A wind turbine blade measurement system and a method of improving accuracy of a wind turbine blade measurement system

A wind turbine blade measurement system for optically determining a torsion of a wind turbine blade installed on a wind turbine, wherein the wind turbine blade measurement system comprises:

• a wind turbine with a number of wind turbine blades, including at least a first wind turbine blade, extending from a hub on a main shaft having a substantially horizontal axis, the number of wind turbine blades together with the hub constituting a rotor, which can be put into rotation by wind,
• wherein the first wind turbine blade comprises:
• a root section located nearest the hub, and
• a plurality of sets of markers provided on an exterior surface of the first wind turbine blade at different spanwise positions on the first wind turbine blade along a spanwise direction of the wind turbine blade, each set of markers comprising a plurality of individual marker points,
• a first camera, which is mounted in a fixed position on a support structure on an exterior surface of the root section of the wind turbine blade, and arranged so as to measure along the spanwise direction of the first wind turbine blade, and
• an auxiliary camera, which is arranged at a position relative to the wind turbine, the auxiliary camera being arranged so as to being able to carry out measurements of the plurality of sets of markers and an orientation of at least one of the support structure and the first camera.

A method of improving accuracy of a wind turbine blade measurement system, wherein the wind turbine blade measurement system comprises:

• a wind turbine blade, which is configured to be mounted to a hub of a wind turbine, the wind turbine blade comprising:
• a root section, which when mounted to the hub, is located nearest the hub, and
• a plurality of sets of markers provided on an exterior surface of the wind turbine blade at different spanwise positions on the wind turbine blade along a spanwise direction of the wind turbine blade, each set of markers comprising a plurality of individual marker points,
• a first camera, which is mounted in a fixed position on a support structure on an exterior surface of the root section of the wind turbine blade, and arranged so as to measure along the spanwise direction of the wind turbine blade, and
• an auxiliary camera, which is arranged at a position outside of the wind turbine blade, the auxiliary camera being arranged so as to being able to carry out measurements of the plurality of sets of markers and an orientation of at least one of the support structure and the first camera,
• wherein the method comprises the steps of:
• a) arranging the wind turbine blade in a substantially zero-load condition,
• b) carrying out a reference measurement of the plurality of sets of markers and calculate zero-load twist angles for each of the spanwise positions on the wind turbine blade using the auxiliary camera, and
• c) carrying out a first measurement of at least a number of the plurality of sets of markers using the first camera and comparing the first measurement with the reference measurement to calibrate the wind turbine blade measurement system.