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Application of dither control for automotive wiper squeal
B. Stallaert1, F. Doucet1, J. Rys1, A. Diallo2, S. Chaigne2, J. Swevers1, P. Sas1
1 K.U.Leuven, Department of Mechanical Engineering,
Celestijnenlaan 300 B, B-3001, Heverlee, Belgium
2 Renault, Direction de la Recherche,
1, avenue du Golf, F-78288 Guyancourt Cedex, France
e-mail: [email protected]
AbstractThis paper describes a feasibility study on dither control for automotive wiper squeal. Wiper squeal is a
tonal noise caused by an unstable vibration due to the friction between wiper blade and windscreen. Dither
control is the superposition of a high-frequency signal, in this case a high-frequency vibration, to stabilize a
low-frequency unstability in a system. A finite element model of the wiper has been developed to facilitate
the choice and design of an actuator system; piezo actuators are applied on the wiper to apply the dither
signal. First experimental results show that wiper squeal can effectively be suppressed by dither control, as
soon as the dither amplitude reaches a certain threshold value.
1 Introduction
Car manufacturers spend a lot of effort in reducing unwanted noise inside cars. While engine noise is
becoming less dominant thanks to optimized engine design and improved isolation, other noise sources are
becoming more important. One of these sources are the wipers, which create two types of noise; reversal
and running noise. Reversal is an impact noise caused when the wiper blade changes direction and flips over.
Running noise is the common name for al types of noise produced by the wiper during movement. This
last type of noise, and more specifically squeal, is investigated in this paper. Dither control is successfully
applied to a squealing wiper, in order to suppress the noise.
Dither control applied to a mechanical system is the superposition of a high-frequency vibration to stabilize
a low-frequency vibration. Experimental and numerical studies have shown its effect on friction induced
vibrations [3, 9]. Recently, dither control has effectively been applied to suppress disk brake squeal [2].
This type of non model-based control is especially interesting for squeal phenomena, since the mechanics of
squeal are complex and have never been captured in a model that takes into account all effects [5]. When
comparing wiper squeal to brake squeal, wiper squeal appears even more complex due to the non-linear
properties of the rubber contact. The success of dither control on brake squeal led to the application to wiper
squeal.
In a first stage, the squeal phenomenon is investigated. From these measurements and earlier studies on
dither control, it appeared that the contact force between wiper and windscreen plays an important role in
the onset of squeal. Therefore, a finite element model is developed to predict this contact force distribution
along the length of the wiper. This can be used to assess the effectiveness of the dither control and to facilitate
the design of the control configuration. After applying piezo actuators on a wiper, an experimental study is
performed to investigate the parameters influencing the dither control.
The main goal of this study is to show the effectiveness of dither control applied on wiper squeal noise. It is
shown that dither control can indeed suppress squeal noise, as long as the contact force variation exceeds a
certain threshold value. Due to amplifier limitations, the dither frequency is limited to 2 kHz.
263
The paper begins with an experimental study on the occurrence of squeal on the wiper, in order to understand
the problem at hand (section 2). Section 3 describes the wiper finite element model, which is developed to
support the optimization of the control configuration. Initial dither experiments and the most important
observations are presented in section 4. Finally, the last section summarizes the most important conclusions.
2 Wiper squeal
The term squeal usually refers to a high frequency (> 1000 Hz) tonal noise [4]. However, due to the mul-
tiplicity of terms used in literature, this paper refers to squeal noise as the noise produced by a wiper, dis-
regarding the frequency content. Measurements have been performed in order to characterize the squeal
noise.
2.1 Measurement set-up
The measurement set-up is shown in figure 1, which is a standard compact three-door car. Both acoustic
and vibration signals are measured; the exterior sound pressure, the interior sound pressure at the position of
the driver’s head and the acceleration of windscreen and wiper. The wiper used in the experiments is of the
uniblade type, consisting of two thin beams with the wiper blade in between. The blade that was used is an
uncoated rubber blade, which is more prone to squeal noise. The wiper used in the experiment, is already
instrumented with dither actuators, in order to be able to make a correct comparison between control on and
control off.
Accelerometer
Microphone
Figure 1: Setup for wiper squeal measurements.
In order to control the preload force on the wiper, a mechanism to adjust this force is required. The mech-
anism, shown in figure 2(a), adjusts the preload in the spring used to pull the wiper against the windscreen.
Measuring the total force was done with a simple dynamometer, as shown in figure 2(b). The wiper arm is
pulled from the windscreen and the force is measured at the instant that the contact is lost.
2.2 Squeal measurements
The first measurement, shown in figure 3, is a time measurement of the exterior noise, to show the presence
of squeal. The measurement clearly shows the difference between the reversal noise and squeal noise. The
first is short and instantaneously and has a typical impact signature. The squeal noise on the other hand
shows growing instability.
264 PROCEEDINGS OF ISMA2006
(a) With the screw, the preload force on the
wiper can be adjusted.
(b) Measuring the force by pulling the
wiper arm from the windscreen with a dy-
namometer. The force is measured when
contact between wiper arm and windscreen
is lost.
Figure 2: Adjusting and measuring the preload force on the wiper.
Time [s]
Sound
pre
ssure
[Pa] Reversal (top) Squeal + reversal (bottom)
17.2 17.4 17.6 17.8 18 18.2 18.4 18.6 18.8 19-0.2
0
0.2
Figure 3: Measured sound pressure outside the car, during one wiping cycle.
Time-frequency analysis of the measured signals allows a more detailed analysis of the squeal noise. The
resulting time-frequency map of the exterior sound pressure is shown in figure 4(a). The recurring cycles are
clearly visible, separated by the reversal of the wiper at the bottom and at the top of the windscreen. Since
the reversal is an impact phenomenon, its frequency content is large. The squeal however, is clearly a tonal
noise at two distinct instances, with a frequency between 110 and 160 Hz. When the right conditions are
met, squeal is observed during every wiping cycle at specific positions of the windscreen. This is important
to assess the effectiveness of the applied control. The onset of squeal is most influenced by the preload force
applied on the wiper. The observed frequency is low for squeal noise. However, as mentioned before, the
term wiper squeal is used here for all self-excited wiper phenomena, whatever its frequency.
As can be expected, the interior sound pressure and the acceleration show the same behavior (figures 4(b)–
(d)). The most obvious difference is the V-shaped noise at approximately 750 Hz and the low frequency
signals at about 50 Hz. The first is identified as the sound of the wiper motor; its frequency content varies
because of the varying motor speed, which on its turn is due to the varying load on the motor. The low
frequency signal is probably related to a low frequency vibration of the wiper which does not influence the
squeal phenomenon, but is clearly visible in the wiper acceleration measurements and less visible in the other
measurements.
3 Wiper finite element model
The experimental results of dither control on brake squeal showed that the contact force variation induced by
dither control, should be some percentages of the nominal value [2]. Due to the stiff structure of a disc brake,
this requirement can be easily met. The flexibility and damping of the wiper rubber however, may complicate
ACTIVE NOISE CONTROL 265
Time [s]
Fre
quen
cy[H
z]
Reversal bottom
Reversal top
Squeal
13 14 15 16 17 18 19
0.8
100
200
300
400
500
600
700
800
900
1000
(a) Sound pressure, outside car.
Time [s]
Fre
quen
cy[H
z]
Wiper motor
13 14 15 16 17 18 19
100
200
300
400
500
600
700
800
900
1000
(b) Sound pressure, inside car.
Time [s]
Fre
quen
cy[H
z]
13 14 15 16 17 18 19
100
200
300
400
500
600
700
800
900
1000
(c) Acceleration, measured on wiper.
Time [s]
Fre
quen
cy[H
z]
13 14 15 16 17 18 19
100
200
300
400
500
600
700
800
900
1000
(d) Acceleration, measured on windscreen.
the application of dither control on wipers. It is therefore relevant to know which contact force variation and
which spatial force distribution can be expected when applying actuators on the wipers. Therefore, a finite
element model is developed which predicts the contact force distribution along the wiper, with and without
actuation.
The model serves multiple purposes; most important is to assess the influence of a certain actuator force on
the contact force. Secondly the model can be used in optimizing the control configuration and control design.
Both a static as a dynamic finite element model are developed and implemented in Matlab. The static model
calculates the resulting contact force distribution along the wiper blade, for a certain rubber type, preload
force and actuation force. The dynamic model is used to calculate eigenmodes and -frequencies. After
validation and updating, a good agreement is obtained with measurements, even when boundary conditions
are varied.
3.1 Overview of the model
Figure 4 shows a photo of the wiper, with two parallel thin beams and the rubber wiper in between. Several
aspects, like the interaction between the two wiper beams and the rubber blade, and the windscreen dynamics,
are not taken into account. This would only be possible with a 3D model. Therefore, several simplifications
are made to develop the model shown in figure 5. The model consists of beam elements, with a non-linear
spring (krub) in every node, representing the rubber. The mass of the beam is taken into account by the
distributed load g. The piezo actuators, which are attached to the wiper system, induce a moment load Mp
at both ends of the actuator [7], while the addition of the actuator itself results in a local stiffening of the
beam, represented by locally increasing the bending stiffness EI , with E the elastic modulus and I the
moment of inertia. The inputs to the model are the preload force Fc and the moment Mp. An overview of
the simplifications is given in following paragraphs.
266 PROCEEDINGS OF ISMA2006
Parallel metal beams
Rubber wiper blade
Figure 4: Close-up photo of the wiper, with two metal beams holding the wiper blade.
...
Windscreen
krub
g Fc MpMp
Piezo actuator
Beam
Figure 5: Schematic representation of the simplified wiper model.
Rigid windscreen The windscreen is modeled as a rigid surface, neglecting the windscreen dynamics and
not taking into account any coupling between windscreen and wiper. Frequency response function
(FRF) measurements of the wiper, from the actuator input to an acceleration signal on the wiper beams,
show however that the windscreen clearly influences the wiper dynamics; the FRF’s are significantly
different when the wiper is positioned at different positions on the windscreen.
By neglecting the windscreen dynamics, the model cannot be used for dynamic calculations with the
wiper on the windscreen. This limits the model to calculations for the wiper in free-free boundary
conditions. However, experimental investigations show that this information in itself already gives
good indications to apply dither control.
Simplified beam model The wiper is modeled as a simplified beam model, neglecting the fact that the wiper
is composed out of two parallel beams with rubber in between. Coupling between the beams as well
as wiper torsion is therefore not modeled. To include these effects, a complex 3D model would be
needed, which would increase the computational burden while bringing relatively little to the model.
The simplification has little effect on the applicability of the model for this feasibility study, since
only normal dither is investigated (section 4.1). Only displacements normal to the windscreen are of
interest, such that the torsional displacement, which results in a vibration of the wiper blade tangential
to the windscreen, is of less importance.
Non-linear wiper stiffness The wiper rubber is modeled as a parallel series of individual springs with non-
linear characteristics. The springs are attached to the nodes of the beam model. This assumption only
neglects the damping introduced by the rubber blade and possible influences caused by the shape of
the blade. The non-linear spring characteristic however, is greatly dependent on this shape and since
this characteristic is experimentally determined, the shape influence is taken into account.
Since the only real assumption introduced by this simplification is the absence of damping, only dy-
ACTIVE NOISE CONTROL 267
namic calculations are influenced. The dynamic part of the model however, is mostly limited to the
calculation of eigenmodes and -frequencies, which are marginally influenced by the absence of damp-
ing.
Uncoupling of the springs In a first phase the coupling of the springs is not included in the model to keep
it as simple as possible. The uncoupled model results in a discontinuous force distribution when force
is applied in a single point. In reality, the force distribution will be continuous and spread out over
several points. A coupled model where the force is taken up by several adjacent springs is therefore
closer to reality. Validation measurements show however, that the uncoupled model is already capable
of predicting the force distribution with acceptable accuracy.
Equivalent shape of the windscreen The deformation of the wiper results from the initial shape of the
wiper, combined with the shape of the windscreen. For simplicity, the wiper is modeled as a straight
beam. The influence of the initial wiper shape is taken into account by adding the initial shape to the
windscreen shape, resulting in an equivalent windscreen shape. Pushing the straight wiper against this
equivalent shaped windscreen causes the correct wiper deformation.
This simplification remains valid as long as the deformations are small and the material behavior
remains linear. Since both the curvature of the windscreen and wiper are small, this simplification is
certainly valid.
3.2 Static wiper model - results
The static model calculates the resulting wiper deformation and contact force distribution along the wiper
for a given preload force. The force can be adjusted with the mechanism described in section 2.1. Due to the
presence of the non-linear spring element, the stiffness matrix is dependent on the deformation. Therefore,
an incremental procedure must be followed where the total force is applied in small steps. In each step, an
iterative procedure calculates the resulting equilibrium.
To evaluate the model, the resulting force distribution is compared with measured data, obtained on a mea-
surement bench where the static force is measured along a line contact. Figure 6 shows the comparison
between measured and simulated force distribution. The contact force is measured in unit force per meter of
rubber.
Before a model update (figure 6(a)), the model is not capable of predicting the measured force distribution.
The model update however, shows that the initial wiper and windscreen shape have an important influence
on the model results. After updating the wiper shape, the overall agreement between simulation and mea-
surement is excellent, as shown in figure 6(b). The largest differences are observed at both ends of the wiper,
where the model is most sensitive to the wiper shape.
To validate the model both the preload force and the rubber material have been changed, resulting in the force
distribution of figure 6(c). Since a different rubber type is used, an additional measurement of the rubber
characteristic was required. Although the simulation and measured data differ locally, the overall agreement
is still acceptable. This and other measurements indicate that both ends of the wiper and the center piece
where the wiper is attached, are most difficult to model correctly.
To assess the capabilities of the model to incorporate the actuator force, a validation measurement is shown
in figure 6(d), where a constant voltage is sent to the piezo actuator. Although the model shows excellent
agreement with the measured data, the importance of this result should not be overestimated. Since the
influence of the actuator force on the force distribution is only a few percent, it is difficult to assess the
significance of a difference between simulation and measurement.
268 PROCEEDINGS OF ISMA2006
x [m]
Fco
nta
ct(x
)[N
/m]
Simulation
Measurement
0.8
0.9
0 0.1 0.2 0.3 0.4 0.50
5
10
15
20
25
30
(a) Before update.
x [m]
Fco
nta
ct(x
)[N
/m]
Simulation
Measurement
0 0.1 0.2 0.3 0.4 0.50
5
10
15
20
25
30
(b) After update.
x [m]
Fco
nta
ct(x
)[N
/m]
Simulation
Measurement
0 0.1 0.2 0.3 0.4 0.50
5
10
15
20
25
30
(c) Validation with other total force Fc and
rubber type.
x [m]
Fco
nta
ct(x
)[N
/m]
Simulation
Measurement
0 0.1 0.2 0.3 0.4 0.50
5
10
15
20
25
30
(d) Validation with actuated piezo-actuator.
Figure 6: Model validation: comparison between measured and simulated force distribution.
3.3 Dynamic wiper model - results
The dynamic model calculates the FRF between the actuator force and the acceleration of a point on the
wiper. Since the main goal is to identify the wiper resonance frequencies, the endpoint of the wiper is chosen
in practice, where all vibration modes are visible. The model calculates both FRF’s in free-free boundary
conditions and in contact with the windscreen. However, due to the simplification of a rigid windscreen (see
paragraph 3.1), the FRF’s for the wiper in contact with the windscreen are not reliable.
Figure 7 shows the comparison of a simulated and measured FRF, for a wiper with free-free boundary
conditions. The model gives a good estimation of the resonance peaks, which is sufficient for the control
design.
ACTIVE NOISE CONTROL 269
Frequency
X/F
Simulation
Measurement
Figure 7: Model validation: comparison between measured and simulated frequency response
function.
4 Dither control
This section gives an overview of the aspects related to dither control on wipers and presents some initial
results.
4.1 Dither background
Dither control is the superposition of a high-frequency signal to stabilize the low-frequency behavior of a
system. Because of its simplicity, dither control is applied in different research fields [6, 8, 10]. In mechanical
systems, the signal is most often a high-frequency vibration. A recent study has shown the positive effect of
dither control on automotive brake squeal [2]. The dither signal, with a frequency up to 20 kHz, was applied
with a piezo stack integrated in the brake. The main advantage and explanation for the widespread use of
dither is its simplicity:
• Dither is an open loop technique, requiring no extra sensors.
• A system or friction model is not necessary which is a major advantage for systems with friction which
are often difficult to model.
• Since dither control is a superposition of a signal, it can often be applied on an existing structure,
requiring only minor changes.
Dither efficiency is determined by the dither amplitude, frequency, signal shape and the location where the
dither signal is introduced in the system.
Dither control comes in two considerably distinct forms, normal and tangential dither [1]. In the first case,
the dither signal is applied normal to the friction surface, its effect being a modification of the friction
by a reduction of the friction coefficient. In the latter case, the dither signal is applied tangential to the
friction surface, its effect being a modification of the influence of friction by averaging the non-linear friction
behavior. Although both directions should be considered beforehand, the application on wipers does not
allow for a straightforward implementation of tangential dither, such that only normal dither is considered.
270 PROCEEDINGS OF ISMA2006
4.2 Dither experiments
To apply the dither signal on the wipers, piezo actuators (patches) are attached to the wiper system, inducing
a bending moment. This results in a high-frequency variation of the contact force between wiper and wind-
screen. Optimizing the actuator location is possible with the wiper finite element model; the configuration
which leads to the largest contact force variation will have the highest chance of effectively suppressing the
squeal noise.
First dither experiments show that dither effectively suppresses squeal noise. The sound pressure measure-
ment in figure 8 illustrates the dither effect. Before and after applying the dither signal, the squeal noise is
clearly visible. Although the squeal noise disappears during the application of the dither control, it is re-
placed by noise generated by the dither control itself, including harmonics of the dither frequency. To avoid
this dither noise, the dither frequency could lie outside the audible frequency range. However, the amplifiers
used in this feasibility study did not allow driving the piezo actuators at such high frequencies.
Time [s]
Fre
qu
ency
Dither signal
Squeal Squeal
0 2 4 6 8 10 12 14 16 18
Figure 8: Exterior sound pressure with and without dither control.
Further investigations show that the efficiency of dither control is influenced by several parameters. It was
observed that a threshold value exists for the dither amplitude and that the efficiency is strongly related to
the preload force and to the dither frequency.
5 Conclusions
The wiper, with an uncoated blade, produces a low frequency tonal squeal noise with a frequency between
110 and 160 Hz, depending on environmental and boundary conditions.
To support the optimization of the control configuration, a wiper finite element model, implemented in
Matlab, is developed. It predicts the contact force distribution between wiper and windscreen, along the
wiper blade, as well as the wiper resonances in free-free boundary conditions. Validation measurements
show a good agreement between simulation and measurement.
Finally, experimental results are presented, showing that dither is effective in suppressing wiper squeal noise.
Some influencing parameters are observed, among which frequency content of the dither signal, dither signal
amplitude and preload force.
This paper has shown that dither control for wiper squeal is possible. However, the squeal noise is replaced
by noise generated by the dither signal. Further research will focus on working around this constraint.
ACTIVE NOISE CONTROL 271
The developed FE model allows to predict which configuration leads to the maximum contact force variation.
This information can be used to enhance the efficiency by an optimal placement of actuators.
Acknowledgements
The research of Bert Stallaert is funded by the Institute for the Promotion of Innovation through Science and
Technology in Flanders (IWT-Vlaanderen).
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