Textbook

Module5 MDPS (Motor Driven Power Steering)

Module5 MDPS (Motor Driven Power
Steering)

5.1

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LESSON

Overview ...........................................................................................................................123
5.1.1 Introduction..................................................................................................................... 123
5.1.2 History ............................................................................................................................ 124
5.1.3 Categorization by Motor Location................................................................................... 125

5.2

Layout ...............................................................................................................................127

Components .....................................................................................................................129

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5.3

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5.2.1 System Configuration ..................................................................................................... 127
5.2.2 Input / Output Elements.................................................................................................. 128

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5.3.1 Torque & Angle Sensor .................................................................................................. 129
5.3.2 ECU................................................................................................................................ 132
5.3.3 Motor .............................................................................................................................. 133
5.3.4 Rack & Pinion................................................................................................................. 134

5.4

Control Logic....................................................................................................................135

5.5

High Vehicle Speed ........................................................................................................ 136
Over Heat Protection...................................................................................................... 137
Over Load Protection ..................................................................................................... 138
Restoration Control ........................................................................................................ 139

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5.4.2
5.4.3
5.4.4
5.4.5


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5.4.1 Low Vehicle Speed......................................................................................................... 135

Maintenance .....................................................................................................................141

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5.5.1 Variant Coding................................................................................................................ 141
5.5.2 SAS Calibration .............................................................................................................. 142

[Learning Objectives]

▪ Describe the history of MDPS.
▪ Describe the system layout and list the locations, mechanisms and functions of components.
▪ Verify the control logic in the current data.
▪ Take the necessary measures after changing parts.

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Module5 MDPS (Motor Driven Power Steering)

5.1


Overview

5.1.1

Introduction

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Motor Driven Power Steering (MDPS) uses an electric motor to assist the steering power. Because of
friction between the front tires and the road, especially in parking, steering power to turn the steering wheel
is required.


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Module5 MDPS (Motor Driven Power Steering)

History

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5.1.2

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Figure 5-1 Hydraulic Power Steering (left) / Electric Power Steering (right)

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MDPS operates the motor according to the steering condition, thus offering an optimal steering
characteristic. On the other hand, power steering uses hydraulic pressure for reduction of steering effort,
enabling the driver to easily operate the steering wheel. EPS-equipped cars are increasing and EPS is
expected to replace hydraulic power steering system for the following reasons.

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▪ The system is environment friendly since no steering oil is used.

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▪ Due to the lower weight and less components the fuel consumption is reduced.

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▪ A speed-responsive function can be implemented.

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Categorization by Motor Location

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5.1.3

Module5 MDPS (Motor Driven Power Steering)

▪ Pinion-type: Suitable for mid-size vehicles

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▪ Rack-type: Suitable for large-size vehicles

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▪ Column-type: Suitable for small to mid-size vehicles

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EPS is divided into three types according to the motor location, Column type, Pinion type, Rack type.
Torque increases as the motor is located closer to the wheels, a characteristic suitable for mid to large-size
vehicles. However, design cost increases as a more complex layout is required.


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Module5 MDPS (Motor Driven Power Steering)

Layout

5.2.1

System Configuration

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5.2

1 Steering Wheel

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The driver turns the steering wheel, the initial input terminal of MDPS, to transfer driving intentions
(direction, speed, etc) to the vehicle.


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2 Torque & Angle Sensor (integrated)

Recognizes the force (torque) and direction of the driver's steering.
3 ECU

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Combines input values (torque, direction, speed, variant code, etc) and applies the desired assist
torque to the motor.
4 BLAC Motor

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The motor rotates according to the signal received from ECU.
5 Shaft

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The combined force of torque created by the driver's steering and the motor's assist torque is applied
to the shaft; the force is then transferred to the pinion gear from the worm gear.
6 Rack & Pinion Gear
Rotational movement is converted to a straight-line movement through the combination of the rack
gear and pinion gear, and this determines the direction of the wheels.

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5.2.2

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Input / Output Elements
Input Elements

Output Elements

▪ Battery Voltage
▪ Vehicle Speed

▪ Motor

▪ Engine Speed

▪ ESP Warning Lamp

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▪ Torque Sensor
▪ Angle Sensor


Function

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Configuration

This input is used for monitoring the system voltage and alternator load in
order to increase the engine idle rpm.

Vehicle Speed

The Control Module calculates the current for the motor based on the vehicle
speed sensor input.

Engine Speed

This signal is received by the Engine Control Module (idle up function). In
order to operate, a engine speed higher than 500rpm needs to be detected
by the MDPSCM.

Torque Sensor

Measures the twisting of the torsion bar.

Angle Sensor

The sensor delivers information about turning speed and the neutral position
of the steering wheel. The amount of restoration current is calculated by the
MDPSCM based on the Steering Angle- and Vehicle.


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Battery Voltage

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Speed sensor signal.

The Warning Lamp is on for approximately 5 seconds when the ignition is
turned on. It is continuously turned on when a fault is detected or during
communication with the Scan Tool.

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Warning Lamp

The torque output of the motor is controlled by varying the current. Low
current = low torque, High current = high torque.


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Module5 MDPS (Motor Driven Power Steering)

Components

5.3.1

Torque & Angle Sensor

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Figure 5-2 MDPS Ass’y

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Some of vehicle model have integrated torque and angle sensor which is more advanced. As shown in the
picture, two portions for the torque and angle detection have been integrated in one unit, referred to as ‘a
torque and angle sensor.’

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1) Torque Sensor

The torque sensor is installed in the middle of the steering column and is composed of a main and a sub
sensor. When the steering wheel is turned, the torsion bar is twisted in proportion to the steering resistance.
The offset of the input- in relation to the output shaft causes a change in magnetic field, thus causing a
change of current. With the change of input signal current the Motor Driven Power Steering Control Module
(MDPSCM) also detects the turning direction of the Steering wheel.

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Module5 MDPS (Motor Driven Power Steering)

This Magnetic field is proportion to torque. The Hall IC detects this change of magnetic field and send
signal in voltage form. The voltage is proportion to rotating angle from -4.5˚ to 4.5˚.

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The torque sensor has two output signal and MDPS ECU compares these signal and for detecting sensor
fault.

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2) Angle Sensor

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As the steering rotates, the main gear may rotate. Two planetary gears with different number of gear teeth
will rotate at the same time and the magnet attached in the planetary gear rotates. The Hall IC detects the

change of rotating angle and outputs the voltage signals to the control module. Finally the control module
will calculate the steering angle by processing these signals. Note that the operating principle is same
as one for the absolute steering angle in the ESC system. Therefore, it is necessary to do the sensor
calibration after replacing the sensor.

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As the steering rotates, the main gear may rotate. Two planetary gears with different number of gear teeth
will rotate at the same time and the magnet attached in the planetary gear rotates. The Hall IC detects the
change of rotating angle and outputs the voltage signals to the control module. Finally the control module
will calculate the steering angle by processing these signals. Note that the operating principle is same
as one for the absolute steering angle in the ESC system. Therefore, it is necessary to do the sensor
calibration after replacing the sensor.

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ECU

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5.3.2

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If the driver moves the steering wheel, a sensor registers the steering torque exerted and sends this
information as an electrical signal to the ECU. This calculates the supporting steering torque and activities
the servo-motor on the basis of the calculated result.

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Motor

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5.3.3

Module5 MDPS (Motor Driven Power Steering)

Currently, brushless AC motors are used the most, and depending on the required steering performance,
the torque generated by these motors is 3 to 6 Nm.

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The direction of rotation of the motor depends on the direction of motion of steering wheel.

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5.3.4

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Rack & Pinion

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As an alternative to a constant reduction ratio on the rack, suitable toothing of the rack allows the ratio to

be varied as a function of travel. In this way, the straight running stability of the vehicle can be improved by
suitably indirect ratio around the center of the steering. At the same time, it is possible with a direct ratio
arrangement in the range of medium and large steering angles to reduce the necessary steering angle
requirement when turning from lock to lock.

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Module5 MDPS (Motor Driven Power Steering)

Control Logic

5.4.1

Low Vehicle Speed

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At low vehicle speed (measured by the vehicle speed sensor) the twisting of the torsioning bar (measured
by the torque sensor) is high. The MDPSCM supplies a high current to the motor in order to reduce the
steering effort for the driver.

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5.4.2

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High Vehicle Speed

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At high vehicle speed the twisting of the torsioning bar is low. The MDPSCM supplies a low current to the
motor in order to increase the steering effort for the driver, thus offering higher steering stability.

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Figure 5-3 Over Heat Protection

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5.4.3

Module5 MDPS (Motor Driven Power Steering)

Over Heat Protection

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Turning the steering wheel left and right continuously causes overcurrents in the motor, which can lead to
internal damage in the motor. Overheat protection reduces motor currents to prevent this from happening.
For this function, temperature is measured not by the temperature sensor, but by the ECU's calculation of

motor running time. Steering feels heavier when overheat protection is enabled.

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5.4.4

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Over Load Protection

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Fully turning the steering wheel by 780 or -780 degrees rapidly increases current consumption. Overload
protection reduces currents to protect the motor. The vehicle is slightly understeered even if the steering
wheel is fully turned when overload protection is enabled.

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Figure 5-4 Restoration Control

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5.4.5

Module5 MDPS (Motor Driven Power Steering)

Restoration Control

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Restoration control is a function that helps the steering wheel return to its starting position after a turn
for straight-line driving. After steering and finishing a turn, the steering wheel automatically returns to its
starting position without having to steer in the opposite direction.

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Maintenance

Figure 5-5 MDPS S/W Management

Variant Coding


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5.5.1

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5.5

Module5 MDPS (Motor Driven Power Steering)

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Variant coding takes place through EPS type recognition in GDS. The steering feel drivers need differs
by region; as such, steering feel is set according to region and vehicle model. Korean drivers prefer a
light steering feel, while European and North American drivers prefer a heavier one. Depending on each
vehicle model and region, a steering feel can be selected by using a diagnostic tool, and the process for

recognizing the specifications must be carried out when the MDPS system is replaced. If the process is not
conducted, an error code(C1702 Specification Recognition Error) is output. If the specification for another
region is selected, a different steering feel is set.

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5.5.2

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SAS Calibration

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This function has to be performed after replacement of the Motor Driven Power Steering Control Module
(MDPSCM) or the Absolute Position Sensor (APS). Successful calibration is displayed under current data's
of the Scan Tool “Calibration Status.”

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