The present invention relates to a power steering apparatus for steering a wheel for steering of an automotive vehicle in accordance with a driver's operation and particularly to an electric power steering apparatus which can independently design treatments of, for example, road surface information and disturbance information and steering safety and also can obtain a safe, comfortable steering performance which is easily tunable.
Steering of an automotive vehicle is performed by transmitting an operation (ordinarily a rotation operation of a steering wheel) of a steering device provided inside a vehicle compartment to a steering mechanism provided outside the vehicle compartment for performing a turning maneuver in the direction of a wheel (ordinarily, front wheel) for steering.
As for such steering mechanisms for automotive vehicles, various types of steering mechanisms such as a ball screw type and a rack-pinion type have been in practical use. For example, the steering mechanism of the rack-pinion type, which is configured such that sliding in an axial direction of a rack shaft extended in a right-and-left direction at a front portion of a vehicle body is transmitted to each of right and left front wheels via a tie-rod and a knuckle arm provided thereto, is constituted such that a pinion, which is fit into a tip end of a rotation shaft (steering column) of the steering wheel extending to outside the vehicle compartment, is meshed with a rack gear formed in a middle section of the rack shaft and, then, the rotation of the steering wheel is converted into sliding in an axial direction of the rack shaft, to thereby perform steering in accordance with a rotation operation of the steering wheel.
Further, in recent years, a power steering apparatus, which is constituted such that an actuator for a steering assistance such as a hydraulic cylinder or an electric motor is provided in a middle section of the steering mechanism, the actuator is driven in accordance with a detection result of a steering force to be added to the steering wheel for steering, a movement (drive) of the steering mechanism in accordance with the rotation of the steering wheel is assisted by an output from the actuator and, then, a labor load of a driver is alleviated, has widely been applied.
Now, an ordinary constitution of the electric power steering apparatus is described with reference to
The control unit 30 is mainly constituted by a CPU. An ordinary function to be executed by a program in the CPU is shown in
The function and an operation of the control unit 30 is now described. The steering torque T to be inputted after detected by the torque sensor 10 is phase-compensated by a phase-compensating device 31 for enhancing stability of a steering system and the thus-phase-compensated steering torque TA is inputted to a steering assisting command value computing device 32. Further, the vehicle speed V detected by the vehicle sped sensor 12 is also inputted to the steering assisting command value computing device 32. The steering assisting command value computing device 32 determines the steering assisting command value I which is a control target value of the electric current to be supplied to the motor 20 based on the inputted steering torque TA and the vehicle speed V. The steering assisting command value I is not only inputted to a subtracting device 30A but also supplied to a differential compensating device 34 of a feed-forward system for enhancing a response speed and, then, deviation (I-i) of the subtracting device 30A is not only inputted to a proportional computing device 35 but also inputted to an integral computing device 36 of a feedback system for improving characteristics thereof. The output from each of the differential compensating device 34 and the integral computing device 36 is inputted to an adding device 30B in an addition manner and, then, an electric control value E which is a result of such addition in the adding device 30B is inputted to a motor drive circuit 37 as a motor drive signal. A motor electric current value i of the motor 20 is detected by a motor electric current detecting circuit 38 and, then, the thus-detected motor electric current value i is inputted to the subtracting device 30A, to thereby be fed back.
On the other hand, the mechanism as shown in
Frequency response characteristics of the control unit 301 is shown in
A conventional electric power steering apparatus is configured such that it can simultaneously design stability of a system and a treatment against road surface information and disturbance information by a robust stabilization compensating device. The robust stabilization compensating device is a compensating device as described in, for example, JP-A No. 8-290778 which has a characteristic formula represented by G(s)=(s2+a1·s+a2)/(s2+b1·s+b2) in which s represents a Laplace operator, removes a peak value of resonance frequency of a resonance system comprising an inertia factor and a spring factor contained in the steering torque T and compensates a phase deviation of the resonance frequency which inhibits the stability and response of the control system.
However, it is difficult, from a standpoint of tuning, to treat a plurality of information and signals in a plurality of frequency bands by a single compensating device. Particularly, when mechanical or electric characteristics is changed even to a small extent, there is a problem in that it takes longer time in tuning. Further, unless by a fairly experienced engineer, there is a problem in that an apparatus of same performance can not be obtained.
The present invention has been accomplished under these circumstances and an object of the present invention is to provide an electric power steering apparatus which is easily tunable, constituted at a low cost and can obtain a safe, comfortable steering feeling.
The present invention relates to an electric power steering apparatus which controls a motor that gives a steering assisting force to a steering mechanism based on an electric current controlling value which is computed from a steering assisting command value which has been computed by a computing device based on a steering torque generated in a steering shaft and an electric current value of the motor and the stated object of the present invention can be attained by being provided with a self-aligning torque estimating section which estimates a self-aligning torque by a disturbance observer constitution and a steering torque feedback section which performs definition of a steering reaction force based on a self-aligning torque estimated value which has been estimated by the self-aligning torque estimating section and feeds the steering reaction force back to the steering torque.
According to the present invention, a self-aligning torque estimating section for estimating a self-aligning torque by a disturbance observer constitution is provided and, then, definition of a steering reaction force is performed based on a self-aligning torque estimated value which has been estimated by the self-aligning torque estimating section and a motor rotation (angle) signal or angular speed signal and, thereafter, the steering reaction force is fed back to a steering torque. Further, according to the present invention, the self-aligning torque is estimated and the resultant self-aligning torque estimated value is fed back to the steering torque together with torque information of a torsion bar. A control unit according to the present invention has a robust property in that stability of a system can be secured regardless of fluctuations of characteristics (for example, resonance frequency) of the system. Still further, the definition of static characteristics of the steering reaction force is determined based on a necessary steering force and the self-aligning torque estimated value and the definition of dynamic characteristics of the steering reaction force is performed such that a gain of transfer function in a frequency band of information which is desirous to be conveyed to a driver is allowed to be large while the gain of transfer function in the frequency band of information which is not desirous to be conveyed to the driver is allowed to be small. For this account, the definition of necessary steering reaction force can easily be performed and a low-cost constitution and a stable, comfortable steering feeling can be realized.
Furthermore, since a motor rotation angle signal (or a motor angular speed signal) and a motor electric current command value are allowed to be used for estimating the self-aligning torque, a constitution of the self-aligning torque estimating section of high precision and low-cost can be realized by using the motor rotation angle signal (or the motor angular speed signal) used for controlling the motor.
The self-aligning torque estimating section 110 comprises a factor 111 (Q/Pn) in which the motor angular speed ω is inputted and treated and a factor 112 (M·Q) in which the motor electric current command value Iris inputted and treated, is allowed to determine a deviation between the output of the factor 111 and the output of the factor 112 by a subtracting device 113 and, then, outputs the result(the deviation) as the self-aligning torque estimated value ES. Q(s) indicates a low-pass filter, while Pn(s) indicates a theoretical model of rack-pinion. The factor 111 is constituted by a transfer function Q(s) and a transfer function Pn−1, while M indicates a transfer function (=1/(T1·s+1)) which, then, forms the factor 112 by being multiplied by Q(s).
The M·Q of the factor 112 is a product of an electric characteristic M of the motor and a low-pass filter Q, while the Q/Pn of the factor 111 is a quotient obtained by dividing the low-pass filter Q by an ideal model Pn. The basis on which the self-aligning torque estimating section 110 can compute the self-aligning estimated torque value ES is as described below. A torque Tm is represented by the following formula (1):
Tm=M(s)×Ir (1)
wherein M(s)=(Kt×Gr)/(T1·s+1).
Further, the motor angular speed ω is represented by the following formula (2):
ω=P(s)×[Tm+Ttb−SAT] (2)
wherein P(s)=1/(Jpt·s+Cpt)
Still further, based on a constitution of the self-aligning torque estimating section 110, the self-aligning estimated value ES is represented by the following formula (3):
ES=M·Q−Q/Pn (3)
Therefore, when the formula (3) is substituted by the formulae (1) and (2), the result comes to be as follows:
Further, when the pinion-base inertial moment Jpt and pinion-base damping coefficient Cpt value of the pinion-rack mechanism 130 are determined such that the relation of Pn(s)=P(s) is held, the following relation can be obtained:
ES=Q(s)×[SAT−Ttb] (5)
Therefore, since the addition result of the adding device 135 is inputted in the steering feedback section 120, the following formula (6) can be obtained:
Therefore, in the range in which Q(s)=1, the following formula can be obtained:
SatE=SAT (7)
From the above description, the relation between the self-aligning torque SAT and the self-aligning torque estimated value ES is represented by the formula (5), while a relation between the self-aligning torque SAT and the addition result SatE is represented by the formula (7).
Further, the filter Q, the motor characteristics M and characteristics of the model Pn can be represented by respective formulae as described below.
When the filter Q(s) uses the angular speed ω and, also, Tq is a time constant, the Q(s) is represented by the following formula:
Q(s)=1/(Tq·s+1) (8)
When it uses the angle θ and, also, b0 and b1 are each a constant, the Q(s) is represented by the following formula:
Q(s)=b1/(s2+b0·s+b1) (9)
Each of them (8) (9) shows a high-cutoff filter. Further, the motor characteristics M(s) and model P(s) can be represented by the following formulae:
M(s)=Kt×Gr/(T1·s+1) (10)
P(s)=1/(Jpn·s+Cpn) (11)
As described above, the addition result SatE is inputted to the steering feedback section 120, while the deviation (AT−Th) between the steering torque Th and the steering reaction force AT which is an output from the steering feedback section 120 are inputted to the control unit 100 and, in the present control system, the steering torque Th and the SAT information are utilized for the feedback control.
Further, according to the present invention, characteristics of the control unit 100 are allowed to be gain and phase characteristics as shown in
In a constitution as described above, the deviation (AT−Th) between the steering torque Th and the steering reaction force AT which is an output from the steering torque feedback section 120 is obtained by the subtracting device 101 and the deviation (AT−Th) is inputted in the control unit 100 and, then, the motor electric current command value Ir which is an output therefrom not only drives the motor 102 but also is inputted in the self-aligning torque estimating section 110 of a disturbance observer constitution. The control unit 100 compensates stability of an entire system and has robust characteristics by securing the stability of the entire system regardless of fluctuations of characteristics (for example, resonance frequency) of the system. Determination of the transfer function K(s) of the control unit 100 may either be performed by PID or a try-and-error method.
An output of the motor 102 is inputted in the adding device 105 via the motor torque coefficient 103 (Kt) and the gear ratio 104 (Gr) and, then, the resultant addition value is inputted in the pinion-rack mechanism 130 (1/(Jpt·s+Cpt)) via the subtracting device 106. An output from the pinion-rack mechanism 130 is inputted in the subtracting device 133 via the integral factor 131 (1/s) and the output of the integral factor 131 is inputted in the factor 132 which indicates dynamic characteristics of the vehicle and, then, the self-aligning torque SAT which is an output therefrom is inputted in the subtracting device 106. Further, the addition result in the subtracting device 133 is outputted via the spring coefficient 134 (Ktb) of the torsion bar.
An output Ttb from the spring coefficient 134 (Ktb) is not only inputted in the adding device 135 but also fed back to the adding device 105, while the motor angular speed ω which is an output from the pinion-rack mechanism 130 is inputted in the self-aligning torque estimating section 110. Then, the self-aligning torque estimated value ES from the self-aligning estimating section 110 is inputted in the steering torque feedback section 120 via the adding device 135. The steering torque feedback section 120 comprises the dynamic characteristic sub-section 121 and the static characteristic sub-section 122 of feeling characteristics of torque which a human being feels.
According to the present invention, the self-aligning torque for the electric power steering and the feedback control system using the steering torque are utilized and the gist thereof, that is, the control unit 100 of the feedback, being characterized by the frequency characteristics (gain and phase) in
The result of the characteristics K(s) of the control unit 100 applied to the case of
In the aforementioned example, the angular speed ω is used for the self-aligning torque estimation. However, it is also possible to perform estimation thereof by using the angle.
Industrial Applicability
According to a power steering apparatus of an automotive vehicle according to the present invention, treatments of road surface information, disturbance information and the like and designing of steering stability can independently be designed, to thereby being capable of providing a low-cost constitution, easy tuning, and a stable, comfortable steering feeling.
Number | Date | Country | Kind |
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2002-1935 | Jan 2002 | JP | national |
Filing Document | Filing Date | Country | Kind |
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PCT/JP03/00067 | 1/8/2003 | WO |