Note: Descriptions are shown in the official language in which they were submitted.
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System for measuring the thickness of a layer of rubber for a tyre
Technical field
[0001] The present invention relates to a system for measuring the thickness
of a layer of
rubber, and more particularly to the measurement of the thickness of remaining
rubber on a
tread of a tyre.
Prior art
[0002] In a known way, the tread of a pneumatic tyre, or more simply a tyre,
regardless
of whether it is to be fitted on a passenger vehicle, a heavy transport
vehicle, a civil
engineering vehicle, or other vehicle, is provided with a pattern comprising,
notably,
pattern elements or elementary blocks delimited by various main, longitudinal,
transverse
or oblique grooves, the elementary blocks also possibly comprising various
finer slits or
sipes. The grooves form channels intended to discharge the water during
running on wet
ground, and define the leading edges of the pattern elements.
[0003] The depth of the tread is at a maximum when a tyre is new. This initial
depth may
vary according to the type of tyre in question, as well as the use for which
it is intended; by
way of example, "winter" tyres generally have a pattern depth greater than
that of
"summer" tyres. When the tyre becomes worn, the depth of the elementary blocks
of the
pattern decreases and the stiffness of these elementary blocks increases. The
increase in the
stiffness of the elementary pattern blocks causes a reduction in some
performance
characteristics of the tyre, such as the grip on wet ground. The water
discharge capacity
also decreases markedly when the depth of the channels in the pattern
decreases.
[0004] It is therefore desirable to be able to monitor the development of the
wear of the
tread of a tyre.
[0005] This monitoring is usually carried out by visual observation of the
tread by the
user or a mechanic, with or without actual measurement with a depth gauge.
However, this
observation is not very easy to carry out, notably on rear tyres which are
harder to access,
and furthermore it is not very precise.
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[0006] Numerous proposals have been made to automate the measurement of the
depth of
tyre patterns. Such devices can be placed on the roadway on which vehicles
run. These
devices usually operate by two techniques, either based on optical systems
with cameras or
lasers, or based on eddy currents.
[0007] The systems based on optical systems are costly, have to be embedded in
the
roadway, and require regular maintenance. Moreover, the measurements are
subject to
interference due to soiling and the presence or spraying of water, mud, snow,
etc.
[0008] Documents US 7,578,180 B2 and WO 2008/059283 propose systems for
measuring the thickness of the tread of a tyre, comprising sensors sensitive
to the eddy
currents generated by an exciting magnetic field in the crown reinforcement of
the tyre.
These systems are placed on a roadway.
[0009] However, it has been found that these measurement systems are not
entirely
satisfactory in some cases. This is because the reinforcement of some tyres is
such that the
crown of the tyre is insufficiently conductive to allow the establishment of
eddy currents.
These measurement systems are therefore unsuitable for measuring the thickness
of the
treads of these tyres.
Brief description of the invention
[0010] One object of the invention is a system for measuring the thickness of
a layer of
rubber material of a tyre, the layer comprising a face joined to an adjacent
reinforcement
made with at least one material having a magnetic permeability greater than
the magnetic
permeability of air, and a free face in contact with the air, the system
comprising a sensor
capable of measuring the distance d between the joined face and the free face
of the layer
of rubber material. This system is characterized in that the sensor comprises
a single
excitation and measurement coil, and in that the excitation frequency and
power are such
that the inductance at the terminals of the coil increases when the distance d
decreases.
[0011] According to one object of the invention, the sensor of the measurement
system
has the advantage of operating in reluctance mode, and therefore with a lower
coil
excitation frequency for a given power than in the case of a similar sensor
operating in a
mode sensitive to eddy currents.
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[0012] Measurement in reluctance mode also makes use of the magnetic
permeability of
the adjacent reinforcement, and has been found to provide high measurement
sensitivity to
any variation of the distance d.
[0013] Preferably, the coil of the sensor is positioned around, or is
surrounded by, a
material having high electrical resistivity and high magnetic permeability.
[0014] The presence of this material with high electrical resistivity and high
magnetic
permeability, such as a ferrite, has the advantage of localizing the magnetic
field lines and
thus providing a more localized measurement of layer thickness.
[0015] The ferrite may be U-shaped.
[0016] The coil is then preferably positioned around one of the lateral
branches of the U.
[0017] Alternatively, the coil may be positioned around the bottom of the U of
the ferrite.
[0018] In this embodiment, the range of the sensor can be improved simply by
increasing
the spacing between the two ends of the U.
[0019] This range may also be increased by increasing the cross section of the
poles
formed by the two parallel bars of the U-shaped circuit.
[0020] According to another embodiment, the excitation and measurement coil is
positioned around an E-shaped material having high electrical resistivity and
high
magnetic permeability.
[0021] In this embodiment, the coil is advantageously positioned around the
central bar
of the E.
[0022] In this embodiment, the range of the sensor can be improved simply by
increasing
the spacing between the central bar of the E and its two outer ends.
[0023] This range may also be increased by increasing the cross section of the
poles
formed by the three parallel bars of the E-shaped circuit.
[0024] According to a third embodiment, the excitation and measurement coil is
positioned around a material having high electrical resistivity and high
magnetic
permeability, the material having an axis of symmetry and being E-shaped as
seen in any
axial section.
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[0025] In this embodiment, known as a "potted" arrangement, the coil is
positioned
around the central axis of the magnetic circuit.
[0026] In this embodiment, the range of the sensor may be increased simply by
increasing the outside diameter of the pot structure, so that the spacing
between the central
pole and the outer pole becomes greater.
[0027] This range may also be increased by increasing the cross section of the
two poles
of the pot structure.
[0028] This axisymmetrical embodiment has the advantage of being insensitive
to the
orientation of the metal cords forming the adjacent reinforcement. The sensor
is therefore
insensitive to the anisotropy of this adjacent layer.
[0029] Advantageously, the excitation and measurement coil is supplied by an
alternating
power source.
[0030] When an alternating power supply frequency is used, it is advantageous
to limit
this to not more than 150 kHz, thereby greatly limiting the generation of eddy
currents in
the adjacent reinforcement of the layer. Additionally, if a frequency of 10
kHz is exceeded,
the conventional noises measured by an antenna in the near field can be
avoided.
[0031] Furthermore, as the supply frequency increases for a given current, the
time
resolution of the measurement improves. Finally, increasing the frequency
makes it
possible to reduce the measurement time, which has a favourable effect on the
power
consumption of the whole system.
[0032] It has been found to be advantageous to use a supply frequency in the
range from
40 to 80 kHz.
[0033] In order to evaluate the distance d between the sensor and the metal
tyre
reinforcement, it is possible to measure the complex impedance or only a part
of this
impedance, such as the resistance or inductance.
[0034] It is also possible to measure the phase or the modulus of the complex
impedance.
[0035] Advantageously, the inductance of the coil is measured.
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[0036] For example, the inductance of the coil may be measured by means of the
following formula:
U = L(di/dt)
[0037] To do this, the coil can be supplied with a known stationary sinusoidal
current,
enabling the derivative with respect to time to be precisely known, and a
device for
measuring the amplitude of the voltage at the terminals of the coil can be
used.
[0038] This device for measuring the voltage amplitude at the terminals of the
coil may
measure the voltage continuously or may use an amplitude demodulation system.
[0039] The measurement system is advantageously positioned in an electrically
non-
to conductive casing whose materials have a magnetic susceptibility equal to
zero or
sufficiently low to be similar to those of air or a vacuum.
[0040] Preferably, the coil has an axis of sensitivity and the casing has a
face for
application against the free face of the layer whose thickness is to be
measured, and the
application face of the casing is perpendicular to the axis of sensitivity of
the coil.
[0041] The casing may be a portable casing.
[0042] In this case, the measurement system according to one object of the
invention may
be used for measuring the thickness of rubber material of a sidewall or of an
internal
rubber element of a tyre. This measurement can be performed during the
manufacture of
the tyre or after the completion of this operation.
[0043] The casing may also be suitable for positioning on, or embedding in, a
roadway.
[0044] In this case, the measurement system is preferably used for measuring
the
thickness of remaining rubber material on a tyre tread.
[0045] Evidently, the excitation and measurement coil of the measurement
system
according to one object of the invention may be formed by a plurality of coils
connected in
series or in parallel.
[0046] The invention is particularly applicable to tyres having metal
reinforcers in their
crowns and/or their carcass plies, such as those to be fitted on vehicles of
the passenger or
SUV ("Sport Utility Vehicle") type, or on industrial vehicles selected from
among vans,
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heavy transport vehicles ¨i.e. light rail vehicles, buses, heavy road
transport vehicles
(lorries, tractors and trailers), and off-road vehicles such as civil
engineering vehicles¨,
and other transport or handling vehicles.
Description of the Drawings
100471 The attached figures show a number of embodiments of a measurement
system
according to one object of the invention, taking as the principal example the
application of
the invention to the measurement of the thickness of tyre treads:
- Figure 1 is a perspective view of a vehicle, a tyre of which is passing
over a casing
comprising a measurement system according to one object of the invention;
- Figure 2 shows a casing with a measurement system;
- Figure 3 shows a cross section of a tyre in contact with the casing of
the measurement
system;
- Figure 4 shows the operating principle of a measurement system in the
case of an air-
cored coil, in the absence (a) and in the presence (b) of a metal plate;
- Figure 5 shows schematically an example of the operation of the measurement
system
in the case of a coil with a U-shaped ferrite element;
- Figure 6 shows an alternative embodiment of the system of Figure 5;
- Figure 7 shows a second embodiment with an E-shaped ferrite element;
- Figure 8 shows a third embodiment with a pot-shaped ferrite element; and
- Figure 9 shows schematically a structure of the electronic circuitry of a
measurement
system.
Detailed description of the invention
100481 Figure 1 shows a vehicle 5 whose tyre 8 is running over a casing 6
comprising a
wear measurement system. The drawing shows a passenger vehicle, but the
measurement
system can also be used for any other vehicle, such as a heavy transport
vehicle or a coach.
The remaining thickness of rubber material on the tread of the tyre 8 is
measured when the
tyre runs over the casing 6, without any need to stop the vehicle or remove
the tyre from
the vehicle.
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[0049] Figure 2 shows a casing 12 according to one of the objects of the
invention. This
casing takes the form of a portable assembly which can be placed on a roadway.
It has a
substantially trapezoidal cross section. The casing comprises two inclined
portions, namely
an access ramp 15 and an exit ramp 16. Between these two portions there is a
substantially
horizontal portion 18. The portion 18 of the casing 12 protects a sensor or a
row of sensors
50 for making the distance measurements. The base 20 of the casing is placed
against the
roadway and gives the casing the necessary stability during the operation of
the system.
The casing 12 also comprises electronic circuitry 40 with a power source which
supplies
the sensors 50 with alternating current. The measurements are made when the
tyre contact
area rests on the horizontal portion 18. This horizontal portion is the face
of the casing
which is applied to the surface of the tyre tread. The casing 12 is made of a
non-conductive
material whose magnetic properties are similar to those of air, to avoid
interference with
the measurements.
[0050] According to other embodiments, the casing may be embedded in a roadway
or
may have suitable dimensions and weight for application against a sidewall or
an internal
rubber element of a tyre.
[0051] The measurement of the thickness of remaining rubber material on a tyre
tread is
illustrated in Figure 3. This drawing shows a partial cross section of a tyre
8 bearing on the
application face 18 of a casing 12. The tyre 8 comprises, notably, a tread 80
with patterns
82, a crown reinforcement 84 consisting of two or more plies of metal
reinforcers (not
shown), and sidewalls 86. The casing 12 comprises an application face 18, a
base 20 and a
row of sensors 50. The running surface 88 of the tread 80 bears against the
application face
18 of the casing 12.
[0052] The sensors 50 measure, as will be explained below, the distance DI
which
separates them from the metal reinforcement 84 of the crown of the tyre 8. DI
has three
components. Two of these components are fixed, namely the distance D2 which
separates
the bases of the patterns 82 from the reinforcement 84, and the distance D3
which
separates the sensors 50 from the application face 18 of the casing 12. One
component is
variable with the degree of wear of the tread, namely d, which corresponds to
the
remaining thickness of the tread. Thus:
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d = D1- D2 - D3
[0053] The distance D2 can be known on the basis of the identification of the
type of tyre
being measured. This identification may be manual or automatic, being
performed, for
example, by retrieving identification data recorded in a transponder such as
an RFID
device incorporated in the tyre structure.
[0054] Figure 4 shows the operating principle of the sensor of a measurement
system
according to one object of the invention.
[0055] Figure 4(a) shows an air-cored coil 10 with an axis of symmetry and
sensitivity A.
When the terminals of the coil are supplied with a direct current, the
magnetic field lines
54 emitted by this device extend in the air all around the coil, as shown
schematically in
Fig. 4(a).
[0056] If a metal reinforcement 14, which is a good magnetic field conductor
and a poor
electrical conductor, such as a crown ply of a tyre consisting of parallel
metal reinforcers
embedded in two layers of low-conductivity rubber material, is brought towards
this
device, the field lines will naturally attempt to pass through this metal
reinforcement rather
than through the air, because the reluctance of air is greater than that of
the metal
reinforcement. A localization of the magnetic field lines 54 through the metal
reinforcement 14 can be observed.
[0057] The result is that the magnetic flux density will increase in the area
located
between the coil 10 and the metal reinforcement.
[0058] Thus the variation of the position of the metal reinforcement 14
relative to the coil
10 can be measured by measuring the variation of the inductance of the coil
10.
[0059] Figure 5 shows a schematic example of the operation of an embodiment of
a
measurement system in the case of a coil and a U-shaped ferrite element.
[0060] The layer 21 whose thickness d is to be measured comprises a layer of
rubber
material 24 adjacent to a reinforcement 22 formed by reinforcers whose
magnetic
permeability is greater than the magnetic permeability of air, such as those
normally used
for carcass plies or crown plies of tyres, notably those of heavy transport
vehicles.
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[0061] The casing 12 of the measurement system comprises a sensor 50 which
comprises
a coil 10 positioned around one of the lateral branches 36 of a U-shaped
ferrite element 30.
The presence of the ferrite 30 makes it possible to localize the flow of the
magnetic field
lines through it and thus localize the measurement area. The two bars of the U
are spaced
apart by a distance 11.
[0062] The casing 12 has its application face 18 bearing against the free face
26 of the
layer 21.
[0063] According to an essential characteristic of the measurement system, the
excitation
frequency and power of the coil 10 are such that the inductance at the
terminals of the coil
increases when the distance d decreases.
[0064] Thus the operating mode of the sensor is a reluctance mode, and is
therefore
related to the magnetic permeability of the different parts of the magnetic
circuit.
[0065] The magnetic permeability of the rubber material is much lower than
that of the
adjacent reinforcement, which is itself lower than that of the ferrite.
[0066] Consequently, the reluctance of the layer 24 of rubber material is much
higher
than that of the adjacent reinforcement 22, which is itself higher than that
of the ferrite 30.
This means that the variation of inductance measured at the terminals of the
coil 10 is
mainly due to the variation of the distance d which is the thickness of the
layer of rubber
material, since any variation in the reluctance of the adjacent reinforcement
caused, for
example, by the number of reinforcers or their construction has only a minor
effect on the
accuracy of the measurement. The accuracy and sensitivity of this sensor in
reluctance
mode are therefore good. The range of the sensor is dependent on the distance
11, which is
the distance between the two bars of the U, and on the cross section of the
poles formed by
these two parallel bars.
[0067] Figures 6 to 8 show alternative embodiments of sensors.
[0068] In Figure 6, the sensor 60 comprises a U-shaped ferrite element 64 and
a coil 62
positioned around the central bar of the U.
[0069] In Figure 7, the sensor 70 comprises an E-shaped ferrite element 74 and
a coil 72
positioned around the central bar of the E.
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[0070] In Figure 8, the sensor 90 comprises a pot-shaped ferrite element 94
with an axis
of symmetry and a central bar positioned substantially along this axis of
symmetry, and a
coil 92 positioned around the central bar of the pot. Figure 8(a) shows a
perspective view
of the sensor, and Figure 8(b) shows a cross section along the axis of
symmetry.
[0071] This pot structure has the advantage of being insensitive to the
anisotropy of the
metallic architecture inside the tyre.
[0072] Figure 9 shows an example of the structure of the electronic circuitry
40 for
measuring the thickness of a layer of tyre rubber.
[0073] This electronic circuitry is formed by a "sensor module" 100 and a
"motherboard"
120. It can therefore be used to measure the thickness of a layer at a single
point.
[0074] In order to extend the principle of this arrangement to a system
consisting of
multiple sensors, it is simply necessary to use a plurality of "sensor
modules", all
connected to the same "motherboard".
[0075] In reluctance mode, the inductance L of the sensor increases when the
distance d
between the sensor and an adjacent layer, formed by metal tyre cords,
decreases. The
purpose of this electronic circuitry is therefore to measure the inductance L
of the sensor
coil, so that this distance between the sensor and the adjacent layer can be
deduced.
[0076] The "sensor module" 100 is formed, in part, by a sensor 102 as
described above,
supplied with a current I which is considered to be the phase reference
(4)=0). This current
1 is generated by a current amplifier 104, which is itself driven by an
oscillator 106 whose
frequency is set by a time base 107. The amplifier, oscillator and time base
form part of the
"sensor module-.
[0077] The voltage U for the phase 4, which is non-zero relative to the
current I,
collected at the terminals of the sensor, is first amplified by the amplifier
108 and then
injected into a double demodulator 110, together with the output signal of the
oscillator
106.
[0078] At the output of the demodulator 110, the signals X and Y are found,
representing
the two complex components describing the voltage at the terminals of the
sensor, such
that:
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U = KVX2 + Y2
where K is a factor dependent on the amplification present along the
electronic circuit.
[0079] The two signals X and Y are then filtered by the filters 112 and
digitized by
means of analogue/digital converters (ADC) 114, and are then injected into the
microcontroller 122 of the "motherboard" 120.
[0080] The microcontroller 122 deduces from X and Y the value of the
inductance L of
the sensor 102. It initially calculates the value of the amplitude of the
voltage U at the
terminals of the sensor, using the above formula.
[0081] In a second stage, it deduces from this value the inductance L of the
sensor coil,
the value of the current I injected into the sensor being known, using the
following
formula:
L = ___________________________________
di 1
I dt
[0082] The motherboard is also provided with a number of additional functional
units,
namely:
= a memory 124 for recording the measurements made by the sensor 102;
= an RFID decoder 126 for identifying the tyre, by means of an antenna 128,
if this
can be done by using the presence of an RFID device incorporated in the tyre
structure;
= a wireless communication module 130 for sending data over a distance, via
a
supplementary antenna 132; and
= a power supply 134 distributing the current required for the whole system
from a
battery 136.
The assembly is able to perform numerous measurements on tyres without a
battery
change, giving the system several years of service life without human
intervention.
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