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.

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° 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° C. to about +50° 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.