau..sub.m is connected to a motor controller 90.
The blending filter determination circuit 68 and adaptive filter 80 are fully described in the above mentioned McLaughlin '403 patent. Basically, a linearized closed loop control system is considered for the design of the blending filter and
adaptive filter for the steering system 10. Rotation of the hand wheel 12 results in an angular displacement of .theta..sub.HW on the steering wheel side of the torsion bar position sensor 40. This angular displacement is differenced with the resultant
angular displacement of the output shaft 18 after it is driven in rotation by the electric assist motor by an angle .theta..sub.m through the gearing ratio represented by r.sub.m /r.sub.p where r.sub.m is the effective radius of the motor ball nut and
r.sub.p is the effective radius of the pinion. One radian of rotation of the ball nut produces r.sub.m inches of travel of the rack. Similarly, one radian of rotation of the pinion produces r.sub.p inches of travel of the rack. The resultant angular
displacement .theta..sub.p times the spring constant K.sub.t gives the torque signal .tau..sub.s. In the closed loop arrangement, the output .tau..sub.s is connected to the low pass/high pass filter circuits.
The torque signal .tau..sub.s is passed through the low pass filter 70 resulting in the low passed assist torque .tau..sub.sl. The high passed assist torque .tau..sub.sh is determined by subtracting the low frequency assist torque from the
torque signal .tau..sub.s. The reason that .tau..sub.sh can be determined in this way is discussed below.
Continuous domain blending filters are chosen such that the sum of the low pass filter G.sub.1 (S) and the high pass filter G.sub.H (S) is always equal to one. The low pass filter is chosen to be a first order filter with a pole at
.omega..sub.b. The high pass filter is defined by the constraint that the sum of the two filters must be one. Therefore, the low and high pass filters can be represented as: ##EQU1## When realizing a set of blending filters in a digital computer, those
skilled in the art will appreciate that it is not necessary to construct separate high and low pass filter stages. Rather, the input to the blending filters .tau..sub.s is passed through the low pass filter resulting in the signal .tau..sub.sl. The
high passed signal is the original input torque minus the low passed portion. This can be thoug | | |
