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2016 – MX-5 – Steering

ELECTRIC POWER STEERING (EPS) CONTROL MODULE

Purpose/ Function

• The EPS Control Module (CM) calculates the optimum assist current based on the steering torque signal from the torque sensor installed to the pinion shaft and the vehicle speed signal sent from the PCM via CAN communication.

• The following steering angle signals are output via CAN communication.

• Steering angle (relative angle) signal

• Set the steering position to 0 degrees when the ignition is switched ON (engine off or on) to output the steering angle when steering from that position.

• Steering angle (estimated absolute angle) signal

• The EPS CM outputs the calculated steering angle value to the CAN as a steering angle (estimated absolute angle) signal for other systems requiring steering angle information.

  NOTE:

  • Before the steering wheel angle neutral position auto learning is completed, the EPS CM receives a steering angle (absolute angle) signal from the steering angle sensor via the start stop unit and outputs the signal to the other modules through the CAN communication (with adaptive front lighting system (AFS)).

  • The EPS CM starts calculation of the steering angle (estimated absolute steering angle) after the steering angle neutral position determination, and outputs it as a steering angle (estimated absolute steering angle) signal to CAN.

• The EPS CM controls the following functions:

Function Table

Control item	Function
EPS motor current control	Normal control Calculates the proper assist current based on the steering torque, and the vehicle and engine speeds, and outputs a target current to the EPS motor. System overheating protection control Monitors the temperature of the motherboard in the EPS CM and controls the motor current gradually to prevent overheating of the system. EPS motor backup control Even if a phase 1 open circuit malfunction in the EPS motor occurs, output current to the EPS motor is controlled so that steering assist is maintained. Torque sensor backup control Even if an error occurs in the torque sensor signal, the torque sensor backup control performs control to maintain steering assist to the extent possible.
On-board diagnostic function	The main part of the system control includes the self-diagnosis function. In case a malfunction occurs, the power steering malfunction indicator light/master warning light illuminates to alert the driver, and a DTC is stored in the EPS CM at the same time. As a result of the on-board diagnosis, when a malfunction is determined, system control is suspended or limited to assure safety while driving.
CAN communication function	Outputs the EPS status signal and the power steering malfunction indicator light illuminates on request via CAN lines. Outputs the steering angle (absolute angle) signal, steering angle (relative angle) signal and steering angle (estimated absolute angle) signal.
Automatic configuration function	When the ignition is switched to ON or the engine is started after the EPS CM have been replaced, the EPS CM reads data from the instrument cluster via CAN communication to perform automatic configuration.
Steering wheel angle neutral position automatic learning function	When the ignition is switched from OFF to ON (engine on) and the vehicle is driven normally, the steering wheel angle neutral position is learned automatically based on the wheel speed signal, lateral-G signal, and yaw rate signal.

Block Diagram

Construction

• The EPS CM is installed to the steering gear housing and integrated with the EPS motor.

• For vehicles equipped with a system requiring steering angle information directly after the vehicle starts to be driven, a steering angle sensor is equipped.

Operation

• The EPS CM calculates the optimum assist current based on the steering torque signal from the torque sensor installed to the pinion shaft and the vehicle speed signal sent from the PCM via CAN communication, and outputs control current to the EPS motor.

• The EPS system control uses the steering angle (estimated absolute angle) signal.

Ignition is switched ON (engine off or on)

• The EPS CM outputs a steering angle (absolute steering angle) signal to CAN based on the signal received from the steering angle sensor via the start stop unit. (With steering angle sensor)

• The following systems perform control using a steering angle (absolute angle) signal until the EPS CM completes steering wheel angle neutral position determination after the vehicle starts to be driven.

• AFS (Adaptive Front lighting System)

• The EPS CM regards the steering position when the ignition is switched ON (engine off or on) as 0 degrees, and outputs the steering angle value steered from that position as a steering angle (relative angle) signal.

Vehicle driving

• When the vehicle starts to be driven, the EPS CM performs steering wheel angle neutral position determination based on the following signals:

• Wheel speed signal: Determines straight-driving if there is no rotation speed difference between the left/right wheels.

• Lateral-G signal: Determines straight-driving if lateral-G is not produced.

• Yaw rate signal: Determines straight-driving if yaw momentum is not produced.

• If the EPS CM completes steering wheel angle neutral position determination, the steering angle neutral position is set. The EPS CM regards the steering wheel angle neutral position as 0 degrees, calculates the estimated absolute steering angle, and outputs the steering angle (estimated absolute angle) signal to CAN.

Ignition is switched off

• The steering wheel angle neutral position is cleared when 1 min or more has elapsed after the ignition is switched off.

EPS Motor Current Control

Normal control

• The optimum assist current is calculated based on the steering force signal from the torque sensor and the vehicle and the engine speed signal from the PCM, and then the control current is output to the EPS motor.

System overheating protection control

• The system overheating protection control lowers the current output to the EPS motor if the steering mechanism is turned from lock to lock continuously or the steering rack turning limit is reached repeatedly.

  NOTE:

  • During system overheating protection control, the assist force will feel weaker due to a decrease in the control current. It is normal for the EPS CM to prevent the EPS motor from burning or seizure and the unit from burning out, and it does not indicate a malfunction.

  • While the assist force weakens during this period of time, steering operation can be performed.

• The system overheating protection control has middle and complete steps, and it can verify current conditions and the history using the PID/data monitor function and snap shot data.

• System overheating protection control (middle): Motor output is 40 % or less (reference)

• System overheating protection control (complete): Motor output is 20 % or less (reference)

• The current output returns to normal if the temperature in the system decreases to the normal operation temperature.

  

EPS motor backup control

• Even if an open circuit malfunction occurs in 1 out of the 3 phases in the EPS motor line, the EPS motor backup control performs control to maintain steering assist.

• If the EPS CM detects a phase 1 open circuit malfunction in the EPS motor, the EPS CM controls output current to the EPS motor using a calculation method different than during normal EPS CM control. As a result, even with a phase 1 open circuit malfunction, the steering assist can be maintained.

  NOTE:

  • If a phase 1 open circuit malfunction is detected in the EPS motor, the EPS CM stores DTC U2011:72 and illuminates the power steering malfunction indicator light.

  • If the steering wheel is operated during EPS motor backup control, the steering wheel operation may feel heavier (resistance).

Torque sensor backup control

• Even if an error is detected in the torque sensor signal, the torque sensor backup control performs control to maintain steering assist.

• If the EPS CM detects a torque sensor signal error, the EPS CM controls output current to the EPS motor using a calculation method different from that during normal EPS CM control, and maintains steering assist.

NOTE:

• When the torque sensor backup control is performed, the EPS CM stores DTCs C200B:02, C200B:1C and C200B:62 or C200C:1C and turns the power steering malfunction indicator light on.

• If the steering wheel is operated during torque sensor backup control, normal assist force may not be generated.

Fail-safe

DTC No.	Fail-safe function
U2300:54	Permits control Power steering malfunction indicator light: Not illuminated
U2300:55	Permits control Power steering malfunction indicator light: Illuminated
U2300:56	Permits control Power steering malfunction indicator light: Not illuminated
U3000:16	Inhibits control Power steering malfunction indicator light: Illuminated
U3000:1C
U3000:28
U3000:41
U3000:46	Permits control Power steering malfunction indicator light: Not illuminated
U3000:47	Inhibits control Power steering malfunction indicator light: Illuminated
U3000:49
U3000:4B	Permits control Power steering malfunction indicator light: Not illuminated
U3000:61	Inhibits control Power steering malfunction indicator light: Illuminated
U3000:64	Permits control Power steering malfunction indicator light: Illuminated
U3000:73	Inhibits control Power steering malfunction indicator light: Illuminated
U3000:96
U3003:16
U3003:17

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