U.S. patent application number 12/769745 was filed with the patent office on 2011-06-09 for method for temperature calibration of blade strain gauges and wind turbine rotor blade containing strain gauges.
Invention is credited to Rene Aschermann, Thorsten Honekamp, Matthias Thulke.
Application Number | 20110135474 12/769745 |
Document ID | / |
Family ID | 44082210 |
Filed Date | 2011-06-09 |
United States Patent
Application |
20110135474 |
Kind Code |
A1 |
Thulke; Matthias ; et
al. |
June 9, 2011 |
METHOD FOR TEMPERATURE CALIBRATION OF BLADE STRAIN GAUGES AND WIND
TURBINE ROTOR BLADE CONTAINING STRAIN GAUGES
Abstract
A method for temperature calibration of a strain sensor for a
rotor blade of a wind turbine is provided, the method including
operating the wind turbine in a mode in which substantially no
bending of the rotor blade due to wind occurs, repeatedly measuring
gravitationally induced bending moments of the rotor blade for a
plurality of temperatures measured at the place of the strain
sensor, determining a temperature dependency of the strain sensor
on the temperature, calibrating the strain sensor based on the
determined temperature dependency of the strain sensor such that
the temperature dependency of the strain sensor is compensated.
Inventors: |
Thulke; Matthias; (Berlin,
DE) ; Honekamp; Thorsten; (Emlichheim, DE) ;
Aschermann; Rene; (Nordhorn, DE) |
Family ID: |
44082210 |
Appl. No.: |
12/769745 |
Filed: |
April 29, 2010 |
Current U.S.
Class: |
416/61 |
Current CPC
Class: |
F05B 2270/328 20130101;
G01L 1/2281 20130101; F03D 17/00 20160501; G01M 5/0016 20130101;
G01M 5/0083 20130101; Y02E 10/721 20130101; Y02E 10/72 20130101;
F05B 2270/303 20130101; F05B 2260/83 20130101; F05B 2270/802
20130101; F05B 2270/808 20130101; G01L 1/2218 20130101 |
Class at
Publication: |
416/61 |
International
Class: |
F01D 25/00 20060101
F01D025/00 |
Claims
1. A rotor blade comprising: a first strain sensor arranged at a
surface of the rotor blade; and, a first temperature sensor
arranged adjacent to the first strain sensor.
2. The rotor blade according to claim 1, further comprising a
second strain sensor arranged at the surface of the rotor blade
circumferentially opposite to the first strain sensor; and, a
second temperature sensor arranged adjacent to the second strain
sensor.
3. The rotor blade according to claim 2, wherein each strain sensor
comprises two strain gauges, each strain gauge being adapted to
measure a bending moment along a direction from blade root to blade
tip; and, the four strain gauges of the first and second strain
sensors are electrically connected to each other to form a full
Wheatstone bridge circuit.
4. The rotor blade according to claim 2, further comprising third
and fourth strain sensors arranged at the surface of the rotor
blade, the third strain sensor being arranged circumferentially
opposite to the fourth strain sensor and circumferentially offset
by about 90.degree. to the first strain sensor; third and fourth
temperature sensors, the third temperature sensor arranged adjacent
to the third strain sensor and the fourth temperature sensor
arranged adjacent to the fourth strain sensor.
5. The rotor blade according to claim 2, wherein the first strain
sensor is arranged at a leading edge of the rotor blade; and, the
second strain sensor is arranged at a trailing edge of the rotor
blade.
6. The rotor blade according to claim 2, wherein the first strain
sensor is arranged at a suction side of the rotor blade; and, the
second strain sensor is arranged at a pressure side of the rotor
blade.
7. A method for temperature calibration of a strain sensor arranged
at a rotor blade of a wind turbine, the method comprising:
operating the wind turbine in a mode in which substantially no
bending of the rotor blade due to wind occurs; repeatedly measuring
gravitationally induced bending moments of the rotor blade for a
plurality of temperatures measured at the location of the strain
sensor; determining a temperature dependency of the strain sensor
from said measured data; calibrating the strain sensor based on the
determined temperature dependency of the strain sensor such that
the temperature dependency of the strain sensor is compensated.
8. The method according to claim 7, wherein measuring
gravitationally induced bending moments of the rotor blade
comprises: measuring bending moments of the rotor blade for a
plurality of azimuth positions of the rotor blade and a plurality
of pitch angles of the rotor blade.
9. The method according to claim 7, further comprising: calculating
a gravitationally induced bending moment of the rotor blade based
on the physical properties of the rotor blade, and a rotor azimuth
position; comparing the calculated bending moment of the rotor
blade with the measured bending moment for said rotor azimuth
position; calibrating the strain sensor by setting a correction
value thus that the measured bending moment equals the calculated
bending moment.
10. The method according to claim 7, further comprising:
determining a minimum value and a maximum value of the bending
moment of the rotor blade; determining a ratio of span and a
difference of offset values; determining if the number of data
points is sufficient; determining a calibrated bending moment which
is equal to a product of the ratio of span value and a difference
of the non-calibrated bending moment and the difference of offset
value.
11. The method according to claim 10, wherein the minimum value and
the maximum value of the bending moment of the rotor blade, and the
bending moment of the rotor blade are determined for a flapwise
direction and an edgewise direction.
12. The method according to claim 7, further comprising determining
a functional dependency of the bending moments measured by the
strain sensor on the temperature by a regression analysis.
13. A method for temperature calibration of a strain sensor
arranged at a rotor blade of a wind turbine, the method comprising:
controlling a temperature of a part of the rotor blade in which
said strain sensor is located; measuring a strain using the strain
sensor; measuring the temperature at the location of the strain
sensor; varying the controlled temperature and repeating the strain
and temperature measurements at a different temperature;
determining a temperature dependency of the strain sensor from said
measured data; and, calibrating the strain sensor based on the
determined temperature dependency of the strain sensor such that
the temperature dependency of the strain sensor is compensated.
14. The method according to claim 13, wherein the temperature is
controlled by means of a heating mat arranged on a part of a
surface of said rotor blade.
15. The method according to claim 14, wherein the heated part of
the surface of the rotor blade is larger than the joining area of
the rotor blade and the strain sensor.
16. The method according to claim 13, wherein the temperature is
controlled by means of a heating fan arranged inside the rotor
blade.
17. The method according to claim 13, wherein the temperature is
controlled by means of a vapor compression refrigeration system
adapted to cool a part of a surface of the rotor blade.
18. The method according to claim 13, further comprising: thermally
insulating the temperature-controlled part of the rotor blade from
a further part of the rotor blade in which the temperature is not
controlled.
19. The method according to claim 13, wherein the temperature is
controlled between about -20.degree. C. to about +50.degree. C.
20. The method according to claim 13, further comprising:
increasing the temperature of a part of the rotor blade from an
initial temperature to a final temperature in a step wise manner;
measuring, in every step, a bending moment of the rotor blade using
the strain sensor; measuring, in every step, the temperature at the
position of the strain sensor; determining a functional dependency
between the bending moment and the measured temperature.
Description
BACKGROUND OF THE INVENTION
[0001] The present disclosure relates to wind turbines.
Particularly, the present disclosure relates to a wind turbine
rotor blade which is adapted to perform a temperature calibration
of a strain sensor and methods for temperature calibration of a
strain sensor arranged at a rotor blade of a wind turbine.
[0002] Although horizontal axis wind turbines are well-established
these days, there is still considerable engineering effort going on
to further improve their overall efficiency, power generating
capability, and robustness.
[0003] Modern wind turbines are designed to produce a maximum
amount of energy in a particular geographical area. Therefore, wind
turbines are operated such that the operational wind speed range is
increased. This increases the loads on almost all parts of a wind
turbine, especially the rotor blades which are typically produced
from light weight materials, like glass or carbon fibers. Excessive
loads will result in fatigue failures of the rotor blades. As the
power generation should be maximized, wind turbine rotor blades are
operated closer and closer to their fatigue limit.
[0004] As fatigue failure of rotor blades should be avoided, there
is a need to know exactly when those fatigue failures will occur.
Typically, fatigue failures will occur at a well-defined stress
within the material of the rotor blades. As the material constants
for the material used to build the rotor blade are known, one can
predetermine the force at which the material will break.
BRIEF DESCRIPTION OF THE INVENTION
[0005] In view of the above, a rotor blade including a first strain
sensor arranged at a surface of the rotor blade, and a first
temperature sensor arranged adjacent to the first strain sensor is
provided.
[0006] According to another aspect of the disclosure, a method for
temperature calibration of a strain sensor arranged at a rotor
blade of a wind turbine is provided. The method includes operating
the wind turbine in a mode in which substantially no bending of the
rotor blade due to wind occurs, repeatedly measuring
gravitationally induced bending moments of the rotor blade for a
plurality of temperatures measured at the location of the strain
sensor, determining a temperature dependency of the strain sensor
from said measured data, calibrating the strain sensor based on the
determined temperature dependency of the strain sensor such that
the temperature dependency of the strain sensor is compensated.
[0007] According to yet another aspect of the disclosure, a further
method for temperature calibration of a strain sensor arranged at a
rotor blade of a wind turbine is provided. The method includes
controlling a temperature of a part of the rotor blade in which
said strain sensor is located, measuring a strain using the strain
sensor, measuring temperature at the location of the strain sensor,
varying the controlled temperature and repeating the strain and
temperature measurements at a different temperature; determining a
temperature dependency of the strain sensor from said measured
data, and calibrating the strain sensor based on the determined
temperature dependency of the strain sensor such that the
temperature dependency of the strain sensor is compensated.
[0008] Further aspects, advantages and features of the present
invention are apparent from the dependent claims, the description
and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] A full and enabling disclosure including the best mode
thereof, to one of ordinary skill in the art, is set forth more
particularly in the remainder of the specification, including
reference to the accompanying figures wherein:
[0010] FIG. 1 is a schematic side view of a wind turbine having
rotor blades according to embodiments described herein.
[0011] FIG. 2 is a schematic drawing of a Wheatstone bridge circuit
used in embodiments described herein.
[0012] FIG. 3 is a schematic drawing illustrating four strain
gauges which are electrically connected to each other for forming a
Wheatstone bridge circuit used in embodiments described herein.
[0013] FIGS. 4, 5, 6, and 7 are schematic longitudinal cross
sectional views of a wind turbine rotor blade according to
embodiments described herein.
[0014] FIG. 8 illustrates a method for temperature calibration of a
strain sensor arranged at a rotor blade of a wind turbine according
to embodiments described herein.
[0015] FIG. 9 illustrates the relationship between measured blade
moment curves and the parameters ratio of span and difference of
offset which are used in embodiments described herein.
[0016] FIG. 10 illustrates a further method for temperature
calibration of a strain sensor arranged at a rotor blade of a wind
turbine according to embodiments described herein.
[0017] FIG. 11 is a schematic longitudinal cross sectional view of
a rotor blade root of a wind turbine according to embodiments
described herein.
[0018] FIG. 12 is a schematic longitudinal cross sectional view of
a rotor blade root of a wind turbine according to embodiments
described herein.
[0019] FIG. 13 is a schematic perspective view of a rotor blade
root of a wind turbine according to embodiments described
herein.
[0020] FIG. 14 is a schematic longitudinal cross sectional view of
a rotor blade root of a wind turbine according to embodiments
described herein.
[0021] FIG. 15 is a schematic perspective view of a rotor blade
root of a wind turbine according to embodiments described
herein.
[0022] FIG. 16 illustrates yet a further method for temperature
calibration of a strain sensor arranged at a rotor blade of a wind
turbine according to embodiments described herein.
DETAILED DESCRIPTION OF THE INVENTION
[0023] Reference will now be made in detail to the various
embodiments, one or more examples of which are illustrated in each
figure. Each example is provided by way of explanation and is not
meant as a limitation. For example, features illustrated or
described as part of one embodiment can be used on or in
conjunction with other embodiments to yield yet further
embodiments. It is intended that the present disclosure includes
such modifications and variations.
[0024] FIG. 1 is a schematic side view of a wind turbine 100. The
wind turbine 100 includes three rotor blades 110 but may have more
or less blades 110 according to other embodiments. The rotor blades
110 are mounted on a rotor huh 120 which is connected to a nacelle
130. Nacelle 130 is fixed on top of a tower 140. Rotor blades 110
include strain sensors which are adapted to measure bending moments
of rotor blades 110.
[0025] The rotor blades of a wind turbine are subject to
considerable forces and moments due to wind load, gravitation, and
centrifugal force. Such blade moments are typically proportional to
a stretching or bending of the rotor blades which can be measured
by strain sensors. Typical strain sensors are strain gauges which
may be realized as flat electrical resistors. The resistance of
those flat resistors changes linearly to length variations of the
sensors. Typically, those strain gauges are bonded or glued onto a
surface of the rotor blades being tested. Strain gauges may measure
tiny relative length variations, as small as 10.sup.-6.
[0026] A fundamental parameter of a strain gauge is its sensitivity
to strain, expressed quantitatively as the gauge factor (GF). The
gauge factor is defined as the ratio of relative change in
electrical resistance (.DELTA.R/R) to the strain which is the
relative change in length (.DELTA.L/L).
GF = .DELTA. R / R .DELTA. L / L ##EQU00001##
For metallic foil gauges, the gauge factor is typically about
2.
[0027] Typically, strain measurements seldom involve strains larger
than 10.sup.-3. Therefore, typically electrical resistance changes
smaller than 1.OMEGA. are to be measured. For measuring such small
electrical resistances changes, strain gauges are typically used in
a Wheatstone bridge configuration including a voltage excitation
source.
[0028] Typically, the resistance strain gauges are bonded to a
surface of the rotor blade in order to detect the strain in the
surface. However, strain gauges exhibit temperature dependency and,
therefore, should be operated such that inaccuracies due to change
in temperature are compensated or at least reduced. This is due not
only to the fact that the resistance of most of the conductive
materials used in bonded resistance strain gauge filaments changes
with temperature, but also because the coefficient of thermal
expansion (CTE) of the strain gauge filament is often different
from that of the structure to which it is bonded and sensitivity to
or measurement of strains resulting from thermal expansion is not
desired and will lead to measurement inaccuracies. Thus, even if
the filament of the strain gauge were not directly
temperature-sensitive because of its change in resistance with
change in temperature, it would still be subject to false strain
indications with temperature unless it has a coefficient of thermal
expansion matched with the CTE of the surface to which it is
bonded. Such matching is difficult and expensive because a strain
gauge matched to steel is greatly in error if bonded to aluminum or
some other material, and vice versa.
[0029] Temperature variations cause various effects in strain
gauges. The measurement object changes in size by thermal
expansion, which is detected as strain. The electrical resistance
of the strain gauge and the electrical resistance of the connecting
wires will change. The resistance changes of the strain gauges are
on the same magnitude as the length changes. Typically, they are
both on the order of 10.sup.-6. To measure such small resistance
changes, strain gauges are typically used in a Wheatstone bridge
arrangement with a voltage excitation source.
[0030] Since temperature variations also result in resistance
variations in a gauge filament, some means should be employed to
cancel the effects of these temperature changes from the strain
measurements. The resistance variations which accompany temperature
variations are caused not only by the thermal coefficient of
resistivity of the wire filament, but also by the differences in
the linear CTEs of the wire and test structure. Thus, the
temperature sensitivity of a particular gauge will vary according
to the material to which it is affixed, and any compensating system
must be usable with a variety of test materials. Such temperature
sensitivity is often experienced as "apparent strain" rather than
as a variation in resistance, since the ultimate error calculation
must relate to the actual strain measurements. The most widely used
temperature compensation device for strain gauges, the above
mentioned Wheatstone bridge, makes use of the electrical system to
which they are electrically connected and also of the mechanical
system to which they are fixed.
[0031] Undesired temperature effects may be compensated to a good
degree using a Wheatstone bridge circuit. However, differences due
to different CTEs of the strain sensor and the underlying material
may not be compensated for completely. This can be done by an
exigent temperature calibration in which strain sensors measure
strain at various locations of the rotor blade while simultaneously
measuring the temperature at those strain sensors.
[0032] FIG. 2 shows a schematic drawing of a Wheatstone bridge
circuit 190 as it will be employed in embodiments disclosed herein.
Wheatstone bridge circuit 190 includes four resistors 181, 182,
183, and 184 which are connected to each other to form a closed
loop of the four resistors 181, 182, 183, and 184 arranged as
depicted in FIG. 2. At the junctions of the resistors 181, 182,
183, and 184 are provided four Wheatstone bridge contacts 191, 192,
193, and 194. Typically, an excitation voltage V.sub.B is applied
between Wheatstone bridge contacts 191 and 194 and a bridge voltage
V is measured between contacts 192 and 193. The bridge voltage V
equals:
V = V B [ R 181 R 181 + R 182 - R 184 R 183 + R 184 ] .
##EQU00002##
[0033] Here, R.sub.181, R.sub.182, R.sub.183, and R.sub.184 are the
resistances of the four resistors 181, 182, 183, and 184. In case
the ratio R.sub.182/R.sub.181 equals the ratio R.sub.183/R.sub.184,
the voltage V is equal to zero in which case the Wheatstone bridge
is said to be balanced. Any change in resistance of any resistor of
the bridge results in a nonzero bridge voltage which typically is
the case if the resistance variations of the four resistors 181,
182, 183, and 184 are not equal.
[0034] In case of wind turbines, the four resistors 181, 182, 183,
and 184 are typically realized by strain sensors or strain-gauges.
There are various possibilities to use a Wheatstone bridge with
strain gauges. It is possible to use only one strain gauge as a
Wheatstone bridge resistor, in which case the Wheatstone bridge
circuit is called a quarter-bridge circuit. If two strain gauges
are used as Wheatstone bridge resistors, the Wheatstone bridge
circuit is called a half-bridge circuit. If all Wheatstone bridge
resistors are strain gauges, the Wheatstone bridge circuit is
called a full-bridge circuit. The measured signal of a full
Wheatstone bridge is linearly proportional to the strain to be
measured.
[0035] Typically, for measuring strain of a rotor blade, two strain
sensors measure strain of opposite sign at the rotor blade root.
If, e.g., a rotor blade root bends towards a definite direction,
strain of opposite sign occurs at circumferentially opposite
locations of the rotor blade root. In case of edgewise bending, the
strain sensors are arranged parallel to the edge-wise direction on
opposing inner walls of the rotor blade root. Each strain sensor
includes two strain gauges which are both aligned along a direction
from rotor blade root to rotor blade tip. The strain gauges of each
strain sensor are arranged on opposing sides of the Wheatstone
bridge circuit 190. In that case, the measured voltage V is linear
proportional to the edgewise strain of rotor blade 110. In that
case, the compensation of the CTE is realized between the strain
gauges of the same strain sensor, so mostly at the same location.
However, other strain gauge arrangements are also possible where
strain gauges at different locations of the rotor blade are CTE
compensated.
[0036] To additionally measure flapwise strain, a further
Wheatstone bridge circuit may be used whose strain sensors are
arranged parallel to the flapwise direction.
[0037] FIG. 3 is a schematic drawing illustrating four strain
gauges which are electrically connected to each other to form a
Wheatstone bridge circuit 190. The Wheatstone bridge resistors 181,
182, 183, and 184 are realized by four strain gauges. The resistors
181 and 183 form a left strain sensor 160 and the resistors 182 and
184 form a right strain sensor 160. The four strain gauges are
aligned along a direction from rotor blade root to rotor blade tip,
the direction being indicated by an arrow in FIG. 3. The left
strain sensor 160 is arranged inside the rotor blade root
circumferentially opposite to the right strain sensor 160. Thus,
the left strain sensor 160 measures opposite equal strain compared
to the right strain sensor 160. As the resistors 181 and 183 on the
left are arranged at the same location of the rotor blade root,
they are typically temperature compensated to a good degree. As the
resistors 182 and 184 on the right are also arranged at the same
location of the rotor blade root, typically they are also
temperature compensated to a good degree.
[0038] The four Wheatstone bridge contacts 191, 192, 193, and 194
connect the four Wheatstone bridge resistors 181, 182, 183, and
184. This results in a Wheatstone bridge circuit 190 as in the
embodiment of FIG. 2 which typically measures edgewise or flapwise
strain of a rotor blade.
[0039] In order to measure edgewise and flapwise bending
simultaneously, two Wheatstone bridge circuits 190 are provided
which are adapted to measure rotor blade bending in substantially
orthogonal directions. This is typically achieved by arranging the
Wheatstone bridge circuits 190 orthogonal to each other.
[0040] According to other embodiments, the Wheatstone bridge
resistors 181, 182, 183, and 184 may be connected to the Wheatstone
bridge contacts 191, 192, 193, and 194 in a different way. The
Wheatstone bridge resistors 181, 182, 183, and 184 may also be
aligned along different directions, e.g. Wheatstone bridge
resistors 181 and 182 may be aligned along a horizontal direction
while Wheatstone bridge resistors 183 and 184 may be aligned along
a vertical direction. According to further embodiments, it is
possible to measure torsion moments of the rotor blade.
[0041] FIG. 4 shows a schematic longitudinal cross sectional view
of a rotor blade 110 of a wind turbine 100 seen from a rotor blade
root 150 to a rotor blade tip. The rotor blade root 150 including
strain sensors 160 and temperature sensors 170 is closer to the
observer of FIG. 4 than an airfoil portion 115 of rotor blade 110.
Wind impinges rotor blade 110 at a leading edge 113 and leaves
rotor blade 110 at a trailing edge 114. One part of the wind
travels around rotor blade 110 on a pressure side 112, another part
of the wind travels around rotor blade 110 on a suction side 111 of
rotor blade 110. Rotor blade 110 typically has a round cross
sectional shape at a flange at rotor blade root 150. This cross
sectional shape changes its outline from a circle to a typical
airfoil shape as one travels from rotor blade root 150 to the
airfoil portion 115 of rotor blade 110.
[0042] Rotor blade 110 includes one strain sensor 160 arranged at
an inner surface of the rotor blade root 150 near pressure side 112
of rotor blade 110. Strain sensor 160 is adapted to measure a
bending moment of rotor blade 110 along a direction from blade root
150 to blade tip. A chord line of rotor blade 110 extends from
leading edge 113 to trailing edge 114. Oscillations of rotor blade
110 in the direction of the chord line are sometimes called
edge-wise oscillations. Oscillations perpendicular to the direction
of chord line between suction side 111 and pressure side 112 are
sometimes called flap-wise oscillations. Accordingly, directions
along those oscillations are called edge-wise and flap-wise
directions.
[0043] In case of flap-wise oscillations of rotor blade 110,
bending of rotor blade root 150 along a direction from rotor blade
root to blade tip is very large at portions near pressure side 112
and near suction side 111. If strain sensors 160 are located at
those portions, flapwise bending of rotor blade root 150 can be
detected with high sensitivity.
[0044] As temperature-related effects are the most common causes of
error in strain measurements, the strain sensor should be
temperature calibrated such that the temperature dependency of the
measured data is eliminated.
[0045] A temperature sensor 170 is arranged adjacent strain sensor
160. Temperature sensor 170 measures the temperature at the
location of strain sensor 160. By doing this, a functional
dependency of the measured strain data on the local temperature may
be obtained. Strain may also be measured for various rotor angle
positions, various pitch angle settings and for different
temperatures. For that measured strain data, a regression analysis
may be performed to compensate strain sensor 160 with regard to
temperature effects.
[0046] FIG. 5 shows a schematic longitudinal cross sectional view
of a rotor blade 110 of a wind turbine 100 seen from a rotor blade
root 150 to a rotor blade tip. The rotor blade root 150 including
strain sensors 160 and temperature sensors 170 is closer to the
observer of FIG. 5 than an airfoil portion 115 of rotor blade
110.
[0047] Rotor blade 110 includes two strain sensors 160 arranged at
an inner surface of the rotor blade root 150. The strain sensors
160 are arranged circumferentially opposite to each other. One
strain sensor 160 is arranged near suction side 111 of rotor blade
110; the other strain sensor 160 is arranged near pressure side 112
of rotor blade 110. Strain sensors 160 are adapted to measure a
bending moment of rotor blade 110 along a direction from blade root
150 to blade tip.
[0048] In case of flap-wise oscillations of rotor blade 110,
bending of rotor blade root 150 is large at portions near pressure
side 112 and near suction side 111. If strain sensors 160 are
located at those portions, flapwise bending of rotor blade root 150
can be detected with high sensitivity.
[0049] For each strain sensor 160, there is provided a temperature
sensor 170 arranged adjacent to said strain sensor 160. The
temperature at the location of a strain sensor 160 is measured by
the temperature sensor 170 arranged adjacent to that strain sensor
160. By doing this, a functional dependency of the measured strain
data on the local temperature is obtained. Strain may also be
measured for various rotor angle positions, various pitch angle
settings and for different temperatures. For that measured strain
data, a regression analysis may be performed to compensate strain
sensor 160 with regard to temperature effects.
[0050] According to some embodiments, each strain sensor 160
includes two strain gauges. The in total four strain gauges of the
two strain sensors 160 are electrically connected to each other for
forming a full Wheatstone bridge circuit 190 with the strain gauges
being the resistors of Wheatstone bridge circuit 190.
[0051] Using such a Wheatstone bridge circuit 190 in connection
with rotor blade 110 of the embodiment of FIG. 5, flapwise bending
of rotor blade 110 may be measured with high accuracy.
[0052] FIG. 6 shows a schematic longitudinal cross sectional view
of a rotor blade 110 of a wind turbine 100 seen from a rotor blade
root 150 to a rotor blade tip. The rotor blade root 150 including
strain sensors 160 and temperature sensors 170 is closer to the
observer of FIG. 6 than an airfoil portion 115 of rotor blade
110.
[0053] Rotor blade 110 includes two strain sensors 160 arranged at
an inner surface of the rotor blade root 150. Strain sensors 160
are arranged circumferentially opposite to each other. One strain
sensor 160 is arranged facing leading edge 113 of the rotor blade
110; the other strain sensor 160 is arranged facing trailing edge
114 of rotor blade 110. Strain sensors 160 are adapted to measure a
bending moment of rotor blade 110 along a direction from blade root
150 to blade tip. As strain sensors 160 of the embodiment of FIG. 6
are arranged along the edge-wise direction, strain sensors 160 are
very sensitive to measure strain along the edge-wise direction.
[0054] According to the embodiment of FIG. 6, for each strain
sensor 160 there is a temperature sensor 170 arranged adjacent to
said strain sensor 160. The temperature at the location of strain
sensor 160 can be measured by the temperature sensor 170. By doing
this, a dependency of the measured strain data on the local
temperature at the position of strain sensor 160 is obtained.
[0055] FIG. 7 shows a schematic longitudinal cross sectional view
of rotor blade 110 of wind turbine 100 seen from rotor blade root
150 to the rotor blade tip. The rotor blade root 150 including
strain sensors 160 and temperature sensors 170 is closer to the
observer of FIG. 7 than an airfoil portion 115 of rotor blade
110.
[0056] Rotor blade 110 includes four strain sensors 160 which are
arranged at an inner surface of the rotor blade root 150 in the
same plane. The two strain sensors 160 at the top and bottom of
FIG. 7 form the first strain sensor pair, while the two strain
sensors 160 at the left and right of FIG. 7 form the second strain
sensor pair. The connection lines of the pairs of strain sensors
160 are orthogonal to each other. One strain sensor 160 is arranged
near suction side Ill of rotor blade 110, one strain sensor 160 is
arranged near pressure side 112 of rotor blade 110, one strain
sensor 160 is arranged facing leading edge 113 of the rotor blade
110 and one strain sensor 160 is arranged facing trailing edge 114
of rotor blade 110. Strain sensors 160 are adapted to measure a
bending moment of rotor blade 110 along a direction from blade root
150 to blade tip.
[0057] The first strain sensor pair at the top and bottom of FIG. 7
is aligned along the flap-wise direction. Therefore, the first
strain sensor pair is sensitive to measure strain along the
flap-wise direction. The second strain sensor pair at the left and
right of FIG. 7 is aligned along the edge-wise direction.
Therefore, the second strain sensor pair is sensitive to measure
strain along the edge-wise direction.
[0058] According to some embodiments, each strain sensor 160
includes two strain gauges. The in total eight strain gauges of the
four strain sensors 160 form two full Wheatstone bridge circuits
190 with the strain gauges being the resistors of the Wheatstone
bridge circuits 190. The four strain gauges of the first strain
sensor pair are electrically connected to each other for forming a
first full Wheatstone bridge circuit 190. The four strain gauges of
the second strain sensor pair are electrically connected to each
other for forming a second full Wheatstone bridge circuit 190.
[0059] According to the embodiment of FIG. 7, for each strain
sensor 160 there is a temperature sensor 170 arranged adjacent to
said strain sensor 160. What has been said about strain sensors 160
and temperature sensors 170 in connection with the embodiments of
FIGS. 5 and 6 also applies to the embodiment of FIG. 7.
[0060] In the following, two methods will be described which use
so-called slow rolls of the wind rotor to calibrate strain sensors
of rotor blades. Typically, slow rolls are characterized by the
fact that the wind rotor is spinning slowly and almost no wind load
is applied to the rotor blades 110. Therefore, substantially no
bending of rotor blades due to wind occurs during slow rolls.
During slow rolls, the rotational speed of the electric generator
is between 90 and 200 revolutions per minute (RPM). This
corresponds to about 1 to 2 RPM of the rotor hub and a wind speed
of about 2 to 3 meters per second (m/s). By contrast, the wind
turbine is typically operated with 12 to 15 RPM when producing
electric energy.
[0061] In a first method which is depicted in FIG. 8, a blade
moment is determined using the factory settings of the used strain
sensors. That blade moment is called measured blade moment
M.sub.measured. For the determination of M.sub.measured a fit
function is used. M.sub.measured is then compared to a calculated
blade moment M.sub.calc. M.sub.calc can be determined to good
accuracy, while the value of M.sub.measured is typically less
accurate as a fit function and factory settings are used hereby.
The fitting parameters of the fitting function are adjusted until
M.sub.measured and M.sub.calc match within a predetermined error
margin. In a second method which is depicted in FIG. 10, strain
data are measured for various blade positions and various
temperatures. Subsequently, a regression analysis of the measured
strain data with respect to the temperature is performed, thus
calibrating the temperature dependency of the used strain
sensors.
[0062] A gravitational or gravitationally induced blade moment is
the bending moment acting on the rotor blade originating from the
gravitational force. In case the rotor blade is pointing to the
ground, no bending moment are acting within that rotor blade. Thus,
the blade moment depends on the angle of rotor blade 110 which is
also called rotor position, rotor blade 110 having a rotor position
of 0.degree. when pointing upwards.
[0063] The calculated gravitational blade moment M.sub.calc is
calculated using the formula
M.sub.calc=m.sub.bladegLsin .alpha.,
wherein m.sub.blade is the mass of rotor blade 110, g is the
standard gravity, which is the nominal acceleration due to gravity
at the Earth's surface at sea level, .alpha. is the rotor position
angle, and L is the length from the rotor axis to the center of
gravitation (COG) of the rotor blade.
[0064] Additionally or alternatively, the calculated gravitational
blade moment M.sub.calc may be determined by simulations which
typically may be multibody simulations. Those simulations take into
account the real orientation of the rotor blade with respect to the
rotor axis, the stiffness of the used material of the rotor blade
and so forth.
[0065] The gravitational blade moment can be measured during slow
rolls with the strain sensors provided in the rotor blades.
Typically, the strain sensors are used in a Wheatstone bridge
circuit as depicted in FIG. 2. In principle, the electrical signal
of the strain sensors used in a full Wheatstone bridge circuit is
linearly proportional to the strain. However, due to different CTEs
of the used strain gauges of the strain sensors and the underlying
material, there is a temperature dependent offset and also a
temperature dependent gain. The dependency of a measured blade
moment M.sub.measured on strain may be expressed to a good
approximation as follows:
M.sub.measured=offset(T)+gain(T)strain,
wherein offset(T) and gain(T) are temperature dependent functions.
The blade moment M.sub.measured is a function of the rotor position
.alpha., and pitch angle. Ideally, M.sub.measured should be
independent of the temperature. But in reality M.sub.measured
typically exhibits a temperature dependency which may be
compensated for by the above mentioned methods of FIGS. 8 and 10.
M.sub.measured may also assume negative values. To determine the
offset value, M.sub.measured is measured at 90.degree. and
270.degree. rotor position, i.e. when the rotor blade is in the 3
o'clock and 9 o'clock positions. These positions can be determined
accurately since the measured strain will exhibit its maximum at
these positions while zero-crossing at the 12 o'clock and 6 o'clock
positions. The offset is determined as the sum of the
M.sub.measured values measured at 90.degree. and 270.degree.
divided by 2.
[0066] FIG. 8 illustrates a method for temperature calibration of a
strain sensor 160 arranged at rotor blade 110 of wind turbine 100.
The method starts by completing the installation of wind turbine
100 in step 300. In the next step 310, slow rolls are performed for
pitch angles of 0.degree. and 90.degree. while blade moments are
measured simultaneously using strain sensors 160.
[0067] In the next step 320, the measured blade moment
M.sub.measured is determined using the measured voltage values of
the strain sensors and the above mentioned formula. The first time
step 320 is run, the factory settings of the strain sensors are
used.
[0068] In the following step 330, it is decided whether the
measured blade moment M.sub.measured equals the calculated blade
moment M.sub.calc. If those values do not match within a
predetermined error margin, the offset value and the gain value of
the above mentioned fit function are modified in step 340 and the
method returns to step 320. The modification of the offset and gain
values in step 340 may be executed manually or by a machine based
algorithm. Now, in step 320, the measured blade moment
M.sub.measured is determined again using the modified offset and
gain values. In case the measured blade moment M.sub.measured and
the calculated blade moment M.sub.calc match within a predetermined
error margin, the method continues with step 350 in which the
determined gain and offset values are outputted.
[0069] FIG. 9 illustrates the relationship between measured blade
moment curves and the parameters ratio of span and difference of
offset which are used in embodiments described herein. In FIG. 9,
rotor blade moments are plotted against the rotor position angle
.alpha.. To get measurement curves as depicted in FIG. 9, the pitch
angles of two rotor blades are set to 65.degree. and the pitch
angle of the third rotor blade is set to 0.degree., 45.degree., and
90.degree., consecutively. The rotor position angle .alpha. in FIG.
9 refers to the third rotor blade. The blade moments of the third
rotor blade is then measured for three 360.degree. turns of the
rotor using strain sensors. The plotted curves in FIG. 9 are
determined by averaging over three turns of the measured rotor
blade moments.
[0070] The solid curve in FIG. 9 is a typical measured curve using
a strain sensor at the temperature T.sub.0 which was used for the
initial temperature calibration of the strain sensor at the
factory. The dotted curve shows the same measurement but for a
different temperature T. Both curves are sinusoidal as the rotor
blade moment is a sine function of the rotor blade angle .alpha..
One notices that the peak-to-peak amplitude of the two curves is
different, being 2M.sub.T for the dotted curve measured at
temperature T and being 2M.sub.0 for the solid curve measured at
temperature T.sub.0. Apart from the different peak-to-peak
amplitudes, the sine of the solid curve is centered at zero whereas
the sine of the dotted curve is centered at a different value. The
difference of those center values is called the difference of
offset value .DELTA. which depends on the temperature difference of
the two measured curves. The ratio of span R is the ratio of the
peak-to-peak amplitudes of the curves measured at temperature
T.sub.0 and T:
R=2M.sub.0/2M.sub.T.
Typically, the value of the ratio of span is about 1.
[0071] The aim of the temperature calibration is to have the same
curve of the blade moment against rotor angle .alpha. for an
arbitrary temperature. To this end, one performs a transformation
on the values of the curves measured at temperatures T different
from the temperature T.sub.0 such that they are transformed into
the value of the curve measured at T.sub.0. Therefore, the
relationship between the calibrated blade moment M.sub.calibrated
and the measured uncalibrated blade moment M.sub.uncalibrated thus
reads as follows:
M.sub.calibrated=R(M.sub.uncalibrated-.DELTA.)
[0072] FIG. 10 illustrates a method for temperature calibration of
strain sensor 160 according to another embodiment. The method
starts by performing slow rolls for pitch angles of 0.degree.,
45.degree., and 90.degree. while simultaneously measuring blade
moments using strain sensors in step 400. In the next step 410, a
minimum value and a maximum value for edgewise and flapwise blade
moments are determined. According to another embodiment, the
minimum value and the maximum value is determined for the edgewise
or the flapwise blade moment. According to further embodiments,
blade moments along arbitrary directions are determined.
[0073] In step 420, a ratio of span R and a difference of offset
value .DELTA. according to the definitions given in connection with
the description of FIG. 9 are calculated. In step 430, it is
determined whether the number of data points is sufficiently large
to perform reliable statistical analysis. Typically, a collection
of about 30 data points measured at different temperatures may be
regarded as sufficiently large. Of course, smaller or larger
threshold values can be chosen depending on the required accuracy
of the regression analysis in the following step. In case the
number of data points is larger than 30, the method continues to
step 440. In step 440, a regression analysis of the measured data
is performed with regard to blade temperature. In case the number
of data points is not larger than 30, the method returns to step
400 where more data points at different temperatures are
measured.
[0074] According to some embodiments, no data is collected if the
measured temperature of temperature sensors located at different
locations differ more than 1 Kelvin (K). For temperature
differences larger than 1 K, the data may be considered not
reliable enough or to produce inconsistent results.
[0075] According to yet further embodiments, for cases with
different temperatures at the strain sensors, a temperature
calibration is also possible. For those cases, the regression takes
those different temperatures into account and performs a
calibration transformation which is similar to the transformation
which was described in connection with FIG. 9 but has a different
functional dependency as described in connection with FIG. 9.
[0076] FIG. 11 shows a schematic longitudinal cross sectional view
of a rotor blade root 150 of a wind turbine having two Wheatstone
bridge circuits which are adapted to measure both edgewise and
flapwise bending. The first Wheatstone bridge circuit includes a
first pair of strain sensors 160, and the second Wheatstone bridge
circuit includes a second pair of strain sensors 160. Both the
first pair of strain sensors 160 and the second pair of strain
sensors 160 are arranged circumferentially opposite to each other,
the second pair of strain sensors 160 being arranged
circumferentially offset by about 90.degree. to the first pair of
strain sensors 160. Sometimes, it is not convenient to arrange the
second pair of strain sensors 160 circumferentially offset by
exactly 90.degree. to the first pair of strain sensors 160 due to
space limitations. In these cases, the angle between the first pair
of strain sensors 160 and the second pair of strain sensors 160 is
typically between 70.degree. and 90.degree., preferably between
80.degree. and 90.degree., or more preferably between 85.degree.
and 90.degree..
[0077] According to one embodiment, strain sensors 160 to the left
and right of FIG. 11 are adapted to measure edgewise bending, while
strain sensors 160 to the top and bottom of FIG. 11 are adapted to
measure flapwise bending of rotor blade 110. Each strain sensor 160
includes two strain gauges so that the embodiment of FIG. 11
includes eight strain gauges in total. The strain sensors 160
depicted at the top and the bottom of FIG. 11 thus form a full
Wheatstone bridge circuit 190 as depicted in FIG. 2. The strain
sensors 160 depicted at the left and the right of FIG. 11 also form
a full Wheatstone bridge circuit 190 as depicted in FIG. 2. For
each strain sensor 160, there is provided a temperature sensor 170
adjacent to that strain sensor 160.
[0078] According to another embodiment, temperature calibration of
the strain sensors 160 includes controlling the temperature of a
part of the rotor blade, e.g. of the rotor blade root 150, to a
constant temperature and to measure the signal of the strain
sensors 160 simultaneously for various controlled temperatures. A
regression analysis may then be performed using the measured data,
and the result may be used to compensate the temperature dependency
of the strain sensors.
[0079] According to one embodiment, the temperature calibration of
the strain sensor is performed in the factory with the rotor blade
not being installed on a wind turbine. Typically, the rotor blade
is mounted force-free. In such a case, there is no strain inside
the rotor blade such that the formula mentioned above in connection
with the description of FIG. 8 simplifies to:
M.sub.measured=offset(T).
[0080] In such a case, the temperature dependency of the offset
value can be calibrated while the temperature dependent gain values
are not measured.
[0081] According to a further embodiment, predetermined forces are
exerted onto the rotor blade. This way, it is possible to generate
strain within the rotor blade. This strain gives rise to an
additional blade moment which is equal to the strain multiplied by
the temperature dependent gain factor. In that case, one may also
perform a temperature calibration of the gain value.
[0082] Typically, the temperature change of the offset value is
about 10 times larger than the respective temperature change of the
gain value. Therefore, the main distribution to the temperature
dependency of the blade moment arises from the offset value. As the
above mentioned force-free mounting using predetermined extern
forces is tedious, one typically omits this measurement and
performs only a temperature calibration of the offset value. In
that case, the largest contribution to the error is calibrated.
This means that a good calibration of the factory settings is
achieved.
[0083] FIG. 12 shows the rotor blade root of FIG. 11 wherein a
heater 200 is arranged inside rotor blade root 150. For a
temperature calibration, the heater 200 heats for some amount of
time, e.g. until all temperature sensors 170 measure the same
temperature. At that point of time, the strain sensors 160 measure
the strain at their respective location at rotor blade root 150.
That step is repeated for various temperatures. The collected data
can then be used to perform a regression analysis with which a
temperature calibration of the strain sensors may be executed. In
order to make sure that the temperature is constant at the
locations of strain sensors 160, the heater 200 may be placed in
the middle of rotor blade root 150.
[0084] FIG. 13 is a schematic perspective view of rotor blade root
150 of a wind turbine 100. FIG. 13 shows the embodiment of FIG. 12
in a perspective view. Strain sensors 160 are arranged in a plane
perpendicular to an axis from a center part of rotor blade root 150
to rotor blade tip. Temperature sensors 170 are arranged next to
the strain sensors 160 in the same plane as the strain sensors 160.
The heater 200 is also arranged in the same plane as strain sensors
160 and temperature sensors 170.
[0085] FIG. 14 shows a schematic longitudinal cross sectional view
of a rotor blade root 150 of a wind turbine 100 having two
Wheatstone bridge circuits 190 which are adapted to measure both
edgewise and flapwise bending. What has been said about rotor blade
root 150, strain sensors 160 and temperature sensors 170 in
connection with FIG. 12 also applies to FIG. 14. The embodiment of
FIG. 14 does not use the heater 200 shown in FIGS. 12 and 13.
Instead, a heating mat 210 is used which is arranged on part of a
surface of rotor blade 110. As seen in a cross sectional view of
FIG. 14 rotor blade root 150 is enshrouded by heating mat 210. What
has been said about the temperature calibration in connection with
FIG. 12 also applies to FIG. 14.
[0086] FIG. 15 is a schematic perspective view of a rotor blade
root of a wind turbine 100. FIG. 15 shows the embodiment of FIG. 14
in a perspective view. Regarding an axis from a center part of
rotor blade root 150 to rotor blade tip, heating mat 210 which is
wrapped around rotor blade root 150 is centered in the plane in
which strain sensors 160 and temperature sensors 170 are
arranged.
[0087] In further embodiments, heater 200 and heating mat 210 may
be exchanged with a cooling device. In particular, these cooling
devices may be adapted to cool down the rotor blade portion
including the strain sensors. Thus, the temperature dependency of
the strain sensors may also be verified for lower temperatures,
e.g. below freezing point. According to some embodiments, however,
it should be noted that the temperature dependency of the strain
sensors may be relatively low for temperatures below 15.degree.
C.
[0088] FIG. 16 illustrates yet a further method for temperature
calibration of a strain sensor 160 arranged at a rotor blade 110 of
a wind turbine 100. The method may be performed using a rotor blade
equipped with a heating device according to any of the embodiments
of the FIGS. 12 to 15.
[0089] The method starts in step 500 by controlling the temperature
of part of rotor blade 110 in which the strain sensor 160 is
located. According to some embodiments, this is done by a heating
mat on the outside of the rotor blade root. According to other
embodiments, this is done by a heater arranged inside the rotor
blade root.
[0090] In step 510, the strain at the location of strain sensor 160
and the temperature at the location of strain sensor 160 are
measured.
[0091] Although only one strain sensor 160 and only one temperature
sensor 170 are shown in FIG. 16 exemplarily, it will be understood
by the skilled reader that also a plurality of strain sensors 160
and a plurality of temperature sensors 170 may be used in some
embodiments. The temperature measurement of the temperature sensor
170 may be time shifted from the strain measurement. The
temperature sensors 170 may also be located at another location as
the strain sensors 160. According to other embodiments, the number
of temperature sensors 170 is not equal to the number of strain
sensors 160. In particular, the number of temperature sensors 170
may be smaller than the number of strain sensors 160.
[0092] In the next step 520, it is determined whether the number of
measured temperature data points is sufficient. This determination
may be done by comparison with a predetermined number of data
points to be measured. According to some embodiments, the data
points are equally distributed within a temperature interval
between two temperatures. The method then starts with either the
lowest or the largest temperature and then continues to measure
towards the other extreme temperature point. According to further
embodiments, this determination is done by statistical
calculations.
[0093] In case the number of measured temperature data points is
not sufficient, the method continues to step 530, while if the
number of measured temperature data points is sufficient, the
method continues to step 540.
[0094] In step 530, the controlled temperature is varied to a
different temperature. After step 530, the method returns to step
510 in which the strain at the location of strain sensor 160 and
the temperature at the location of strain sensor 160 are
measured.
[0095] If it was determined in step 520 that the number of measured
temperature data points is sufficient, the method continues to step
540. In step 540 a temperature dependency of the strain sensor from
the measured data is determined. According to some embodiments,
this determination is done by regression analysis methods.
According to other embodiments, a temperature dependency of the
measured signal of the strain sensor is determined. This
determination may also be determined by regression analysis.
[0096] In the last step of the method, step 550 the strain sensor
is calibrated based on the determined temperature dependency. After
step 550 the method ends.
[0097] According to some embodiments, no data is collected if the
measured temperature of temperature sensors located at different
locations differ more than 1 Kelvin (K).
[0098] Typically, this method of the embodiment of FIG. 16 is
performed with a single rotor blade which is not yet installed in a
wind turbine. Hereby, the temperature calibration of the strain
sensor is done before installation of the wind turbine. As the
rotor blades are not yet installed, this calibration may be done
more thoroughly than after installation of the wind turbine.
Therefore, one readily may use the wind turbine with a complete set
of calibrated rotor blades which is advantageous.
[0099] This written description uses examples, including the best
mode, to enable any person skilled in the art to make and use the
described subject-matter. While various specific embodiments have
been disclosed in the foregoing, those skilled in the art will
recognize that the spirit and scope of the claims allows for
equally effective modifications. Especially, mutually non-exclusive
features of the embodiments described above may be combined with
each other. The patentable scope is defined by the claims, and may
include such modifications and other examples that occur to those
skilled in the art. Such other examples are intended to be within
the scope of the claims if they have structural elements that do
not differ from the literal language of the claims, or if they
include equivalent structural elements with insubstantial
differences from the literal language of the claims.
* * * * *