F

E

B
A

D
C

1.

The electric motor

A

2.

Harmonic gear

drives torque tube

3.

Torque tube

4.

Torsion bar

5.

Lever

14

E

C
D

B

drives harmonic gear

drives torsion bar
moves lever

moves coupling rod

B

via a toothed belt

C

D

E
F

677_014

Harmonic drive gear
Design and function
Harmonic Drives are a strain wave gearing system used
wherever zero backlash, precision and high reliability are
required. They are ideal for the operation of the actuators
of the active suspension system. As the main component,
they transmit the rotation of the electric motor into relatively small angles of rotation and high torque to the
torque tube/torsion bar which are converted into the vertical motion of the coupling rod. Harmonic drives are used
extensively in robotic devices because of their precision (no
backlash), reliability and long service life. Harmonic drives
were first used by Audi in the dynamic steering system of
the Audi A4.

By inserting the wave generator into the flex sleeve, the
flex sleeve takes on the elliptical shape of the wave generator. When the wave generator rotates, it causes the flex
sleeve to radially deform. The external teeth of the flex
sleeve only mesh with the internal teeth of the circular ring
gear in two places around the circumference of the ring
gear as the wave generator rotates.

Harmonic drive component sets consist of only three components: wave generator, flex sleeve and circular ring gear.
The elliptical shaped wave generator is the driven element
of the gear set and has specially designed ball bearings.
The flex sleeve is a high strength torsionally stiff yet flexible shell-like component with external teeth which reliably
transmits high loads.
The outer circular ring gear has internal teeth. It is
installed over the flex sleeve. The rigid ring gear has two
teeth more than that of the flex spline.

Flex sleeve and wave generator

402_011

Ring gear with internal teeth

Wave generator with special ball bearings

Flex sleeve with external teeth

677_018

15

When the electric motor is actuated, the wave generator
rotates. Because the number of teeth on the flex sleeve and
on the ring gear are not the same, the teeth on the flex
sleeve do not mesh exactly with the teeth on the ring gear.
The teeth on the flex sleeve engage the tooth flanks of the
ring gear in a laterally offset manner. This eliminates
backlash.

The force acting on this tooth flank produces a minimal
continuous rotational movement of the ring gear.
All the teeth on the circumference of the ring gear mesh in
a time-shifted manner due to the “rotation” of wave
generator.

Laterally offset meshing of flex sleeve and ring gear teeth

W = 0°
R = +0°

W = 90°
R = +0.88°

W = 180°
R = +1.76°

W = 270°
R = +2.65°

402_013

W = 360°
R = +3.53°

Wave generator = W
Ring gear = R
402_014

All the teeth on the circumference of the ring gear mesh in
a time-shifted manner due to the “rotation” of wave
generator. This produces a continuous rotational movement
of the ring gear at a defined ratio without any backlash.

16

This illustration shows the difference in movement
(measured in degrees) between the wave generator and the
ring gear. The ratio can be changed during the design
process by changing the number of teeth.

Electric motor
Design and function
The drive source for the actuator is a star connected
permanently excited 48 Volt AC motor with five pole pairs
with electronic commutation (brushless). The maximum
power of the motor is approximately 2.0 kW; this is
however only briefly required within a few milliseconds. The
average power required is relatively low and is between
approximately 10 and 200 watts, depending on the driving
style and the road conditions.
The motor is activated by AC voltages between
0 and 48 Volts with a phase rotation of 120°. This
generates currents in the stator coils in alternating
directions (see illustration below). Corresponding magnetic
fields are generated around the coils through which the
current is flowing. Their polarity changes each time the
direction of the current changes. This generates a magnetic
field with rotating polarities which surrounds the rotor. This
magnetic field exerts corresponding force effects on the
permanent magnets which are permanently connected to
the rotor, thereby generating a torque which causes the
rotational movement of the rotor. Depending on the
activation (rotational direction of the magnetic field),
turning clockwise or counter-clockwise can be achieved.

The position of the rotor is detected by a sensor. This
sensor is on the end of the rotor, on the opposite end to the
belt pulley. There is a permanent magnet in the hollow
shaft of the rotor. Its position is measured by a
magnetoresistive sensor. The magnetoresistive measuring
principle is based on the fact that the electrical resistance
in ferromagnetic metals changes due to the effects of
external magnetic fields. Analyzing the changes in
resistance allows the position of the magnet in the rotor
shaft, and therefore also the angle of the rotor, to be
determined.

48V -DC voltage
(negative)

48V -DC voltage
(positive)

Sensor wire: rotor
position sensor control module

48V cable for Front
Suspension Stabilization
Actuator (AC voltage)

676_015
17

In section 2, the greatest voltage is at V so the current
changes direction and flows from V to U. The rotating
magnetic fields generated by these alternating currents
cause corresponding force effects on the permanent
magnets of the rotor, which then generate torque.

The image shows the voltage curve of the phases. Electric
currents are generated by corresponding differences in
potential between the individual phases. The flow of
current through the coils is shown as an example in section
1 of the graph. As the phase voltage U is the greatest in
this section, the induced current flows from U to V and W
via the star point.

48V
U

Phase U
Phase V
Phase W
0
Current flow

1

2

90°
U

V, W

V

U, W

120°
W

180°
U, V

U

270°
V, W

V

U, W

360°
W

U, V

U
V

W

18

676_017

Suspension stabilization control modules
The electric motors are activated on a per axle basis by
separate control modules. The control modules are
supplied 12V Terminal 30 power which enables their
participation in the vehicle Networking system and a 48V
power supply (Terminal 40) for the power path.

Both Suspension Stabilization Control Module 1 J1152 and
Suspension Stabilization Control Module 2 J1153 are
controlled by Drivetrain Control Module J775. They
communicate via a sub-bus system.
The control modules are installed between the actuators on
the subframes of the front and rear axles. They are flush
mounted with the surrounding components which provides
external protection.

Suspension Stabilization
Control Module 1 J1152

677_019

Front axle unit: the control module, actuators and integrated connector console with wiring.

Suspension Stabilization
Control Module 2 J1153
677_020

Rear axle unit: the control module, actuators and integrated connector console with wiring.
19

Drivetrain Control Module J775
Drivetrain Control Module J775 (2nd generation) includes
the regulating software for the active suspension in addition to the regulating software for other suspension system
features and the corresponding sensors. It acts as the
master control module for the Suspension Stabilization
Control Modules. J775 communicates with the other data
bus nodes via FlexRay and with the Suspension Stabilization Control Modules via a sub-bus.
The regulating software is modular and mainly contains the
modules described below. A more detailed description of
the functions can be found on the following pages in the
“System functions” chapter.
Drivetrain Control Module J775

Body control module
The aim is to pacify the vehicle body and to neutralize road
excitation of up to approximately 5 Hz. Vehicle body vibrations have a frequency range of approximately 1-5 Hz.
Preview module
This module evaluates data from the front camera to detect
the surface of the road ahead. This enables the vehicle to
react to upcoming road excitation (for example, speed
bumps or bumpy road surfaces) in time, providing a significant increase in comfort.
Pitch module
The pitch of the vehicle (rotational movement about the
vehicle’s transverse axis) when moving off and braking is
reduced. This positively influences the vehicle’s dynamics
and comfort by shortening the braking distance and better
supporting the vehicle body.

677_021

Roll module
The roll of the vehicle (rotational movement about the
vehicle’s longitudinal axis) in corners is reduced. This positively influences mainly driving dynamics and driving safety
due to better steering response and neutral vehicle handling.
Crash module
If an impending side impact is detected, that side of the
vehicle is raised very quickly. Instead of impacting the door
area, the colliding object hits a larger proportion of the
more rigid sill area, which can reduce the effects of the
accident.
Elevated entry module
When a door is opened, the vehicle body is raised to make it
easier to get in/out of the vehicle.

677_022

J775 detects the rotational movement of the vehicle around the main axes and the vertical acceleration of the vehicle body.
Additional data is read in to determine the full current status of the driving dynamics (for example, the lateral acceleration
from the Airbag Control Module, the vehicle speed from the ABS Control Module.
20

System functions
Basic function
The regulation of the stabilization rates, air springs and
dampers is closely linked. The air spring system assumes
the task of balancing the load and implementing both
manual and automatic changes to the specified vehicle
level. The damping required due to the actuators is taken
into account when the damping forces are calculated.

The basis for the calculation of the coupling force/tension
of the torsion bars required is both the Audi drive select
setting chosen by the driver and the relevant driving state
or the vehicle dynamics determined by the relevant
sensors. Corresponding sensors are implemented in J775
itself. Torques are measured about the x axis (roll torques),
the y axis (pitch torques) and the z axis (yaw torques). The
yaw torques are used to estimate the lateral acceleration;
these figures are then compared with the measured values
from the Airbag Control Module. Special software determines the vehicle speed. The vehicle level is determined, as
with adaptive air suspension (AAS), by four separate level
senders. The vertical movements of the vehicle body are
measured by an acceleration sensor, which is also part of
the running gear control unit.

Drivetrain Control Module J775 calculates the torque with
which the torsion bar should be tensioned for each wheel
individually and almost in real time. This torque corresponds to a defined angle of rotation at the output of the
harmonic drive gear.
These angle figures are provided as control inputs to Suspension Stabilization Control Modules 1 and 2 for the front
and rear axles. The control modules achieve these positions
by activating the electric motors of the actuators. The
measured values from the rotor position sensors form the
basis for determining the angles.
The total ratio for the angle of rotor rotation to the angle
of rotation at the drive gear output is 189:1 (belt drive of
rotor – drive gear input: 1:2.36: drive gear input – drive
gear output: 80:1). This means that the rotor of the electric
motor must turn 189 times to achieve one rotation at the
drive gear output.

Driver Assistance Systems
Control Module J1121

The actuators are always supplied with current during
active operation. There are phases of electrodynamic
damping in every control operation. The permanently
excited rotors of the electric motors are then “driven”. The
rotating magnetic fields generated as a result induce electric voltages in the stator coils. The electric currents generated are used for recuperation of the 48 Volt battery. The
maximum recuperation is approximately 3 kW; this may
however only be effective very briefly.

Drivetrain Control Module
J775

Actuators on
front axle

Active suspension - software
Sensors for
vehicle dynamics

Front camera

Preview module

Activation
of actuators

Measured values
from rotor position
sender

Suspension
Stabilization
Control Module 1
J1152

Additional external
messages/signals

Vehicle level sender
Angle at output of
harmonic drive gear

Body control module

Vehicle level sender

Pitch module
Audi drive select
module

Vehicle level sender

Roll module

Suspension
Stabilization
Control Module 2
J1153
Activation
of actuators

Actuators on
rear axle

Vehicle level sender

Measured values
from rotor position
sender

Additional internal
signals