Method for tasting a rotor blade of a wind power
installation and test apparatus
The invention relates to a method for testing a rotor
blade of a wind power installation, which covers a
rotor blade coverage area during operation of the wind
power installation, and to a test apparatus for
carrying out the method. A second aspect of the
invention relates to a test apparatus for wind power
installations.
Rotor blades of wind energy installations are highly
loaded components and must be regularly checked for
structural faults. In the case of rotor blades mounted
on a wind energy installation, a test such as this is
time-consuming and costly, since they are difficult to
access. This applies in particular to wind energy
installations on the open sea.
US 4,854,724 A discloses a method for non-destructive
examination of weld beads and spot welds. A
thermography method is used in this case, in which the
spot weld to be examined or the weld bead and its
surroundings is or are heated, and the temperature
profile and the cool-down response are followed using a
thermal imaging camera. This has the disadvantage that
the method cannot be used when the object is moving.
The relevant wind power installations would therefore
have to be stopped in order to check the rotor blades
in this way.
US 6,419,387 Bl likewise discloses a thermography
method in which small parts of the surface of a
workpiece which is to be examined are heated, and the
temperature profile and the cooling-down response are
then observed. US 6,419,387 Bl proposes a raster
method, by means of which even relatively large-area
components can be examined quickly and cost-
effectively. Disadvantageously, even this is not
possible during operation of a wind power installation.
The same applies to WO 2006/074 938 Al, which likewise
discloses a thermographic method for examination of a
component for structural damage. This method can also
not be carried out during operation of a wind power
installation.
WO 03/069324 Al discloses a method by means of which it
is possible to examine on a thermographic basis whether
adhesive which is heated during curing is present at
all the intended points. The method described in
WO 03/069324 Al is used exclusively for inspection in
the production process, and can therefore likewise not
be used when a rotor blade is rotating during operation
of the wind power installation.
The invention is based on the object of providing a
method for testing a rotor blade of a wind power
installation, by means of which disadvantages in the
prior art can be reduced. A further aim is to provide a
test apparatus which can be used to carry out the
method.
The invention achieves the object by a method having
the following steps:
(a) transmission of an aiming light beam, in
particular of an aiming laser beam which has an
aiming light beam power density, in an aiming
light beam direction at the rotor blade coverage
area,
(b) detection of any reflection of the aiming light
beam on an impact point on the rotor blade by a
detection apparatus,
(c) directly after the detection of the reflection,
electrically controlled transmission of a
measurement laser beam with a measurement laser
beam power density which is greater than the
aiming light beam power density, at the impact
point, such that the rotor blade is heated at the
impact point,
(d) measurement of a temperature change at the impact
point, and
(e) repetition of steps a) to d) for a plurality of
impact points.
Before the aiming light beam is transmitted in method
step a) , the aiming light source is aligned with the
rotor blade coverage area. The aiming light beam which
is then transmitted either strikes a rotor blade and is
at least partially reflected and thrown back, or passes
through between two rotor blades. In order to ensure
that the aiming light beam does not cause any damage in
its further path in the latter case, the aiming light
beam has a relatively low aiming light beam power
density which, for example, is less than 1 mW/mm2,
preferably less than 0.1 mW/mm2.
The proportion of the light of the aiming light beam
which is thrown back after the aiming light beam has
been reflected on a rotor blade is detected in method
step b). The measurement laser beam is transmitted
immediately after detection of the light of the aiming
light beam thrown back from the rotor blade.
Advantageously, there is less than 10 ms between the
detection of the reflected light of the aiming light
beam and the transmission of the measurement laser
beam, particularly preferably less than 5 ms. This
ensures that the test is carried out as quickly as
possible. Furthermore, this short time period ensures
that the aiming laser beam and the measurement laser
beam strike the rotor blade at the same point, even
where the rotor blade can be caused to move slightly,
for example to vibrate, for example because of the
wind, even when the wind power installation is
stationary.
The measurement laser beam has a measurement laser beam
energy density which is higher than the aiming light
beam energy density, and, for example, is more than
50 mW/mm2. This ensures that the measurement laser beam
strikes the rotor blade to be tested.
The measurement laser beam strikes the rotor blade to
be tested. The rotor blade is therefore heated at the
impact point. After a certain time period which, for
example, is 10 s, the measurement laser beam is
switched off. The heating of the rotor blade is thus
stopped at the impact point. In the next method step,
the temperature distribution on the surface of the
rotor blade is measured at the impact point of the
measurement laser beam and around it. Details relating
to the measurement and the evaluation of the data
obtained are described further below. Once the
temperature distribution has been measured, method
steps a) to d) are repeated at a different impact point
on the rotor blade. The rotor blade can therefore be
tested over a wide spatial range.
A spatial temperature distribution is advantageously
measured. After the measurement laser beam has been
switched off, the temperature distribution is measured
on the rotor blade surface, on a position-resolved
basis over an area whose diameter corresponds, for
example, to five times the diameter of the measurement
laser beam at the impact point.
At least one time temperature distribution is
advantageously also measured. In this case, after the
measurement laser beam has been switched off, the
temperature distribution is measured on the surface of
the rotor blade which is to be tested, on a position-
resolved basis at different times.
Inhomogeneities are advantageously determined from the
spatial and/or time temperature distributions, and a
signal is output when a predetermined threshold value
for the inhomogeneity is exceeded. After the
measurement laser beam has been switched off, the
energy supplied to the rotor blade surface by the
measurement laser beam is dissipated into the material
of the rotor blade. The temperature at the surface of
the rotor blade therefore falls. The rate at which the
thermal energy supplied by the measurement laser beam
is dissipated, and therefore also the rate at which the
temperature on the rotor blade surface falls, depends
on the thermal conductivity of the rotor blade
material. A rotor blade which is composed of a
homogeneous material also has a homogeneous thermal
conductivity. This means that the supplied heat is
transported at the same speed in all directions, as a
result of which the temperature likewise falls with the
same characteristic at all points.
The measurement laser beam strikes the rotor blade to
be tested at an impact point. At this point, the
measurement laser beam ideally has a circular cross
section, whose radius is, for example, 15 mm. The
measurement laser beam power density is not constant
within this cross section, but decreases from the
center of the cross section toward the edge. This means
that more energy was transmitted to the rotor blade to
be tested in the center of the impact point. The
temperature distribution on the rotor blade surface is
therefore also not constant within the impact point.
This also falls from the center point of the impact
point toward the edge. In this case, the radius of the
measurement laser beam cross section is the distance
from the center of the cross section in which the
impacting measurement laser beam power is half the
measurement laser beam power that strikes the center of
the cross section.
Since the heat supplied through the measurement laser
beam is transported away, and the temperature on the
rotor blade surface therefore decreases at the same
rate in all directions in the case of homogeneous
materials, the temperature measured at one position is
dependent only on the distance from the center of the
impact point. The data obtained by a measurement of the
spatial temperature distribution on the surface of the
rotor blade with an intact rotor blade is therefore
essentially rotationally symmetrical around the center
point of the impact point. If, for example, one point
now remains hot for considerably longer than other
points at the same distance from the center of the
impact point, this could be caused, for example, by an
air enclosure located underneath this point. This air
enclosure has a considerably lower thermal conductivity
than the material surrounding it, as a result of which
the thermal energy which was supplied by the
measurement laser beam here cannot be transported away
into the material so quickly.
A possible measured value by means of which
inhomogeneities can be measured is the difference
between a maximum and a minimum temperature which are
measured on a ring around the center of the impact
point. If this difference exceeds a previously defined
threshold value, a signal is output. By way of example,
this is output online or by radio to a control center
or a computation center.
,In order to measure the temperature change over time, a
plurality of measurement results of the spatial
temperature distribution are carried out at different
times. The decrease of the temperature at one specific
point over time is calculated therefrom. The heat flux
can be deduced from this, and the thermal conductivity
can be deduced from the heat flux. For a homogeneous
medium, the thermal conductivity is spatially constant.
The decrease of the temperature at one position on the
rotor blade over time therefore depends only on the
distance of this position from the center point of the
impact point. If this decrease is now considerably less
at one point than at other points which are at the same
distance from the center of the impact point, this
indicates an air enclosure under the surface of the
rotor blade. By way of example, inhomogeneities are
determined by determining the decrease of the
temperature over time along a ring whose center point
is the center point of the impact point. If the
difference between the maximum value and the minimum
value of this decrease exceeds a previously defined
threshold value, a signal is output.
The temperature distribution is advantageously measured
using a thermal imaging camera. This is adjusted such
that it detects an area on the rotor blade surface at
whose center the impact point of the measurement laser
beam is located. By way of example, the area to be
measured is a circle, having a diameter which
corresponds to five times to ten times the diameter of
the impact point.
The aiming light beam direction is advantageously
changed after the temperature distribution has been
measured. This ensures that a different impact point is
tested in the next method run. In this case, care must
be taken to ensure that both the aiming laser beam and
the measurement laser beam then strike the rotor blade
at a different angle. The aiming light beam in
particular is in consequence reflected at a different
angle, which can considerably change the intensity of
the light to be detected after reflection. In order to
take account of this, the sensitivity of the detection
of the aiming light beam after reflection on a rotor
blade is increased or decreased.
The method is advantageously carried out while the
rotor blades are stationary. This results in
particularly high measurement accuracy.
The measurement laser beam is advantageously at a
wavelength which is beyond the human visible spectrum.
This ensures that the measurement laser beam does not
cause any damage, or causes minimal damage, when, for
example, it does not strike a rotor blade because of a
malfunction. However, a hazard can occur even when
testing points on the rotor blade surface which the
measurement laser beam does not strike at right angles.
In this case, as in the case of the aiming light beam,
a proportion of the incident light is reflected. Since
the measurement laser beam has a high measurement laser
beam power density, as described above, this results in
a considerable hazard, which can be minimized by
optimum choice of the wavelength of the measurement
laser beam.
The rotor blade to be tested is preferably attached to
a hub, and the aiming light beam is transmitted to
impact points at different radial distances from the
hub. This ensures that the rotor blade to be tested is
tested comprehensively. As already described, the
sensitivity of the apparatus used to detect the
reflected light of the aiming light beam can be matched
to a possibly different impact angle, and therefore
reflection angle, of the light.
The measurement laser beam and the aiming light beam
are preferably transmitted on a common beam path. This
ensures that a transmitted measurement laser beam
strikes a rotor blade since, obviously, the aiming
light beam has also been reflected.
The aiming light beam and the measurement laser beam
are preferably transmitted from a measurement apparatus
which is mounted on a second wind power installation.
In this case, at least in the relatively close vicinity
of the hub to which the rotor blade to be tested is
attached, the aiming light beam strikes the rotor blade
essentially at right angles, as a result of which the
maximum intensity of the aiming light beam can be
reflected. Furthermore, the process ensures, in
particular, that the measurement laser beam cannot
located at eye level of any passers-by or animals,
where it could cause damage.
A test apparatus according to the invention for wind
power installations comprises an aiming light source
which has an aiming light beam power density, a
detection apparatus, which is designed to detect any
reflection of the aiming light beam on a rotor blade of
the wind power installation, a measurement laser, which
has a measurement laser power density which is greater
than the aiming light beam power density, and is
designed to output a measurement laser beam in a
measurement laser beam direction, and a temperature
measurement apparatus for measuring the temperature
distribution at the impact point, as well as an
electrical controller, which is connected to the aiming
light source, to the detection apparatus, to the
measurement laser and to the temperature measurement
apparatus, and is designed for carrying out one of the
methods mentioned above.
In particular, diode lasers and solid-state lasers are
suitable for use as aiming and measurement lasers.
However, other laser types can also be used. The aiming
light beam power density is, for example, less than
1 mW/mm2, advantageously less than 0.1 mW/mm2. In
contrast, the measurement laser beam has a measurement
laser beam power density which, for example, is 50
mW/mm2. The electrical controller ensures that the
method can be carried out automatically. A preset
routine which scans the entire surface of the rotor
blades by skilful choice of the impact points can be
implemented easily, thus considerably reducing man
hours and therefore the costs involved.
The aiming light source can advantageously be adjusted
in the aiming light beam direction, and the measurement
laser can be adjusted in the measurement laser beam
direction, in a motorized manner. This allows the
direction to be adjusted considerably more accurately
than if the laser had to be adjusted by hand and,
furthermore, this improves the reproducibility. For
example, it is easily and precisely possible to once
again set a point on a rotor blade at which the
detected data is subject to errors of for which the
detected data needs to be checked.
In a wind farm with two wind power installations, one
of the described test apparatuses is advantageously
mounted on at least one of the wind power
installations, and these apparatuses are designed to
carry out a method as described above. This is
particularly advantageous for wind farms on the high
seas since they can be accessed only with difficulty,
as a result of which it is complex and costly to test
the rotor blades of these wind power installations.
Furthermore, the rotor blades of wind power
installations on the high seas have to be checked
considerably more frequently since they are subject to
wear considerably more quickly because of more extreme
weather conditions and the continuous influence of
salt.
One exemplary embodiment of the invention will be
described in more detail in the following text with
reference to a drawing, in which:
Figure 1 shows two wind energy installations, one of
which is equipped with a test apparatus
according to the invention, for carrying out
a method according to the invention.
Figure 1 shows a first wind power installation 1 and a
second wind power installation 2, having a respective
tower 10.1 and 10.2, and a respective pod 11.1 and
11.2. The pods 11.1 and 11.2 each have a hub 12.1 and
12.2, about which the rotor blades 13.1, 13.2, 13.3 and
13.4 attached thereto rotate. The two wind power
installations 1 and 2 are separated by a distance R.
The distance R is generally between 200 and 900 meters.
A test apparatus 14 according to the invention is
mounted on the pod 11.2 of the second wind power
installation 2 and has an aiming laser, a detection
apparatus, an aiming laser and a temperature
measurement apparatus 15. Figure 1 shows only the
temperature measurement apparatus 15 of these
components of the test apparatus 14.
At the start of the method for testing the rotor blade
13.1 for structural faults, the test apparatus 14 uses
the aiming laser contained in it to emit an aiming
laser beam 20 in the direction of the area covered by
the rotor blades 13.1 and 13.2 of the wind power
installation 1. The aiming laser beam in this case
strikes the impact area 16 on the rotor blade 13.1 of
the wind power installation 1, where at least a portion
of the light is reflected, and is thrown back in the
direction of the test apparatus 14 on the pod 11.2 of
the wind power installation 2. This proportion of the
aiming laser beam 14 that is thrown back is detected by
the detection apparatus contained in the test apparatus
14. For this purpose, it is not necessary to transmit a
laser beam at the start, and in principle any method is
suitable which allows the position of the rotor blade
to be measured to be found. The use of the combination
of an aiming laser beam 20 and a measurement laser beam
21, which are preferably transmitted on one beam path,
ensures greater accuracy and reproducibility, however.
If the detection apparatus contained in the test
apparatus 14 has found a reflection of the aiming laser
beam 20 on the impact area 16 on the rotor blade 13.1,
a measurement laser beam 21 is emitted. Figure 1 shows
the aiming laser beam 20 and the measurement laser beam
21 slightly offset. This is possible, but they are
advantageously transmitted on one beam path.
The measurement laser beam 21 strikes the rotor blade
13.1 at the impact point 16. As a result of the high
power density of the measurement laser beam 21, the
temperature of the rotor blade 13.1 at the impact point
16 is increased, and the heat created there is
dissipated into the material of the rotor blade 13.1.
The rate at which this happens and the spatial extent
which is reached depend on the thermal conductivity of
the material of the rotor blade 13.1. This is changed
by structural faults, for example cracks or air
enclosures, thus resulting in a different temperature
profile being found when the impact point has
structural faults. The temperature distribution at the
impact point 16 on the rotor blade 13.1 is measured via
the temperature measurement apparatus 15 which, for
example, may be a thermal imaging camera which is
integrated in the test apparatus 14. In this case, a
spatial distribution of the heat energy and the time
profile of the heat transport are measured.
Inhomogeneities and structural faults can be discovered
from the comparison of the data determined in this way
with the known data for an intact rotor blade.
v Since the method can be carried out over a relatively
long distance R, a suitable test apparatus can also be
mounted on the ground, or, for example, in a mobile
form in a car. This allows the method to be used
flexibly, and there is no need to obtain a specific
test apparatus for each wind power installation, thus
considerably reducing the costs of the maintenance
method.
We claim:
1. A method for testing a rotor blade of a wind power
installation, which covers a rotor blade coverage
area during operation of the wind power
installation, having the following steps:
(a) transmission of an aiming light beam, in
particular of an aiming light beam which has
an aiming light beam power density, in an
aiming light beam direction at the rotor
blade coverage area,
(b) detection of any reflection of the aiming
light beam on an impact point on the rotor
blade by a detection apparatus,
(c) directly after the detection of the
reflection, electrically controlled
transmission of a measurement laser beam with
a measurement laser beam power density which
is greater than the aiming light beam power
density, at the impact point, such that the
rotor blade is heated at the impact point,
(d) measurement of a temperature distribution at
the impact point, and
(e) repetition of steps (a) to (d) for a
plurality of impact points.
2. The method as claimed in claim 1, characterized in
that a spatial temperature distribution is
measured.
3. The method as claimed in one of the preceding
claims, characterized in that a temperature
distribution is measured at at least two different
times.
4. The method as claimed in one of claims 2 or 3,
characterized in that the temperature distribution
is measured using a thermal imaging camera.
5. The method as claimed in one of claims 2 to 4,
characterized by the following steps:
determination of inhomogeneities from the
spatial and/or time temperature distribution,
and
outputting of a signal when a predetermined
threshold value for at least one of the
inhomogeneities is exceeded.
6. The method as claimed in one of the preceding
claims, characterized by the following step:
after the measurement of the temperature
distribution, variation of the aiming light
beam direction.
7. The method as claimed in one of the preceding
claims, characterized in that the method is
carried out while the rotor blades are stationary.
8. The method as claimed in one of the preceding
claims, characterized in that the measurement
laser beam is at a wavelength which is beyond the
human visible spectrum.
9. The method as claimed in one of the preceding
claims, characterized in that
the rotor blade is attached to a hub, and the
aiming light beam is transmitted to impact
points at different radial distances from the
hub.
10. The method as claimed in one of the preceding
claims, characterized in that the measurement
laser beam and the aiming light beam are
transmitted on a common beam path.
11. The method as claimed in one of the preceding
claims, characterized in that the aiming light
beam and the measurement laser beam are
transmitted from a measurement apparatus which is
mounted on a second wind power installation.
12. A test apparatus for wind power installations
having:
(a) an aiming light source which has an aiming
light beam power density,
(b) a detection apparatus, which is designed to
detect any reflection of the aiming light
beam at an impact point on a rotor blade of
the wind power installation,
(c) a measurement laser, which has a measurement
laser power density which is greater than the
aiming light beam power density, and is
designed to output a measurement laser beam
in a measurement laser beam direction,
(d) a temperature measurement apparatus for
measuring a temperature distribution at the
impact point, and
(e) an electrical controller, which is connected
to the aiming light source, to the detection
apparatus, to the measurement laser and to
the temperature measurement apparatus, and is
designed to carry out a method as claimed in
one of claims 1 to 11.
13. The test apparatus as claimed in claim 12,
characterized in that the aiming light beam
direction of the aiming light beam can be
adjusted, and the measurement laser beam direction
of the measurement laser can be adjusted, in a
motorized manner.
14. A wind farm having at least two wind power
installations, characterized in that a test
apparatus as claimed in claim 12 is mounted on one
of the wind power installations and is designed to
carry out a method as claimed in one of claims ' 1
to 11 on a rotor blade of another wind power
installation.

The invention relates to a method for testing a rotor blade (13.1) of a wind
power plant (1), which sweeps across a rotor blade surface to be covered
during operation of the wind power plant, comprising the following steps:
emitting a target light beam (20), in particular a target light beam having a
power density, in a direction of the target light beam onto the rotor blade
surface to be covered; detecting a possible reflection of the target light
beam by a detection device at a point of incidence (16) on the rotor blade;
electrically controlled emission of a measurement laser beam (21) which
has a power density that is larger than the power density of the target
light beam immediately after detection of the reflection at the point of
incidence so that the rotor blade is heated at the point of incidence;
measuring a temperature distribution at the point of incidence; and
repeating the steps (a) to (d) for several points of incidence.