Development of a robot-aided modal analysis measurement method using laser Doppler vibrometry

Abstract

In the present master's thesis, a robot-assisted modal analysis method is developed for the company V2i, specialized in vibration identification. The main goal is to position a laser Doppler vibrometer (LDV) on an industrial robot in order to, on the one hand, gain time in the point-to-point movement of the LDV and eventually automating the whole process, and on the other hand, get the ability to perform precise 3D measurements. Indeed, Euler angles are tricky to set manually with a satisfactory degree of accuracy.

A brief literature review is done on the different calibration methods and the existence of this type of application. After installing the robot, the risk assessment due to its presence in the company along with the needed safety installations are also established.

The main part of the work treats the specific subject of tool and part calibration. Indeed, when fixing a LDV on a robot head, the problem is that the beam has no physical end. Thus, the calibration phase needed to define the positions and orientations of the laser beam and the studied part with respect to the robot frame becomes quite different from a physical tool calibration. In this work, several solutions for these two calibrations are proposed and tested. A selection is then made. The retained method consists in combining, on the one hand, a common method for physical tool calibration based on physical contact, and on the other hand, the LDV and part calibration procedure. To this end, a plastic 3D printed tip is attached to the robot end effector with the LDV. When placing the tip in such a way that the laser beam touches its extremity, calibrating the LDV boils down to calibrating a simple tip. Similarly, defining the position of the part with respect to the robot boils down to touching it in several locations with the tip and computing the transformation matrix between the robot frame and the part frame. A validation of this method in terms of repeatability and accuracy as a function of the number of palpated points is performed and gives satisfactory results. To the author's knowledge, no other article treats the calibration problem of a LDV on a robot head with such a both simple and efficient method.

Afterwards, modal analyses using both mono- (1D) and multi-directional (3D) measurements are conducted on an academic compressor wheel. The interest is double: firstly, one is able to show that the calibration precision is sufficient to perform an accurate modal analysis; secondly, one is able to show the advantage of using the robot to perform 3D measurements by comparing with the 1D results. Finally, a discussion on the perspectives of automation and inherited time gain is carried out.