ELECTRIC POWER STEERING (EPS) CONTROL MODULE CONSTRUCTION/OPERATION


BHE061367880T01

Construction

• The EPS control module is located in the engine compartment, on the underside of the PCM.

• The module calculates the proper assist current based on the steering force signal from the torque sensor installed on the steering gear and linkage, and the vehicle and engine speed signals from the PCM using CAN communication line, and then outputs the control current to the EPS motor.

Function Table

Control items

Function

Motor current control
• Calculates the proper assist current based on the steering force, and vehicle and engine speeds, and outputs a control current to the EPS motor.
System overheating prevention control
• In order to prevent system overheating, motor current is controlled according to turning limit or motor output control.
Self-diagnosis function
• A function that allows important parts of the control system to perform self-diagnosis. In case a malfunction occurs, the EPS warning light illuminates to alert the driver, and at the same time a DTC is stored in the EPS control module.
• As a result of the self-diagnosis, when a malfunction is determined, system control is suspended or limited to prevent any dangerous occurrence while driving.

1
Torque sensor
2
PCM
3
Steering force signal
4
Vehicle speed signal (CAN)
5
Engine speed signal (CAN)
6
EPS control module
7
On-board diagnosis function
8
Motor control current
9
Idle speed increase request signal (CAN)
10
EPS warning light on request signal (CAN)
11
EPS motor
12
EPS warning light

Operation

Motor current control

• The proper assist current is calculated based on the steering force signal from the torque sensor, and the vehicle and engine speed signals from the PCM, and then the control current is output to the EPS motor.
• Also, various control corrections are performed according to vehicle driving conditions.
– Base current control
• The base current is the fundamental current amount for driving the EPS motor and is calculated based on the steering force and vehicle speed signals.

1
Motor control current
2
Vehicle speed (high)
3
Steering torque

– Inertia correction control
• The inertia correction control compensates for the influence caused by the motor body of revolution inertia (insufficient torque when starting, torque continuance when stopping). The correction current to the base current is either increased or decreased according to vehicle conditions.
• Inertia correction control is calculated based on the steering force and vehicle speed signals, and the motor speed.

1
Corrective current
2
Vehicle speed (low)
3
Vehicle speed (high)
4
Steering force derivative

– Damping correction control
• The damping correction control reduces slight vibrations transmitted from the road surface (kickback) using the motor control current. The correction current to the base current is either increased or decreased according to vehicle conditions.
• The damping correction control is calculated based on the steering force and vehicle speed signals, and the motor speed.

System overheating prevention control

• To prevent a malfunction of the system caused by overheating (due to motor over-speed or other factors), the steering mechanism turning limit and cumulative value of the motor current are detected and the current output to the EPS motor is controlled accordingly.
– Turning limit control
• The turning limit control detects the turning limit of the steering mechanism and when it is determined that the limit has been reached, current output to the EPS motor is reduced by approximately one-half.

1
Motor output (%)
2
Return
3
Turning limit detected
4
Not at turning limit
5
Time

• Then, when it is determined that it is no longer at the turning limit, the motor current is returned to normal.
– Motor output limit control
• The motor output limit control detects the cumulative value of the current output to the motor, and if the steering mechanism is turned from lock to lock continuously (or similar repetitious operation), current output to the EPS motor is lowered.
• The motor current will gradually return to normal after the steering torque is detected at 0 N·m {0 kgf·cm, 0 in·lbf} or the ignition switch is turned off. A maximum of 15 min is required to return to normal conditions.

1
Motor output under limited control
2
Motor output (%)
3
Cumulative current observed
4
Turning back and forth starts
5
Control starts
6
Time
7
Motor output returning
8
Current controlled to return (max. 15 min)
9
Return starts
10
Return to normal