This document describes an active vibration control system developed by TrelleborgVibracoustic to reduce unwanted vibrations in electric vehicles with range extender engines. The system uses an electrodynamic actuator and electronic control unit to detect vibrations from the combustion engine and generate counter-vibrations to cancel them out, improving interior noise and vibration levels. Testing of a prototype system on a vehicle found it effectively reduced booming noises from the unbalanced four-cylinder range extender engine, particularly its dominant second-order vibrations transmitted through the right-hand side engine mount. The active approach provides a lighter and more flexible solution than conventional passive vibration damping methods.

1. AUTHORS
Dipl.-Math. Enrico Kruse
is Director Product Innovation at
TrelleborgVibracoustic in Weinheim
(Deutschland).
Andrew Harrison, M.B.A.
is Senior Program and Technical
Manager at TrelleborgVibracoustic
in Farmington Hills (USA).
Active Vibration Control Technology for
Electric Vehicles with Range Extender
TrelleborgVibracoustic developed an active
system to reduce undesired vibrations in
electric cars with Range Extender. These
vibrations occur through three- or four-
cylinder engines used for range extension.
The system raises comfort levels by tackling
vibrations of the combustion engine through
the use of specific counter-vibrations.
DEVELOPMENT ACOUSTICS | NVH
14

2. PRINCIPLES
Combustion engines generate a number of
excitations which manifest in the form of
vibrations and noise, passing through the
air (airborne noise or vibration) or through
the vehicle structure itself (structure-
borne noise or vibration). Structure-borne
noise moves through the engine mounts
and vehicle structure into the interior of
the vehicle where it is perceived either as
vibration on the steering wheel, seat rail
or floor panel, or as disruptive noise. The
frequencies of these disturbances often
depend on the engine speed, and it is vir-
tually impossible to eradicate them before
they reach the interior by following a con-
ventional (passive) approach.
Nowadays less cylinders, turbo charg-
ing, downsizing, downspeeding and the
removal of balance units are common-
place measures to reduce fuel consump-
tion and CO2 emissions while increasing
the specific output of said engines. How-
ever, all these measures typically also
generate or exacerbate NVH (noise,
vibration, harshness) issues inside and
outside the cabin. The goal of this pro-
ject was to use active vibration cancella-
tion measures to find a better balance
between CO2 reduction and noise and
vibration requirements.
VIBRATION COMFORT IN ELECTRIC
VEHICLES WITH RANGE EXTENDER
Electric vehicles with combustion
engines for range extension present a
particular challenge in relation to inte-
rior noise and vibration comfort. The rel-
atively quiet and smooth electrical drive
is in contrast to the louder and harsher
operation of the combustion engine
which can cause disturbance and thus
counteract one of the main advantages of
an electrical drive.
The situation is often worsened by the
pressure to reduce weight in order to
maximise range in electrical operation.
Moving away from passive approaches for
reducing noise, such as isolation, fixed
frequency dampers and balance shafts,
can lead to a reduction in weight, but also
to increased noise and vibration levels.
The target vehicle for this investigation
was a range extended hybrid electric car
from a major OEM. The internal combus-
tion range extender was a four-cylinder
gasoline engine without balance shafts.
The hybrid powertrain was mounted in
the vehicle using a standard pendulum
style setup for laterally installed engines:
load carrying mounts above the power-
train center of gravity (right hand side
and left hand side) and a single torque
link at a lower, more central position. The
original setup showed several areas of
high interior noise levels (“booming”)
with a significant contribution from the
four-cylinder engine’s 2nd order – the
order that would otherwise have been
canceled by the balance shafts. The chal-
lenging 2nd engine order interior noise tar-
get was derived from a same-class com-
petitor vehicle also sporting a four-cylin-
der gas engine but with balance shafts
and switchable engine mounts.
AVC SYSTEM AS SOLUTION
APPROACH
An AVC (active vibration control) system
was applied to tackle these 2nd engine
order NVH issues. In comparison to bal-
ance shafts an AVC solution is lighter
and smaller, practically frictionless and
more adaptive to environmental and pro-
duction variations. All vehicle tests were
carried out at the customer site in an
anechoic chamber on a dynamometer to
ensure controlled test conditions.
Active vibration control technology
detects and tackles specific structure-
borne vibrations and resulting noise, and
improves the NVH level of a vehicle. The
basic principle involves offsetting incom-
ing sine vibrations with counter-vibra-
tions of the same frequency and ampli-
tude, but of opposite phase, generated by
means of an actuator, FIGURE 1.
This phase-delayed force and its fre-
quency are determined in an electronic
control module through a series of com-
plex algorithms. They are based on an
adjustment table in the case of an open-
loop system, or on a feedback sensor
(microphone and/or accelerometer) in
the case of a closed-loop system.
The system is also able to detect and
offset several specific excitation adjust-
ments at once. The flexibility of this
active approach is another important
benefit. Due to this, the system can be
integrated into the vehicle even late in
the development process. This helps to
reduce cost, weight and complexity, for
example through savings in relation to
balance shafts, isolating materials or
active noise suppression systems, as well
as optimised engine mounts.
The AVC system consists of three com-
ponents, FIGURE 2. The electronic control
unit (ECU) processes input signals and
determines the corresponding output
response to the vehicle. The electrody-
namic actuator generates an output iner-
tial force which is passed through the
vehicle structure. The input sensor (either
microphone or accelerometer) measures
vehicle responses, either in g or dB.
SYSTEM COMPONENTS: ACTUATOR
The actuator is the system component
that generates the counter vibrations. It
has to be small enough to fit yet strong
enough to provide sufficient force output
over the entire operating frequency range.
The electrodynamic actuator chosen
for this investigation consists of a mass
suspended elastically between two steel
springs, FIGURE 3. The mass contains
a strong permanent magnet made of
neodymium or samarium cobalt mate-
rial. This mass is moved back and forth
through the electromagnetic field gener-
ated by a surrounding coil. The coil
system, the moving mass, the magnet
type, the spring path and the springs
are all set up in accordance with the
frequency response required and the
force required over this frequency. Vibra-
tions in cars typically occur within a fre-
quency range of 15 to 480 Hz and with
a peak force value of 50 to 200 N. Ulti-
mately, the specific design is a balance
FIGURE 1 Disruptive vibration (dark blue) and counter-vibration (light blue)
06I2015 Volume 117 15

3. between performance requirements, cost
and installation space.
In principle, the electrodynamic shaker
can be placed anywhere provided there is
a flat enough surface and sufficient room.
It can also work under any inclination
angle. Obviously, in this project it needs
to be placed where it results in the big-
gest reduction in interior noise. It is also
evident to place the actuator on the body
side rather than on the powertrain itself.
That way the engine mounting system
can provide the first layer of isolation and
the AVC only needs to deal with the
remaining vibrations. There were signifi-
cant time constraints in this project, so a
comprehensive transfer path analysis
from the powertrain mounts to the driver
sensor points (namely the driver’s ear,
but also seat track, steering wheel, etc.)
could not be carried out. However, it was
quite clear that the unbalanced four-cyl-
inder engine would generate a significant
amount of 2nd order engine shaking. The
acting point of this shaking force is the
middle of the crankshaft, i.e. closer to the
right hand side (RHS) mount than to the
left hand side (LHS) one. Vibrations at
the powertrain mounts in all directions
under several operating conditions were
checked. As can be seen in the example
in FIGURE 4, vibrations in RHS Z proved
to be dominant, as anticipated. Vibra-
tions in other directions or at other
mount locations were much lower.
Removing the RHS mount completely
(and supporting this side of the power-
train to ground to eliminate this vibra-
tion path) dropped the 2nd order content
of the interior noise in some engine
speed ranges by up to 5 to 15 dB. This
served as another indicator that placing
the actuator upright and on the rail close
to the RHS powertrain mount should be
a good start.
SYSTEM COMPONENTS: ECU
The electronic control unit (ECU) con-
sists of a piece of source code containing
the main algorithms for active vibration
control as well as a piece of interface
software enabling a computer to be con-
nected in order to support vehicle devel-
opment measures. Depending on the
available installation space in the vehicle
as well as on actuator sizes and system
type (open-loop or closed-loop), the con-
troller can be set in an isolated position
or integrated in the actuator housing.
SYSTEM COMPONENTS:
INPUT SENSORS
The error (or feedback) sensor can be
either a microphone or an accelerometer.
Multiple input sensors can also be used
(and even multiple output signals gener-
ated) in the case of a complex vehicle
issue like the one at hand. In order to
specifically target the interior noise, an
additional microphone was placed inside
the cabin close to the driver‘s ear. The
FIGURE 3 Electrodynamic actuator
FIGURE 2 Active vibration control technology (AVC) in vehicles
DEVELOPMENT ACOUSTICS | NVH
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4. accelerometer which usually covers a
range of up to 1.2 g can basically be
installed anywhere in the car. The sen-
sor and the ECU can also be installed
inside the actuator, thereby reducing
costs and avoiding the complexity of an
additional component.
SYSTEM OPTIONS
The system is configured individually
based on customer requirements and
specific NVH issues to be resolved.
Closed loop systems use a continuous
feedback loop employing an accelerome-
ter or microphone as the error sensor.
The engine’s unbalanced 2nd order shak-
ing forces are the target in this project.
These excitations are purely mass and
inertia generated, i.e. they depend on the
engine speed but not on the engine load.
In such a case a closed-loop feed-back is
not necessary and an open loop system
may suffice.
In this much simpler and cost-efficient
approach, the determination of the drive
signal to the actuator is performed sim-
ply by cross checking a lookup table
which has listed the output amplitude
and phase shift required by the actuator
at any given point in the engine rpm
range. In theory this lookup table can
indeed be quite complex and include
other determining factors available on
the vehicle CAN (controller area net-
work) bus, such as ambient temperature,
vehicle loads, battery voltage and so on.
It should be noted that an error sensor
(accelerometer or microphone) is still
needed at the position of issue to train
the system and populate the lookup
table, just not to operate the system once
it is initiated.
IMPLEMENTATION INTO PRACTICE
In a first set of investigations an acceler-
ometer was placed right next to the
shaker, on the rail close to the RHS power-
train mount. The vertical vibrations at
that point were taken as the input signal
for the electronic controller. With this
setup a broadband reduction of the local
vibrations was achieved. However, this
improvement did not materialise in the
cabin. The most important signal, the
2nd order driver side interior noise,
remained largely unaffected.
A further optimisation (e.g. through
algorithm tuning, shaker inclination or
sensor repositioning) of this setup would
have been possible but it was deemed
more effective to use a microphone next.
Eventually, a driver side microphone was
used as the error signal to control the
actuator. The actuator itself was still
placed near the RHS powertrain mount
in pure vertical direction.
FIGURE 5 and FIGURE 6 illustrate the
measurement results for the final opti-
mised system setup. Due to the lack of
balance shafts, the range extender pro-
-30
-20
-10
0
10
20
30
40
1000 1500 2000 2500 3000 3500 4000 4500
Acceleration[dB(m/s2
)]
Engine speed [rpm]
RHS X
RHS Y
RHS Z
LHS Z
FIGURE 4 2nd order powertrain
side vibration levels during
engine run-up (RHS= right
hand side, LHS= left hand side)
25
30
35
40
45
50
55
60
65
70
1500 2000 2500 3000 3500 4000 4500 5000
Interiornoise[dB(A)]
Engine speed [rpm]
AVC off
AVC on
Target
25
30
35
40
45
50
55
60
65
70
1500 2000 2500 3000 3500 4000 4500 5000
Interiornoise[dB(A)]
Engine speed [rpm]
AVC off
AVC on
Target
FIGURE 5 Sample noise pressure level of engine order 2 on driver‘s side – without AVC (red) and with AVC (blue)
FIGURE 6 Sample noise pressure level of engine order 2 on passenger‘s side – without AVC (red) and with AVC (blue)
06I2015 Volume 117 17

5. duces a dominant vibration of engine
order 2, which is perceived in the form of
booming noises in the interior of the
vehicle (red curve). The customer
requirement was to reduce the interior
noise below the level of a benchmark
vehicle equipped with balance shafts
and switchable engine mounts. By using
the AVC system, interior noise (blue line)
was reduced significantly by up to 20 dB.
As illustrated by the measurement
results, the customer‘s requirements
(black dashed line) were exceeded over
the entire rotational speed range for all
passengers.
SYSTEM INTEGRATION IMPROVES
COST-EFFECTIVNESS
Be it for three-, four-, five- or six-cylin-
der, cylinder deactivation, missing bal-
ance shafts or electric vehicles with
combustion engines as range extenders,
the technology illustrated here can bring
about a significant reduction in noise
and vibration in many different kinds of
vehicles. With the growing number of
hybrid vehicles, downsizing concepts
with three- or four-cylinder engines, and
further lightweight design techniques
throughout the entire vehicle, active
solutions like these provide a response to
the vibration control challenges of
energy-saving vehicle concepts. In this
specific investigation it was shown that
even a single actuator AVC system with
open loop control is already sufficient to
meet the customers NVH requirements.
It is able to reduce interior noise over a
wide range of frequencies to levels other-
wise only achievable through balance
shaft units or the like. This is especially
remarkable since the AVC has no direct
impact on the air borne noise paths
between the engine and the driver’s ear.
Future developments will focus on inte-
grating sensors, controllers and actuators
into a single unit. This system integra-
tion will further improve cost-effective-
ness and reduce weight, while helping to
widely establish active approaches on
the market.
REFERENCE
[1] TrelleborgVibracoustic (Hrsg.): Schwingungs­
technik im Automobil, Vogel Business Media,
Würzburg, 2015
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