Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2001-03-12
2002-08-13
Lerner, Avraham (Department: 3618)
Motor vehicles
Steering gear
With electric power assist
C701S042000
Reexamination Certificate
active
06431307
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric power steering system which assists a steering operation by driving an electric motor according to a steering torque and transmitting a driving force generated by the electric motor to a steering mechanism. The invention further relates to a control system for such an electric power steering system.
2. Description of Related Art
In an electric power steering system for assisting a steering operation by utilizing a driving force generated by an electric motor, the electric motor is driven on the basis of a steering torque applied to a steering wheel, and the driving force generated by the electric motor is transmitted to a steering mechanism.
The electrical construction of a prior art electric power steering system is shown in FIG.
3
. The electric power steering system controls an electric motor
58
for applying a steering assist force to a steering mechanism by a controller
50
comprised of a microcomputer. An output signal of a torque sensor
57
for detecting a steering torque applied to a steering wheel is inputted to the controller
50
. The analog signal outputted from the torque sensor
57
is converted into a digital signal by an analog/digital (A/D) convertor
51
. The controller
50
determines a control command value corresponding to an electric current to be applied to the electric motor
58
on the basis of the steering torque T in a digital form, and drives the electric motor
58
via a motor driving circuit
55
on the basis of the control command value.
The controller
50
is adapted to perform every computation therein on a digital basis. That is, the controller
50
performs software-based computations according to predetermined operation programs, thereby functioning as an assist controlling section
52
, an inertia compensating section
53
and an adder section
54
.
The assist controlling section
52
determines a target current value I according to the steering torque T. The inertia compensating section
53
outputs a compensation current value &Dgr;I according to a time-based differential value &Dgr;T of the steering torque T. The compensation current value &Dgr;I is an electric current value for compensating for a response delay occurring due to the inertia of the steering mechanism and the electric motor
58
. The adder section
54
adds the compensation current value &Dgr;I from the inertia compensating section
53
to the target current value I from the assist controlling section
52
to provide the control command value I+&Dgr;I. The motor driving circuit
55
is controlled on the basis of the control command value I+&Dgr;I, whereby the electric motor
58
generates the steering assist force according to the steering torque T with a satisfactory responsiveness.
FIG. 4
is a diagram showing a relationship between the time-based differential value&Dgr;T of the steering torque T and the compensation current value&Dgr;I. The inertia compensating section
53
outputs the compensation current value &Dgr;I which is, for example, proportional to the time-based differential value&Dgr;T of the steering torque T. However, all the computations to be performed in the controller
50
are program-based digital computations, so that the time-based differential value &Dgr;T is merely a discrete value on a one-bit basis and, hence, the compensation current value&Dgr;I is also a discrete value. Therefore, the compensation current value &Dgr;I changes stepwise with respect to the time-based differential value &Dgr;T.
Where the steering torque T changes sufficiently rapidly, the electric motor
58
can properly be controlled even if the compensation current value &Dgr;I changes stepwise with respect to the time-based differential value &Dgr;T. Where the steering torque T changes very slowly with time, on the contrary, the target current value I outputted from the assist controlling section
52
changes stepwise by a very small amount in a longer cycle as shown in FIG.
5
A. Accordingly, a pulse indicative of the compensation current value &Dgr;I is generated for a time-based differential value of &Dgr;T=1 bit in a relatively long cycle, as shown in
FIG. 5B
, by the inertia compensating section
53
. In this case, the control command value I+&Dgr;I obtained by the addition of the target current value I and the compensation current value &Dgr;I in the adder section
54
appears as a pulse in a pulsed waveform in a relatively long cycle as shown in FIG.
5
C. When a pulsed electric current is applied to the electric motor
58
according to the control command value, resonance occurs between the electric motor
58
and the steering mechanism, resulting in uncomfortable vibrations and noises.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an electric power steering control system which can effectively suppress the vibrations and the noises.
It is another object of the present invention to provide an electric power steering system which can effectively suppress the vibrations and the noises.
The electric power steering control system according to the present invention is a control system for an electric power steering system which is adapted to generate a steering assist force by an electric motor driven on the basis of a steering torque applied to an operation member, and comprises: a target current determining circuit for generating a target current value according to the steering torque; a compensation value determining circuit for generating a compensation current value according to a time-based differential value of the steering torque; a waveform transforming circuit for performing a waveform transformation to convert the compensation current value generated by the compensation value determining circuit into a modified compensation value which sinusoidally changes with time; a circuit for generating a control command value by superimposing the modified compensation value generated by the waveform transforming circuit on the target current value determined by the target current determining circuit; and a motor driving circuit for driving the electric motor on the basis of the control command value.
With this arrangement, the compensation current value is generated according to the time-based differential value of the steering torque, and then subjected to the waveform transformation thereby to be converted into the modified compensation value which sinusoidally changes with time. Then, the modified compensation value obtained through the waveform transformation is superimposed on the target current value determined according to the steering torque for the generation of the control command value. Even if the steering torque changes slowly with time so that the compensation current value changes in a pulse form, the control command value does not change in a pulse form but gradually sinusoidally changes. Therefore, uncomfortable vibrations and noises of the electric motor or the steering mechanism can be prevented by controlling the driving of the electric motor on the basis of the control command value.
The present invention is particularly advantageous where the target current determining circuit, the compensation value determining circuit and the like are adapted to determine the target current value and the compensation current value in a digital data form on the basis of the steering torque in a digital data form.
The target current determining circuit, the compensation value determining circuit, the waveform transforming circuit and the like may perform their functions on a software basis by causing a microcomputer to execute predetermined programs.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiments with reference to the attached drawings.
REFERENCES:
patent: 4574903 (1986-03-01), Hashimoto et al.
patent: 4624334 (1986-11-01), Kelledes et al.
patent: 4651840 (1987-03-01), Shimizu et al.
pat
Koyo Seiko Co. Ltd.
Lerner Avraham
Luby Matt
Rabin & Berdo P.C.
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