Pitch and Roll Angle Calibration for Scanning LiDARs to Reduce Uncertainties in Measurement Height

Introduction

The application of scanning LiDARs for the measurement of wind speed is a relatively new technology in the wind energy sector and also has potential relevance for various topics,(e.g. measuring the wind speed for power performance test for offshore wind turbines). Scanning LiDARs offer a customizable scanning geometry and are, therefore, able to measure wind speed and wind direction at a position that is not centered on its own position. For such situations, the knowledge of the elevation angle is crucial, as it directly influences the measurement height. For instance, for power performance tests of offshore wind turbines of typical size, an accuracy higher than 0.1° is necessary. Independent from the displayed elevation angle of the device, the inclination of the device must be known. The device measures its own orientation within the earth’s gravitational field using two built-in electronic inclinometers. In order to achieve a high accuracy of these measurands, these sensors must be calibrated prior to the measurement. A calibration procedure, including detailed uncertainty assessment, has been developed in the testing laboratory of Deutsche WindGuard.

Approach

In order to assess the accuracy of the pitch and roll inclinometers of the LiDAR, the displayed inclination of the device for a given elevation angle (0°) is compared to the measured inclination while the inclination of the device is varied manually. Thus, the true position of the laser beam is determined within a given distance from the device, using a detection construction. The inclination is then measured by two redundant methods:
• Direct measurement of the angle between laser lens and real laser beam location with a theodolite;
• Indirect measurement of the angle, applying a trigonometric function using the vertical displacement of the laser beam and the horizontal distance between the exit from the lens and the detection construction.


Main body of abstract

Both measurement principles require a clear definition of the reference height, in order to define an inclination angle of 0°. It is assumed that this reference height is the exit point of the laser beam. A rotating laser is utilized to transform this height to the detection plate (approx. 80m distance from laser lens). In order to avoid the common and often time-consuming problem of locating the invisible laser beam (λ=1550nm) , an additional visible laser device is mounted to the scanning head of the tested LiDAR and aligned with the invisible laser beam after having found it once. This first location is identified by iteratively blocking and unblocking the laser beam with the detection construction. Blockage of the laser beam occurs when the LiDAR’s carrier-to-noise ratio changes significantly in the relevant range gate. For the subsequent measurements, the location of the visible laser beam was identified.
By varying the tested measurand (pitch or roll) via adjustment of the screws in the legs and subsequently identifying the position of the laser beam, the slope and offset of the pitch and roll inclinometer can be determined.
In order to address pitch and roll inclinometers, the procedure described above is conducted for both measurands independently, by setting the azimuth angle of the device to 0° (pitch) or 90° (roll).


Conclusion

A new method of calibrating pitch and roll angle for scanning LiDARs has been developed. The challenge of identifying the invisible laser beam is avoided by the application of an adjustable visible laser beam. Redundant measurement principles verify the reliability of the results.


Learning objectives
The results of devices tested thus far lead to the assumption that calibration of the built-in inclinometers is recommended, in order to achieve a high accuracy in elevation angle and consequently in measurement height.