U.S. patent application number 13/368561 was filed with the patent office on 2012-08-09 for balancing of wind turbine parts.
Invention is credited to Steffen Bunge.
Application Number | 20120200699 13/368561 |
Document ID | / |
Family ID | 45566905 |
Filed Date | 2012-08-09 |
United States Patent
Application |
20120200699 |
Kind Code |
A1 |
Bunge; Steffen |
August 9, 2012 |
Balancing of Wind Turbine Parts
Abstract
A wind of the type having a tower and a nacelle with a rotor
rotatably connected to the nacelle for rotating about a rotor axis
and having a plurality of equally spaced blades has the rotor
balanced by firstly taking a measurement of torsional vibration and
then by using photographic techniques to analyze dynamic imbalance
caused by differences in the angle of attack of the blades. The
torsional vibration is detected using two sensors at positions
mirrored exactly in distance to the left and right of the rotor
axis and detecting vibration in the axial direction. The angle of
attack is measured by analyzing images of the tip of the blade
where, during the analysis, distortion in angles at different
locations in the image are corrected, in dependence upon a prior
analysis of an image taken by the camera relative to a known
image.
Inventors: |
Bunge; Steffen; (Pinawa,
CA) |
Family ID: |
45566905 |
Appl. No.: |
13/368561 |
Filed: |
February 8, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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61440500 |
Feb 8, 2011 |
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Current U.S.
Class: |
348/142 ;
348/E7.085; 73/455 |
Current CPC
Class: |
F03D 13/20 20160501;
Y02E 10/728 20130101; F05B 2270/8041 20130101; F03D 17/00 20160501;
F03D 13/35 20160501; F03D 1/0675 20130101; F05B 2270/802 20130101;
F05B 2260/96 20130101; Y02E 10/72 20130101 |
Class at
Publication: |
348/142 ; 73/455;
348/E07.085 |
International
Class: |
H04N 7/18 20060101
H04N007/18; G01M 1/16 20060101 G01M001/16 |
Claims
1. A method of aerodynamic balancing a rotor of a wind turbine, the
wind turbine comprising a tower and a nacelle mounted to the top of
the tower, the rotor being rotatably connected to the nacelle for
rotating about a rotor axis and having a plurality of equally
spaced blades, the method comprising: positioning a camera below
the rotor; rotating the rotor so that each blade in turn is in a
reference position in which a tip of the blade is directed toward
the camera; capturing an image of the tip of each blade in turn in
the reference position; carrying out an analysis of the image to
determine a blade angle of each blade; in the event that a blade
angle of one of the blade is different from that of the other
blades by a blade angle difference greater than a predetermined
angle, adjusting said one of the blades to reduce the blade angle
difference; and during the analysis, correcting angles at different
locations in the image, in dependence upon a prior analysis of an
image taken by the camera relative to a known image.
2. The method according to claim 1 wherein different points of the
image of the tip are identified in the image and the angles between
the different points are corrected in dependence upon their
location in the image.
3. The method according to claim 1 wherein easily recognizable
contour lines or reference points of the image of the tip are used
which are selected so that they are identical on every blade.
4. The method according to claim 1 wherein the accuracy of the
angle of attack is in the range of .+-. 1/10 of a degree to .+-.
2/10 of a degree.
5. The method according to claim 1 wherein during the analysis,
correction is made for trapezoidal distortion in the image.
6. The method according to claim 5 wherein during the analysis,
correction is made for trapezoidal distortion in the image caused
by the angle between the optical axis during taking the photos
compared to the vertical axis of the tower.
7. The method according to claim 5 wherein during the analysis,
correction is made for trapezoidal distortion in the image caused
by a measurement line created on a sloping away contour line, that
is sloping away from the camera.
8. The method according to claim 1 wherein for the capturing of the
image of the tip of each blade in turn, one of the blades is
selected to be first imaged.
9. The method according to claim 8 wherein the blade is selected to
be first imaged by determining any one which has cone angle
deviation and by selecting as the first one which has less cone
angle deviation.
10. The method according to claim 9 wherein the blade is selected
to be first imaged by applying a rotor lock to locate each blade in
turn as close as possible to the vertically downward position and
by taking an image of each blade in turn from a remote position to
determine those that are closest in position to each other at the
vertically downward position and selecting as the first to be
imaged one of those which are closest.
11. The method according to claim 1 including measuring the
torsional vibration of the system before and after the
adjustment.
12. The method according to claim 1 wherein the torsional vibration
of the system is detected using two sensors at positions mirrored
exactly in distance to the left and right of the rotor axis and
detecting vibration in the axial direction.
13. The method according to claim 12 wherein the use of the two
axial sensors in the axial direction and at the mirrored or exactly
symmetrical distance from the axis eliminates all axial vibration
by subtracting both axial signals and doubling the torsional signal
at the same time.
14. The method according to claim 1 wherein in a wind farm of a
plurality of wind turbines, the torsional vibration of each is
measured and a number of the turbines is selected for analysis of
the angle of attack having a vibration beyond a predetermined set
value to avoid analyzing all of the blades of all of the
rotors.
15. A method of aerodynamic balancing a rotor of a wind turbine,
the wind turbine comprising a tower and a nacelle mounted to the
top of the tower, the rotor being rotatably connected to the
nacelle for rotating about a rotor axis and having a plurality of
equally spaced blades, the method wherein the torsional vibration
of the system is detected using two sensors at positions mirrored
exactly in distance to the left and right of the rotor axis and
detecting vibration in the axial direction.
16. The method according to claim 15 wherein the use of the two
axial sensors in the axial direction and at the mirrored or exactly
symmetrical distance from the axis eliminates all axial vibration
by subtracting both axial signals and doubling the torsional signal
at the same time.
17. A method of aerodynamic balancing a rotor of a wind turbine,
the wind turbine comprising a tower and a nacelle mounted to the
top of the tower, the rotor being rotatably connected to the
nacelle for rotating about a rotor axis and having a plurality of
equally spaced blades, the method comprising: measuring the
torsional vibration of the system; adjusting the angle of attack of
the blades by: positioning a camera below the rotor; rotating the
rotor so that each blade in turn is in a reference position in
which a tip of the blade is directed toward the camera; capturing
an image of the tip of each blade in turn in the reference
position; carrying out an analysis of the image to determine a
blade angle of each blade; in the event that a blade angle of one
of the blade is different from that of the other blades by a blade
angle difference greater than a predetermined angle, adjusting said
one of the blades to reduce the blade angle difference; and
re-measuring the torsional vibration of the system to ensure that
it is reduced by the adjustment.
18. The method according to claim 17 wherein the torsional
vibration of the system is detected using two sensors at positions
mirrored exactly in distance to the left and right of the rotor
axis and detecting vibration in the axial direction.
19. The method according to claim 18 wherein the use of the two
axial sensors in the axial direction and at the mirrored or exactly
symmetrical distance from the axis eliminates all axial vibration
by subtracting both axial signals and doubling the torsional signal
at the same time.
20. The method according to claim 17 wherein in a wind farm of a
plurality of wind turbines, the torsional vibration of each is
measured and a number of the turbines is selected for analysis of
the angle of attack having a vibration beyond a predetermined set
value to avoid analyzing all of the blades of all of the rotors.
Description
[0001] This application claims the benefit of priority under 35 USC
119(e) of Provisional Application 61/440,500 filed Feb. 8,
2011.
[0002] This invention relates to a method of aerodynamic balancing
a rotor of a wind turbine of the type comprising a tower and a
nacelle mounted to the top of the tower, the rotor being rotatably
connected to the nacelle for rotating about a rotor axis and having
a plurality of equally spaced blades around the axis.
BACKGROUND OF THE INVENTION
[0003] Wind turbines in HAWT design (horizontal axis) consist of
four main parts as a structure, the base, the tower, the nacelle
and the rotor with one or more blades.
[0004] The blades are mounted at fixed angularly spaced positions
around the axis. The turbine includes a wind detection system which
analyses the wind speed and direction repeatedly so as to
repeatedly adjust the angle of the nacelle around a vertical axis
of the tower, that is the angle of the rotor axis relative to the
wind direction, and to adjust the angle of attack of the blades
around the longitudinal axis of the blade relative to the wind
speed.
[0005] Turbines if out of balance will cause significant vibration
in the tower and can if sufficient rapidly deteriorate the
structure or individual components to a position where it cannot be
repaired or leave the turbine in an unsafe condition to operate in.
Periodic checking of the state of balance must therefore be carried
out.
[0006] Some manufacturers apply specific hardware and use methods
such as theodolites or other template based methods to ensure ideal
alignment between blades, without recording pictures but adjusting
in real time.
[0007] Several attempts have been made to measure the angles remote
from the ground with simple photograph based measurements but are
not satisfactory. In most cases no suitable procedures were taken
to verify measurements. In other cases vibration measurements have
revealed results inconsistent with the measurements taken.
[0008] One attempt for the determination of a blade angle is
described in patent application WO 2009/129617 (Jeffrey et al)
published 29 Oct. 2009, the disclosure of which is incorporated
herein by reference, or to which reference may be made for details
not described herein.
[0009] When followed, the described methods in that patent
application are unsatisfactory in that the measurements taken will
include errors which up to now have not been determined so that
this method has not lead to a satisfactory solution. Furthermore,
the methods are of limited practical value and may provide a
serious risk to the safe operation of a wind turbine when used as
the base of further decision making.
[0010] In addition to analyzing the aerodynamic related moments and
the mass moments, it is necessary to try to measure effectively the
actual vibration that is occurring to determine whether any
adjustments which are made are improving the total balance
situation or are not. If the analysis of the aerodynamic related
moments and the mass moments is based on faulty data, it is very
possible that any adjustments made actually create a deteriorating
situation so that the amount of vibration worsens.
[0011] Typically the evaluation of torsions vibration modes is made
with two lateral sensors; but this is not suitable or reliable if
the center of the torsional movement is not accurately determined.
The arrangement of the nacelle mass, the hub and rotor mass as well
as all other components mounted on the tower causes a situation
where the center for torsional movement is not identical with the
center of the tower so that this manner of measuring balance is
inaccurate and can lead to faulty conclusions.
[0012] Furthermore it is known that rearrangements of heavier parts
in the wind turbine as well as exchange of parts, for example the
use of a lighter generator, can easily shift this center and lead
to misleading readings on torsional vibration evaluation.
[0013] Up to now therefore balancing of turbines of this type has
been ineffective and in some cases has lead to a situation where
the results are confusing or even worsen the situation.
[0014] In view of the very high capital cost of this equipment,
methods to accurately and inexpensively maintain the turbine in
balance are very important.
SUMMARY OF THE INVENTION
[0015] It is one object of the present invention to provide a
method of aerodynamic balancing a rotor of a wind turbine of the
type comprising a tower and a nacelle mounted to the top of the
tower, the rotor being rotatably connected to the nacelle for
rotating about a rotor axis and having a plurality of equally
spaced blades.
[0016] According to the invention there is provided a method of
aerodynamic balancing a rotor of a wind turbine,
[0017] the wind turbine comprising a tower and a nacelle mounted to
the top of the tower, the rotor being rotatably connected to the
nacelle for rotating about a rotor axis and having a plurality of
equally spaced blades,
[0018] the method comprising:
[0019] positioning a camera below the rotor;
[0020] rotating the rotor so that each blade in turn is in a
reference position in which a tip of the blade is directed toward
the camera;
[0021] capturing a minimum of one image of the tip of each blade in
turn in the reference position;
[0022] carrying out an analysis of the image to determine a blade
angle of each blade;
[0023] in the event that a blade angle of one of the blade is
different from that of the other blades by a blade angle difference
greater than a predetermined angle, adjusting said one of the
blades to reduce the blade angle difference;
[0024] and during the analysis, correcting angles at different
locations in the image, in dependence upon a prior analysis of an
image taken by the camera relative to a known image.
[0025] Preferably different points of the image of the tip are
identified in the image and the angles between the different points
are corrected in dependence upon their location in the image.
[0026] Preferably easily recognizable contour lines or reference
points of the image of the tip or maximum width of the blade are
used which are selected so that they are identical on every
blade.
[0027] Preferably the accuracy of the angle of attack is in the
range of .+-. 1/10 of a degree to .+-. 2/10 of a degree.
[0028] Preferably during the analysis, correction is made for
trapezoidal distortion in the image.
[0029] Preferably during the analysis, correction is made for
trapezoidal distortion in the image caused by the angle between the
optical axis during taking the photos compared to the vertical axis
of the tower.
[0030] Preferably during the analysis, correction is made for
trapezoidal distortion in the image caused by a measurement line
created on a sloping away contour line, that is sloping away from
the camera.
[0031] Preferably for the capturing of the image of the tip of each
blade in turn, one of the blades is selected to be first imaged by
applying a rotor lock to locate each blade in turn as close as
possible to the vertically downward position and by taking an image
of each blade in turn from a remote position to determine those
that are closest in position to each other and to the vertically
downward position and selecting as the first to be imaged one of
those which are closest.
[0032] Preferably the method includes measuring the torsional
vibration of the system before and after the adjustment.
[0033] Preferably the torsional vibration of the system is detected
using two sensors at positions mirrored exactly in distance to the
left and right of the rotor axis and detecting vibration in the
axial direction.
[0034] Preferably the use of the two axial sensors in the axial
direction and at the mirrored or exactly symmetrical distance from
the axis eliminates all axial vibration by subtracting both axial
signals and doubling the torsional signal at the same time.
[0035] Preferably in a wind farm of a plurality of wind turbines,
the torsional vibration of each is measured and a number of the
turbines is selected for analysis of the angle of attack having a
vibration beyond a predetermined set value to avoid analyzing all
of the blades of all of the rotors.
[0036] According to the invention there is provided a method of
balancing a rotor of a wind turbine,
[0037] the wind turbine comprising a tower and a nacelle mounted to
the top of the tower, the rotor being rotatably connected to the
nacelle for rotating about a rotor axis and having a plurality of
equally spaced blades,
[0038] the method wherein the torsional vibration of the system is
detected using two sensors at positions mirrored exactly in
distance to the left and right of the rotor axis and detecting
vibration in the axial direction.
[0039] According to the invention there is provided a method of
aerodynamic balancing a rotor of a wind turbine,
[0040] the wind turbine comprising a tower and a nacelle mounted to
the top of the tower, the rotor being rotatably connected to the
nacelle for rotating about a rotor axis and having a plurality of
equally spaced blades,
[0041] the method comprising:
[0042] measuring the torsional vibration of the system; adjusting
the angle of attack of the blades by: [0043] positioning a camera
below the rotor; [0044] rotating the rotor so that each blade in
turn is in a reference position in which a tip of the blade is
directed toward the camera; [0045] capturing an image of the tip of
each blade in turn in the reference position; [0046] carrying out
an analysis of the image to determine a blade angle of each blade;
[0047] in the event that a blade angle of one of the blade is
different from that of the other blades by a blade angle difference
greater than a predetermined angle, adjusting said one of the
blades to reduce the blade angle difference;
[0048] and re-measuring the torsional vibration of the system to
ensure that it is reduced by the adjustment.
[0049] The main sources of the inaccuracies are lens distortion and
trapezoidal distortion and can be corrected with the methods
described hereinafter.
[0050] In tests it could be shown that the errors obtained without
the methods described herein can easily be shown to be in the range
of several degrees.
[0051] The errors in analysis will therefore lead to wrong
corrections of angles of the blades which will increase wear and
tear of all wind turbine components through increased vibration
limits.
[0052] The errors are in addition influenced by an offset angle
caused by trapezoidal distortion. The power curve of a wind turbine
will be negatively affected if adjusted with results not following
the procedure described.
[0053] However every blade angle adjustment should be accompanied
by a vibration measurement for ultimate verification of any
adjustment made according to determined angle deviations between
the blades.
[0054] The present disclosure as set out hereinafter provides a
method to achieve sufficient measurements of angles for the purpose
described with minimized errors caused by lens distortion and
trapezoidal distortion. To minimize systematic errors the procedure
describes ways to establish which blade should be selected as the
first blade to be analyzed.
[0055] The result for any angle measurement following the described
procedure represents the true physical angle within general
measurement errors in the range of .+-. 1/10 of a degree to .+-.
2/10 of a degree or better.
[0056] The procedure describes how to establish and use suitable
measurement equipment and methods.
[0057] FIG. 1 of the above application shows that there is no
awareness of distortions caused by perspective (see FIGS. 7, 8 and
10). As well there is no awareness of distortion of the camera lens
itself. Both together will cause wrong readings to be obtained thus
invalidating any corrections made.
[0058] There is provided no indication as to how to effect
selection of the first blade described which will lead to
additional perspective distortion when any cone angle deviation or
partition deviation is present, which is very often the case in
wind turbines of this type.
[0059] For analysis, wind turbines rotor can be simplified and
described as a rotor disc. The rotor blades cover only an area of
that disc. The disc or all forces in it need to be balanced so that
the sum of all forces is centered and identical with the center of
rotation of the rotor shaft itself.
[0060] With the exception of a one blade rotor where a counter mass
not a blade is used to achieve a balanced status in regards to the
mass moment, on every other rotor with two or more blades the mass
moments as well as aerodynamic related moments need to be balanced.
As result the residual moments or the residual imbalance is
minimized for safe operation of the turbine.
[0061] Both the mass moments and the aerodynamic related moments
and their residual components in a rotor system are not individual
measurable but are united for each rotor in an integral imbalance
vector.
[0062] To determine and remove the mass imbalance of any given
rotor with two or more blades, the effects of the aerodynamic
related moments need to be first eliminated or minimized. If the
aerodynamic part is not properly investigated, the result of a
balancing procedure which provides the size and position of counter
weights is flawed to an unknown degree and the result is that the
turbine remains out of balance or worsens.
[0063] The aerodynamic part of the integral in many cases is
treated as constant as a mass moment but in fact changes with the
wind speed due to change in lift force. Thus balancing based on
mass moments only, without the consideration of aerodynamic
influences, is only valid for the same wind conditions that the
balancing was performed on. The integral imbalance can be smaller
or bigger at any other wind condition. The turbine therefore may be
stable at one wind speed but unacceptably out of balance at other
wind speeds showing increased vibration levels.
[0064] The main procedure to eliminate aerodynamic differences
between the blades is to ensure that the angle of attack for every
blade is the same or deviates only in given limits.
[0065] Therefore those angles of wind turbine blades need to be
determined and deviations between the blades need to be evaluated.
The desired accuracy is in the range of .+-. 1/10 of a degree to
.+-. 2/10 of a degree. A standard industry value for limits on
blade angle vibrations is 0.60 degree between individual blades
(see Germanischer Lloyd
Guideline-for-the-Certification-of-Wind-Turbines-Edition-2010-1(4.3.4.1
General influences) and IEC 61400-13(4.6 Sensor accuracy and
resolution)). For some turbine manufacturers it might be bigger or
smaller.
[0066] The absolute angle of attack of the airfoil of a wind
turbine blade is not always directly visible or can be identified
as such, however easily recognizable contour lines or reference
points can be used as substitute, provided they are selected so
that they are identical on every blade. Depending on the
measurement goal this substitute can, but not necessarily has to
have, a known position or relation to the angle of attack of the
airfoil.
[0067] Aerodynamic imbalances caused by damaged or missing blade
elements are not removed with the described procedure and are not
subject of the patent application.
[0068] For initial calibration the most blades have zero degree or
similar marks which need to be lined up with marks at the hub body
or any other suitable reference. However those marks can get lost
over time or are found to be wrongly placed initially. Because of
the constant movement in the system (pitch system) the calibration
in it can get lost over time or is incorrect as the result of a
faulty pitch system. There is therefore a need to verify those
calibrations in a fast and efficient manner.
[0069] The evaluation of torsions vibration modes with two lateral
sensors is not suitable or reliable if the center of the torsion
movement is not given. While for a 2d parameter for the location of
the center of the torsional movement, the first position or
parameter is given to be along the main shaft axis the position on
the second parameter crossing the main shaft axis horizontal at 90
degree is unknown. The arrangement of the nacelle mass, the hub and
rotor mass as well as all other components mounted on the tower
does in all cases implement that the center for torsional movement
is typically not identical with the center of the tower, but
unknown in its location.
[0070] Furthermore the practice has shown in the past that
rearrangements of heavier parts as well as exchange (e.g. lighter
generator) can easily shift this center and lead to misleading
readings on torsional vibration evaluation.
[0071] For verification of torsional vibration either a sensor
measuring torsion directly or two sensors in axial position
mirrored exactly in distance to the left and right of the main
shaft axial axis shall be used.
BRIEF DESCRIPTION OF THE DRAWINGS
[0072] FIG. 1 is a front elevational view of a blade of a wind
turbine and showing particularly the Angle of Attack at the
blade.
[0073] FIG. 2 is a schematic illustration of a general camera
calibration setup for use in the present invention.
[0074] FIG. 3 is a schematic illustration of a calibration grid of
squares with a marked center for use in the present invention.
[0075] FIG. 4 shows the grid of FIG. 3 in a situation where the
Center of distortion (CD)=center of picture (CP) with influence of
an equally distributed pincushion distortion
[0076] FIG. 5 shows the grid of FIG. 3 in a situation where there
is a one dimensional deviation for center of distortion (CD) vs
center of picture (CP) with influence of an unequally distributed
pincushion distortion
[0077] FIG. 6 shows the grid of FIG. 3 in a situation where there
is a two dimensional deviation for center of distortion (CD) vs
center of picture (CP) with influence of an unequally distributed
pincushion distortion
[0078] FIGS. 7A and 7B show schematically a side elevational view
of the wind turbine showing angles used during the photograph an
din calculating distortion correction.
[0079] FIG. 8 is a schematic illustration of the angles between
object planes used in calculating trapezoidal distortion.
[0080] FIG. 9 is a schematic plan view of a wind turbine showing
the location of sensors to detect vibrations.
[0081] FIG. 10 is a schematic illustration of an image of the
angles of FIGS. 7 and 8 used in calculating trapezoidal
distortion.
DETAILED DESCRIPTION
[0082] In FIGS. 7A and 7B is shown a conventional wind turbine.
This includes a nacelle 11 mounted on a tower 9 underneath the
nacelle. A main shaft 13 connects the drive train to the hub and
rotor assembly of the hub body 14 carrying the blades 15 which are
typically three blades arranged at 120 degrees. The blades 15 are
mounted at fixed angularly spaced positions around the axis. The
turbine includes a wind detection and control system 8 which
analyses the wind speed and direction repeatedly so as to
repeatedly adjust the angle of the nacelle 11 around a vertical
axis 9A of the tower, that is the angle of the rotor axis 13
relative to the wind direction, and to adjust the angle A (FIG. 1)
of attack of the blades 15 around the longitudinal axis of the
blade relative to the wind speed.
[0083] Turning now to FIG. 9, a conventional wind turbine shown
including a nacelle 11 mounted on a cylindrical tower underneath
the nacelle. A main shaft 13 connects the drive train to the hub
and rotor assembly of the hub body 14 carrying the blades 15 which
are typically three blades arranged at 120 degrees.
[0084] An axial drive train axis 16 crosses the center of the tower
12 and defines a lateral direction 17 and an axial direction 18
together with a torsional direction 19 (subject to be shifted along
axis 16).
[0085] A vibration sensor 21 can be located on one side of the axis
16 with its measurement axis or axis of sensitivity axial, that is
arranged parallel to the axis 16. A second vibration sensor 22 can
be located on the other side of the axis 16 with its measurement
axis or axis of sensitivity axial, that is arranged parallel to the
axis 16. The second sensor 22 is located at a mirrored position
relative to sensor 21 in reference to the axis 16.
[0086] Other possible locations of vibration sensors 23 with its
measurement axis lateral crossing the center of the tower as
described in vibration sensor with measurement axis lateral
crossing the center of the tower (as described in "Germanischer
Lloyd
Guideline-for-the-Certification-of-Wind-Turbines-Edition-2010-1"
page 2-12 2.3.2.7.2) to sufficiently read vibrations for purpose of
determination of an mass unbalance. Sensor 24 is an additional or
integrated sensor for the revolution of the main shaft 13.
[0087] However the use of sensor 23 is unsuitable for torsional
vibration reference as the center of torsion could be shown to be
as much as 1.15 m ahead of the center of the tower. This creates a
torsional vibration influence for both sensors in the two lateral
sensor option as described in the above Patent Application
WO29129617A1, leading to inaccurate torsion evaluation.
[0088] The use of the two axial sensors 21 and 22 in the axial
direction and at the mirrored or exactly symmetrical distance form
the axis 16 can eliminate all axial vibration by subtracting both
axial signals and doubling the torsional signal at the same time.
The measurement axis is only aligned with and will pick up the
tangential part of the torsional vibration direction parallel to
the drive train and tower axis. Even if the sensors are not mounted
directly to the left and right of the tower axis lateral (which
will be unknown in most cases) but shifted to the rear or front on
the nacelle they are still only influenced by the same vibrations
as long as they still have the same distance to the lateral axis 16
of the system which can be readily determined.
[0089] This makes the system independent and reliable to work with
different weight distributions such as standard and light weight
generators for otherwise identical turbines to be tested.
[0090] A single axial sensor 21 or 22 which used alone is
influenced by axial tower vibration caused by aerodynamic "blade
passing the tower" excitation as well as by potential mass
unbalances. This is due to the fact that all modern wind turbine
rotors main shafts are tilted (usually 5.degree. or 6.degree.) and
the mass imbalance is rotating on a flat elliptical orbit aligned
with the axial direction of the drive train.
[0091] The use therefore of two sensors 21 and 22 arranged as
stated with their output combined provides a measure of vibration
dependent on aerodynamic imbalances and independent of mass
imbalance.
[0092] In the method of the present arrangement therefore an
initial measurement is taken of the aerodynamic imbalances of the
rotor. If this imbalance is less than a predetermined vibration
value, a decision may be made to take no more measurements.
[0093] In the event that a decision is made based on this initial
measurement to make an analysis of the dynamic imbalance of the
rotor, as set out hereinafter, a first one of the blades is
selected for first analysis. This blade is moved to the position in
FIG. 7 for the image of the blade tip to be taken and analyzed to
determine a blade angle of attack. Each blade in turn is then
analyzed to determine its angle of attack so that a difference of
the angle can be determined. Typically it can be found that one of
the blades is distinctly different in angle from the others two so
that it is clear that the adjustment of that blade is necessary by
adjusting the set point of the angle of attack.
[0094] The adjustment system 8 is of course re-setting the require
angle of attack of the blades on a repeated basis dependent on the
wind speed. The system for this adjustment is well known and widely
used on such turbines. Each blade has an individual adjustment 8A,
8B, 8C so as to calibrate the blades relative to the common
adjustment system 8.
[0095] Thus, on determination of the necessity to adjust the angle
of attack based on the above measured dynamic imbalance by the
sensors 21 and 22, the analysis and adjustment of the angle of
attack using the method more clearly set out hereinafter allows the
dynamic balance to be adjusted. On completion of this adjustment,
the above sensors are again tested to ensure that an improvement in
balance and hence a reduction in vibration has occurred
[0096] Thus for example in a wind farm of a plurality of wind
turbines, the dynamic imbalance of each is measured using the
sensors 21 and 22. Based on these measurements, a low number of the
turbines may be selected for analysis of the angle of attack. These
may be the ones having the worst measured vibration. In this way
the analysis can be applied only to those having a vibration beyond
a predetermined set value to avoid analyzing all of the blades of
all of the rotors.
[0097] FIG. 1 shows a symbolized view in a picture taken to assess
blade angles, deviation thereof or absolute angles of attack
thereof. A deviation between each blade can be described with a
positive or negative angle. The direction "+" or "-" may vary and
might be specified in the pitch control system. The same angles
cannot only be used for comparison to each blade but also in
reference to an objects which has a known geometric alignment to
the rotor plane to determine the absolute angle of attack. Those
objects are usually part of the nacelle 11 or a visible piece of
main shaft 9. While the camera C is standing still the objects in
FIG. 1 will not be at identical positions during a set of pictures
within the picture due to movement of the turbines tower and head
section and therefore the use of the outer limits of the picture
frame have proven to be insufficient as reference for angle
measurements.
[0098] FIG. 2 shows the general setup for a camera to go through
the calibration process. The camera C is mounted on a suitable
mount like a tri-pod T and is lined up with the optical axis to the
center of a grid made G of squares. Both the camera and the grid
need to be perfectly levelled horizontal and vertical.
[0099] FIG. 3 shows the general idea of a calibration grid. It
contents of squares and has a marked center, easy to find through
the cameras finder. A real calibration picture would usually have
more and smaller squares, typically in the size of 2.5.times.2.5
cm.
[0100] In FIG. 4 where the Center of distortion (CD)=center of
picture (CP), equally distributed lens distortion is shown as it
would be in a picture taken from the calibration grid. Ideally the
center of distortion and the center of the picture are identical,
which allows a relatively easy compensation of such effects.
[0101] In FIG. 5 where one dimensional deviation for center of
distortion (CD) vs. center of picture (CP), it is quite possible
that some cameras do have a non-equally centered distortion. In the
figure the center of distortion is shifted to the left. Still some
software might still have option for compensating this effect.
[0102] In FIG. 6 where two dimensional deviation for center of
distortion (CD) vs. center of picture (CP), it is quite possible
that some cameras do have a non-equally centered distortion in two
dimensions. In the figure the center of distortion is shifted to
the left and up. This would be typical for a "soft mounted" or
removable optical system, so that CD can be anywhere when ever the
camera is shut down and switched on again. Cameras with this
behavior are just not suited for the purpose.
[0103] As the result of distortion shown in FIGS. 4,5 and 6 lines
being known to be perfectly parallel in FIG. 3 but being at any
given position within the pictures influenced like in FIGS. 4, 5
and 6 would potentially create a variety of physically not existing
angles (Ghost Angles) and result in false measurements.
[0104] In FIG. 7 showing angles during photograph, this figure is
crucial for understanding potentially severe errors for measuring
absolute angles of attack due to trapezoidal distortion. Every
modern wind turbine has a tilted rotor plane axis XB relative to
the nacelles axis XA or XC.
[0105] The optical axis from the camera is therefore usually tilted
against the towers axis XY and YT too, away from the tower.
[0106] In the majority of cases the rotor blade cannot be
positioned with the blades axis straight down and parallel to the
tower axis YT, which causes another tilted angle Y0,Y1 or Y2 of the
cameras axis relative to the tower axis to the left.
[0107] The main items causing errors are camera lens distortion,
trapezoidal perspective distortion of measured areas and air
temperature gradient distortions along the blade. Heat gradients in
or close to designated measurement areas can be present in any
pictures. The index of refraction of air decreases as the air
temperature increases on heated surfaces like the blades, nacelle
or tower thus causing distortion in any picture taken while this
condition exists. Air temperature gradient distortions can in most
cases be relatively easy be identified as areas of the pictures
might appear like smeared, in part missing or objects of known
geometry are severely deformed.
[0108] The removal of camera lens distortion and trapezoidal
perspective distortion is possible to be corrected with the
described methods. Temperature gradient distortions along the blade
are not possibly to be corrected with methods described but deem
any pictures taken with those effects to be not usable.
[0109] The following steps of methods need to be taken for the
measurement targets. [0110] I. Calibration of the camera (all types
of cameras including film and digital cameras and camcorders).
[0111] II. Establishing objects or reference marks with a known
geometrical position relative to the drive train axis 13 preferably
at the nacelles floor visible within the measurement picture to be
taken. [0112] (only necessary if the absolute angle of attack needs
to be determined) [0113] III. Taking pictures or series of
individual blades in designated positions [0114] IV. Applying lens
distortion correction methods with data gained and "I." on pictures
or isolated video stream pictures. [0115] V. Establishing objects
and references in the designated pictures to be measured with lines
individual points or geometric objects. [0116] VI. Determine the
position of objects and references in the designated pictures to be
measured. [0117] VII. Determine the trapezoidal distortion for the
objects and references established under II [0118] VIII. Direct
measure or calculate angles between the objects targeted to
represent angles for analysis and comparison as final result of the
measurement or as intermitted result for further data processing.
If the absolute angle of attack was the measurement target the
angle has to be determined between the blade related measurement
object and the for trapezoidal distortion corrected reference
line.
[0119] The following is a description of individual steps as part
of methods need to be taken for the measurement targets.
[0120] For serious measurements especially when to be sold as
result and base for further decision making every measurement
equipment needs to be calibrated to insure most accurate
results.
[0121] All cameras use objectives to project a 2D picture of any
object which is in its view to a electronic chip or film material.
Those objectives do always have a lens distortion, which will
create "Ghost Angles" exceeding the desired accuracy in a described
measurement.
[0122] The Calibration process documents the distortion and is the
base for countermeasures to remove this type of distortion for each
individual camera in combination with the used objective.
[0123] The parameter gained as to describe the lens distortion for
the pictures taken with a specific camera need to be applied later
in the process to measurement pictures taken to remove those
effects and making the pictures suitable for accurate
measurements.
[0124] Step 1 For the method described a camera needs to be
designated. Each camera needs to go trough the calibration process
separately. A typical digital camera for this purpose should have 6
megapixel or better and an optical zoom of 10.times. or better.
[0125] Step 2 It needs to be assured that the optical system of the
camera is suited for the measurements. This does include a
necessary zoom function. In certain cases the camera was used in a
similar process as described and would be known to full fill those
requirements. If requirements are fulfilled proceed with Step 6
[0126] Step 3 With any new camera a test picture needs to be taken
ideally at the type of turbine to be measured. Cameras might not be
universal to be used for a variety of turbines.
[0127] Step 4 The camera can be classified as sufficient if all
designates measurement areas can be seen with full optical zoom
(digital zoom disabled). If requirements are fulfilled proceed with
Step 6
[0128] Step 5 If the optical zoom is to strong and does zoom in to
much, it needs to be found out whether the camera can be restricted
to a fixed lower rate for the optical zoom in the camera's setup
menu.
[0129] The zoom level needs to be constant for all pictures to
avoid additional measurement errors. The steps which can be applied
manually with the camera are not accurate enough in the most
cases.
[0130] If this can not be achieved the camera is not suited for the
task and a different one needs to be designated. Return to Step
1
[0131] Step 6 With the camera designated for the task a calibration
setup needs to be established under controlled environments,
usually inside. The camera needs to be mounted on a tri-pod or
similar.
[0132] Step 7 If there is a suitable calibration picture available
the camera should be pointed straight to it using maximal (or
alternatives Step 5) optical zoom. Both the calibration picture and
the camera need to aligned horizontal. The center of the camera
screen (picture) should be directed to the calibration pictures
center. If there is a suitable calibration picture available
proceed with Step 9
[0133] Step 8 If no suitable calibration picture is available one
needs to be created. It should consist at least from squares with a
distinctive center. The squares can usually be about 2.5.times.2.5
cm in a distinctive color. If the calibration picture is fixed
mounted it needs to be made sure that it is perfectly horizontal.
The overall calibration picture should be big enough to cover more
then the camera will be able to cover in a picture with full
zoom.
[0134] Step 9 Take a series of calibration pictures. Shut down the
camera move it away from the tri-pod shake it, mount it again and
take more pictures.
[0135] Step 10 Analyzing the pictures is done best by comparing the
calibration picture deformation to a distortion free grid provided
with some software overlapping the picture or by drawing perfectly
horizontal and vertical lines as an overlay to the picture.
[0136] Step 11 It needs to be identified whether the distortion is
centered and equal in all 4 quarters of the picture for all
pictures including pictures taken after the camera was shut off and
moved. For this purpose quarters side by side can be mirrored and
overlapped for this matter. If distortion is centered and equal in
all 4 quarters proceed with Step 14.
[0137] Step 12 If the distortion is not centered in one dimension,
which means only to the side or only up or down it might not be
feasible to go ahead with an calibration of the camera. However
some programs do offer lens correction with "off center"
distortion.
[0138] Since some optical systems for cameras are "soft" mounted
the distortion might not be constant for all pictures taken in this
process. Proceed to Step 15.
[0139] Step 13 If the camera does show no stable distortion for all
pictures or the distortion is not centered at all, the camera
should be discarded for use in this process. Proceed to Step 1
[0140] Step 14 For regular distortion software or mathematical
routines can be used to remove the distortion horizontal and
vertical and gain parameter to do this. Proceed to Step 16
[0141] Step 15 For distortion of center, the center for the
distortion needs to be investigated so that the exact position can
be gained from the calibration process. Software or mathematical
routines can be used to remove the distortion horizontal and
vertical and gain parameter in this regards.
[0142] Step 16 All parameter need to be saved and are dedicated
only to the camera used in this process.
[0143] Step 17 repeat all steps in the process of recalibration the
camera to ensure quality and to detect potential change or damage
to the camera after use in a suitable time frame.
Taking Pictures or Series of Individual Blades in Designated
Positions
[0144] The process of taking pictures is the next step to gain raw
pictures for further processing of gaining data of the blades
angles in respect to the airfoil, a reference or each other.
[0145] It has to be made sure that the blades with no geometrical
issues such as cone angle deviations are to be taken to position
the camera for the purpose of taking all necessary photographs.
[0146] Step 18 It needs to be verified that the screen on the
camera does provide a grid overlay with sufficient small square
sections.
[0147] Step 19 If the camera does not provide a sufficient overlay
grid one has to establish one by drawing fine horizontal and
vertical lines on the screen. It is of advantage to mark those with
grid coordinates.
[0148] Step 20 Essential for positioning the blade it needs to be
investigated whether the rotor lock does have the same partition as
the number of blades. For instance on a three blade rotor the rotor
lock needs to have 120 degree positions, or whole number dividers
of it like 60 degree, 30 degree and so on. Those positions should
be as close as possible to a straight down position. If possible a
picture away from the turbine should document the angle of the
blade relative to the tower for further measures to compensate
trapezoidal perspective distortion if absolute angles are the goal
of the measurement.
[0149] This only applies for rotor lock systems at the main "low
speed" shaft. Rotor lock systems at the high speed side do normally
not provide and equal partition position due to the gear box
ratio.
[0150] If not possible proceed with Step 35.
[0151] Step 21 If the turbine does provide a sufficient rotor lock
system the first blade needs to be brought in position as straight
down as possible with the rotor lock applied.
[0152] Step 22 Approximate a suitable camera position to take
pictures in the designated area of the blade needed to perform the
measurements.
[0153] Step 23 Take on picture of the first blade in position and
in the desired pitch angle.
[0154] Step 24 Repeat taking one picture of every other blade in
the same position with the same desired pitch angle with applied
rotor lock.
[0155] Step 25 Compare the position of the blade or for the
measurement relevant area on the cameras screen relative to the
grid on the cameras screen.
[0156] Step 26 Do all blades match in vertical position (up and
down) in the picture? If not this would indicate a cone angle
issue. Do all blades match in horizontal (left, right) position in
the picture?
[0157] If not this would indicate a 120 degree partition deviation
issue. If all positions match proceed with Step 29.
[0158] Step 27 If the positions of all blades do not match, the
blades closest in position to each other need to be identified.
Those blades need to be marked down as regular. Non regular blades
do have a slight difference in perspective while the other blades
should be in optimal position.
[0159] Step 28 Bring one of the regular marked blades in position
straight down, with the rotor lock applied and in the desired pitch
angle. Optimize the cameras position for best position to take all
further pictures. Proceed with Step 31.
[0160] Step 29 Bring any blade in a straight down position with the
rotor lock applied and in the desired pitch angle. Optimize the
cameras position for best position to take all further
pictures.
[0161] Step 31 Take the designated amount of pictures of the first
blade in position in the designated pitch angle position.
[0162] Step 32 Bring every other blade in the same position and
desired pitch angle and with the rotor lock applied for every
blade, as it was done with the first blade and take the designated
number of pictures.
[0163] Step 33 While leaving the tripod in position, check all
pictures taken for clarity and focus. This should be done on a
bigger screen then the cameras screen for instance on a laptop
screen.
[0164] Step 34 If all pictures have sufficient clarity and focus in
the areas measurements are about to be performed proceed with Step
72. If the pictures lack enough clarity or focus proceed at Step
26.
[0165] Step 35 When a rotor lock as described in Step 20 is not
available it needs to be investigated if the blade tip does provide
a distinguishable feature identical for each blade. Such feature
can be a drainage hole at the blades tip or a from the blades
surface slightly upraised lighting protector puck. If such features
are not available proceed with Step 49.
[0166] Step 36 Position the first blade as straight down as
possible and apply the rotor break and or rotor lock.
[0167] Step 37 Approximate a suitable camera position to take
pictures in the designated area of the blade needed to perform the
measurements.
[0168] Step 38 Take on picture of the first blade in position and
in the desired pitch angle. Mark the distinguishable feature with a
vertical line on the cameras screen or make a note off the position
in the camera screen grid.
[0169] Step 39 Repeat taking one picture of every other blade in
the same position with the same desired pitch angle with the rotor
brake and or rotor lock applied. Move every other blade with the
distinguishable feature to the vertical mark or position from the
first blade.
[0170] Step 40 Compare the horizontal position of the blade or for
the measurement relevant area on the cameras screen relative to the
grid on the cameras screen.
[0171] Step 41 Do all blades match in vertical position (up and
down) in the picture? If not this would indicate a cone angle
issue. If positions match proceed with Step 44.
[0172] Step 42 If the positions of all blades do not match, the
blades closest in position to each other need to be identified.
Those blades need to be marked down as regular.
[0173] Step 43 Bring one of the regular marked blades in position
straight down, the distinguishable feature at the vertical mark,
with the rotor brake and or rotor lock applied, in the desired
pitch angle. Optimize the cameras position for best position to
take all further pictures. Proceed with Step 45.
[0174] Step 44 Bring any other blade in position straight down, the
distinguishable feature at the vertical mark, with the rotor brake
and or rotor lock applied, in the desired pitch angle. Optimize the
cameras position for best position to take all further
pictures.
[0175] Step 45 Take the designated amount of pictures of the first
blade in position in the designated pitch angle position.
[0176] Step 46 Bring every other blade in position straight down,
the distinguishable feature at the vertical mark, with the rotor
brake and or rotor lock applied, in the desired pitch angle, as it
was done with the first blade and take the designated number of
pictures.
[0177] Step 47 While leaving the tripod in position, check all
pictures taken for clarity and focus. This should be done on a
bigger screen then the cameras screen, for instance on a laptop
screen.
[0178] Step 48 If all pictures have sufficient clarity and focus in
the areas measurements are about to be performed proceed with Step
72. If the pictures lack enough clarity or focus proceed at Step
41.
[0179] Step 49 Position the first blade as straight down as
possible and apply the rotor break and or rotor lock. Pitch the
blade into a 90 degree position.
[0180] Step 50 Approximate a suitable camera position to take
pictures in the designated area of the blade needed to perform the
measurements.
[0181] Step 51 Take on picture of the first blade in position and
in the desired pitch angle. Mark the horizontal position of the
blade with a vertical line or note the position on the cameras
grid.
[0182] Step 52 Take on picture from each other blade with pitch
angle at 90 degree at the same vertical line or mark.
[0183] Step 53 Compare the vertical position of the blade or for
the measurement relevant area on the cameras screen relative to the
grid on the cameras screen.
[0184] Step 54 Do all blades match in vertical position in the
picture? If not proceed with Step 60.
[0185] Step 55 Bring any other blade in position straight down,
pitched at 90 degree, with the rotor brake and or rotor lock
applied. Line the blade up with the mark made before or with the
grid position. Optimize the cameras position for best position to
take all further pictures.
[0186] Step 56 Take the designated amount of pictures of the first
blade pitched to 0 degree or the designated pitch position.
[0187] Step 57 Bring every other blade pitched to 90 degree in the
same vertical position at the mark or grid position. Then pitch
blade to 0 degree or the designated pitch position and take the
designated amount of pictures
[0188] Step 58 While leaving the tripod in position, check all
pictures taken for clarity and focus. This should be done on a
bigger screen then the cameras screen, for instance on a laptop
screen.
[0189] Step 59 If all pictures have sufficient clarity and focus in
the areas measurements are about to be performed proceed with Step
72. If the pictures lack enough clarity or focus proceed at Step
55.
[0190] Step 60 Determine the two blades closest together in
position and mark them down as regular
[0191] Step 61 Mark down any noticeable vertical deviation of the
other blades on the cameras screen relative to the grid on the
cameras screen.
[0192] Step 62 optimize the camera position setup, starting with
one regular marked blade in a straight down position.
[0193] Step 63 Take the designated amount of pictures of the first
blade pitched to 0 degree or the designated pitch position.
[0194] Step 64 Bring every other regular blade pitched to 90 degree
in the same vertical position at the mark or grid position. Then
pitch blade to 0 degree or the designated pitch position and take
the designated amount of pictures.
[0195] Step 65 Determine if the vertical deviation of the
non-regular blade is above the regular position. If the position is
above the regular position proceed with Step 67
[0196] Step 66 Bring each non regular blade pitched at 90 degree
stopped to the left in the screen with the amount marked under Step
61. Proceed with Step 68
[0197] Step 67 Bring each non regular blade pitched at 90 degree
stopped to the right in the screen with the amount marked under
Step 61 .
[0198] Step 68 Take the designated amount of pictures of the first
blade pitched to 0 degree or the designated pitch position.
[0199] Step 69 make sure all non-regular blades are photographed.
If non-regular blades are left proceed with Step 65
[0200] Step 70 While leaving the tripod in position, check all
pictures taken for clarity and focus. This should be done on a
bigger screen then the cameras screen, for instance on a laptop
screen.
[0201] Step 71 If all pictures have sufficient clarity and focus in
the areas measurements are about to be performed proceed with Step
72. If the pictures lack enough clarity or focus proceed at Step 62
.
[0202] Step 72 Store all pictures marked as raw pictures in a
suitable manner.
Applying Lens Distortion Correction Methods with Data Gained Under
"I." on Pictures or Isolated Video Stream Pictures
[0203] Any measurements in the pictures gained and not corrected
for lens distortion would be deemed to be effected by errors and
therefore dangerous to be used for any decision making.
[0204] All pictures to be used for measurements have to be
corrected for lens distortion with the parameters gained during the
camera calibration process.
[0205] With the process described below the pictures are lifted
from the status of raw data into usable data for any further
measurement. However effects caused by trapezoidal distortion are
not removed by it.
[0206] All pictures taken or isolated under Step 18 and following
and to be used in measurements to follow, have to be pre-processed
before.
[0207] Step 73 All pictures have to be reviewed on a suitable
screen. This is to identify the pictures with the highest quality.
Not all pictures taken might be usefully and would if used only
raise the general uncertainty when a measurement error is
statistically gained.
[0208] Step 74 It has to be determined if obvious and significant
signs of heat gradients in or close to designated measurement areas
are present in any pictures. The index of refraction of air
decreases as the air temperature increases as this happens on
heated surfaces like the blades, nacelle or tower. Usually
deformation of the shape of the blades root which should be a
perfect circle is one indication. If this can be ruled out proceed
with Step 76 .
[0209] Step 75 If significant signs of heat gradients in or close
to designated measurement areas are visible then those pictures
need to be discarded. It may ultimately mean that the whole session
of taking pictures needs to be re-done.
[0210] Step 76 The more pictures are taken the more different the
pictures are in overall quality and can show a lack thereof in all
or single designated measurement areas. This needs to verified and
only the best pictures are to be evaluated. Proceed with the
highest quality pictures Step 78 .
[0211] Step 77 All pictures with a lack of quality in one or more
designated measurement areas will be discarded.
[0212] Step 78 All pictures to be taken for further processing and
measurements need to be backed up in there original state.
[0213] Step 79 The software which was used for Step 1 and following
the camera calibration needs to be opened, alternative an
equivalent software can be used. All pictures are loaded individual
or together.
[0214] Step 80 The parameter gained during the camera calibration
process to remove the lens distortion are now applied to each
picture to compensate and remove such effects.
[0215] Step 81 All pictures are now to be saved distinguishable
from there original state or file.
[0216] The pictures are now ready to get any measurements performed
to determine deviations between the blades angles to each other but
not to a reference.
Applying Methods of Correcting Trapezoidal Perspective
Distortion.
[0217] If the measurement goal is the absolute angle of attack to
determine the pitch angle in reference to the rotor plane the
correction of lens distortion is not sufficient enough since the
geometric relation between the camera position reference planes and
measurement planes are still under the influence of trapezoidal
perspective distortion in an unknown extend.
[0218] There are two cases of trapezoidal distortion caused by
differences in perspective to the object lines plane in the
processes for determination of blade angle measurements in regards
to the deviation of those between the individual blades and or
absolute angles.
[0219] The main effect to be corrected is caused by the apparent
disposition and therefore angle between the optical axis (Y0, Y1 or
Y2, FIG. 7 and FIG. 8) during taking the photos which should be
identical with a designated part of the blades axis compared to the
vertical axis of the tower (XT, XY FIG. 7 and FIG. 8). and the axis
of the nacelles body (XA, XC FIG. 7 and FIG. 8).
[0220] To correct the trapezoidal distortion, reference points,
holes or marks have to be established (e.g. FIG. 10).
[0221] The key feature of those is that there position to each
other is known. They should in the best case present the corners of
a rectangle as shown in FIG. 9. It would consist of the sides a,
a', b and b'.
[0222] It needs to big enough to detect trapezoidal distortion
sufficiently, which does mean it should be possible to detect
widening effects in the size of 0.5 degree or smaller depending on
the desired accuracy, while the usual effect is in the range of up
to 5 degree.
[0223] To achieve a useful position for it, the drive train axis XB
which is 90 degree to the rotor plane though the center of rotation
of the rotor plane needs to be projected to the nacelles floor or
any other suitable surface to gain the axis XB'. XA or XC (FIG. 7)
is not necessary projections of the drive trains axis.
[0224] The rectangle to be established should have the lines b and
b' perfectly parallel to XB'.
[0225] After the lens correction is performed under step 73 for
each picture to be measured, the trapezoidal distortion of the
reference rectangle can to be determined.
[0226] With suitable software the rectangle can be adjusted to
match the original correlations between a and a' as well as b and
b'.
[0227] The parameter gained with this can be applied to correct the
angle for the reference to fulfill the requirements for a suitable
measurement of attack.
[0228] The reference is now true in a known angle to the rotor
plane which would describe zero degree pitch.
[0229] Another effect of trapezoidal distortion is caused by a
measurement line created on a sloping away contour line. Sloping
away does mean away from the camera. This needs to be addressed and
compensated with detailed knowledge of the blades design.
[0230] In the blades tip area the slope can be typically up to 45
degree or more which would cause a widening of the angle by the
factor of 2.
[0231] To eliminate those distortions, not knowing blade design
features, measurements need to be taken for angles close to or at
the tip and at the maximum width of the blade. Even so the accuracy
at the tip might not be as good as at the maximum width because of
the shorter measurement line, the deviations between both
measurements should be close or identical. If this is not the case
it needs to be investigated which measurement is affected most by
trapezoidal distortion. Usually the measurement which does show the
bigger deviation with is under the influence of widened angles
caused by trapezoidal distortion.
* * * * *