Electric power steering controller

Details Electric power steering controller Electric power steering controller

2001-05-18

2003-06-10

Sircus, Brian (Department: 2836)

Electrical transmission or interconnection systems

Vehicle mounted systems

Automobile

C318S293000

Reexamination Certificate

active

06577024

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an automotive electric power steering controller for detecting whether a relay has melted and welded at the start of operation.
2. Description of the Related Art
FIG. 9
shows the structure of a known electric power steering controller disclosed, for example, in Japanese Patent Laid-Open No. 64289/1993. This controller has an electric motor
1
for delivering an assisting torque to the steering wheel (not shown) of a vehicle and a battery
2
for supplying motor current IM to the motor
1
, for driving it.
A capacitor
3
having a large capacity of about 1000 &mgr;F to 3600 &mgr;F absorbs the ripple component of the motor current IM. A shunt resistor
4
is used to detect the motor current IM. A bridge circuit
5
is made up of plural semiconductor switching elements Q
1
-Q
4
such as FETs (Field Effect Transistors) for switching the motor current IM according to the magnitude and the direction of the assisting torque. A relay
6
passes or cuts off the motor current IM according to the need.
A driver circuit
7
switches semiconductor switching elements Q
1
-Q
4
and drives the motor
1
via the bridge circuit
5
and actuates the relay
6
. A motor current detection means
8
detects the motor current IM from both ends of the shunt resistor
4
. The driver circuit
7
and the motor current detection means
8
form an interface circuit for a microcomputer (described later). A torque sensor
9
detects the steering torque T applied to the steering wheel. A vehicle speed sensor
10
detects the vehicle speed V of the vehicle.
The aforementioned microcomputer (CPU) is indicated by numeral
12
and calculates the assisting torque, based on the steering torque T and the vehicle speed V. The microcomputer
12
feeds back the motor current IM and creates a driver signal corresponding to the assisting torque. This microcomputer
12
applies driving signals to the driver circuit
7
. One of the driving signals consists of an instruction D
0
indicating the direction of rotation for controlling the bridge circuit
5
. The other driving signal consists of an amount of controlling current I
0
.
The microcomputer
12
comprises a motor current determining means
13
, a subtractor means
14
, and a PID arithmetic means
15
. The motor current-determining means
13
creates the instruction D
0
indicating the direction of rotation of the motor
1
and an instruction Im indicating an amount of motor current corresponding to the assisting torque. The subtractor means
14
calculates the current deviation &Dgr;I of the motor current IM from the instructed motor current Im. The PID arithmetic means
15
calculates amounts of correction of proportional (P) term, integral (I) term, and derivative (D) term from the current deviation &Dgr;I and creates an amount of controlling current I
0
corresponding to a pulse width modulation (PWM) duty ratio.
The microcomputer
12
further includes an A/D converter, a PWM timer circuit, and other components (none of which are shown) and has a self-diagnosing function. When the system is started, the microcomputer judges whether a contact of the relay
6
has melted and welded. Also, the microcomputer makes a fault diagnosis on the system. If no fault is found, the microcomputer actuates the relay
6
and supplies electric power to the bridge circuit
5
. During operation of the system, the microcomputer constantly makes a self-diagnosis to judge whether the system is operating normally. If a trouble occurs, the microcomputer causes the driver circuit
7
to open the relay
6
, thus cutting off the motor current IM.
The operation of this electric power steering system is described by referring to FIG.
9
. The microcomputer
12
accepts the steering torque T and the vehicle speed V from the torque sensor
9
and the vehicle speed sensor
10
, respectively. The motor current IM is fed back to the microcomputer
12
from the shunt resistor
4
. The microcomputer
12
creates the instruction D
0
for the direction of rotation of the power steering and the amount of controlling current I
0
corresponding to the amount of the assisting torque, and sends these to the driver circuit
7
.
Under steady-state operating conditions, the driver circuit
7
closes the relay
6
that is normally open. When the instruction D
0
for the direction of rotation and the amount of controlling current I
0
are input, the microcomputer creates a PWM driving signal and sends it to the semiconductor switching elements Q
1
-Q
4
of the bridge circuit
5
.
Thus, the motor current IM is supplied from the battery
2
to the motor
1
via the relay
6
, the shunt resistor
4
, and the bridge circuit
5
.
The motor current IM is detected via the shunt resistor
4
and via the motor current detection means
8
and fed back to the subtractor means
14
in the microcomputer
12
so that the motor current IM agrees with the instructed motor current Im.
As a result, the motor
1
is driven by the motor current IM and produces a desired amount of assisting torque in a desired direction. The motor current IM contains a ripple component due to switching operation of the PWM drive of the bridge circuit
5
. However, the current is smoothed by the large-capacity capacitor
3
.
Generally, in this kind of electric power steering system, detection is made to see whether a contact of the relay has melted and welded before closure of the relay when the steering system is started. A known method of detecting whether the relay contact has melted and welded uses a voltage Vrc at the relay contact to which a load is connected. If the contact of the relay
6
has melted and welded, the relay contact voltage Vrc becomes equal to the battery voltage via the relay
6
. Therefore, the presence or absence of the melting of the relay can be detected from the magnitude of the relay contact voltage Vrc. However, when the relay is open, the relay contact voltage Vrc becomes equal to the charging voltage for the smoothing capacitor
3
. Consequently, where the electric power steering controller is restarted immediately after turned off, for example, the detection is performed when the smoothing capacitor
3
is not yet fully discharged. That is, the detection whether melting has occurred is done while the relay contact voltage Vrc is high. Hence, fault of the relay
6
, i.e., melting and welding, may be misdiagnosed.
To avoid this, in the prior art electric power steering system described above, when melting and welding of the relay contact are detected, the microcomputer must wait until the smoothing capacitor
3
is fully discharged. This prolongs the starting time of the electric power steering system. A readily conceivable method of shortening the discharge time of the smoothing capacitor
3
is to insert a resistor in parallel with the smoothing capacitor
3
before discharging is performed. In this method, however, the electric current consumed when the electric power steering controller is in operation increases. Furthermore, a discharging resistor having a large power loss is necessary.
SUMMARY OF THE INVENTION
The present invention has been made to solve the foregoing problems. It is an object of the present invention to provide an electric power steering controller in which a smoothing capacitor is discharged with low power consumption to thereby shorten the discharge time, thus shortening the starting time.
An electric power steering controller in accordance with the present invention is adapted to give an assisting torque from an electric motor to a steering member for a vehicle, said electric power steering controller comprising:
a relay containing a first contact at a side of the electric motor and a second contact at a side of a DC power supply and switching a circuit between the electric motor and the DC power supply;
a smoothing capacitor connected with said first contact;
a voltage detecting means for detecting a voltage at said first contact; and
a discharging circuit having a switching means disposed in a dischargi

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