Accuracy and noise analyses of 3D vibration measurements using laser Doppler vibrometer Hossam Khalil, Dongkyu Kim, Joonsik Nam, Kyihwan Park ⇑ School of Mechanical Engineering, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, South Korea article info Article history: Received 10 April 2016 Received in revised form 29 July 2016 Accepted 3 September 2016 Available online 6 September 2016 Keywords: Accuracy of 3D vibration measurement 2D vibration measurement Laser Doppler vibrometer (LDV) Laser scanning vibrometer (LSV) Angle analysis abstract The in-plane and out-of-plane vibration components are used for 3D vibration measurements. The latter can be calculated by using three laser scanning vibrometers (LSVs) or by moving a single LSV to three dif- ferent locations. These vibration components are calculated from the vibration signals measured at each of the three locations and the angles between the local coordinates and the LSV locations. The accuracy of the in-plane and out-of-plane vibration components can be degraded depending on the measurement angle. In addition to accuracy, the noise contained in the LSV can be amplified depending on the measure- ment angle. Hence, it is necessary to implement an analysis methodology for the angles, which is con- ducted for 2D vibration measurements first before extended to 3D. Finally, experiments are performed for both 2D and 3D at small and appropriate angles, and the elicited results are compared to those elicited using a 3D accelerometer. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Accelerometers have been traditionally used for vibration mea- surements in conventional modal testing. These contact-type sen- sors have several disadvantages, including loading effect that change the natural frequencies of light and flexible structures. These sensors cannot measure vibration signal unless they are attached to the object. Ultimately, there is a high sensitivity to electromagnetic interference which makes it difficult to measure the vibration of electronic components. Furthermore, in order to obtain the mode shape of the entire surface, it is necessary to use multiple sensors [1]. These problems can be solved using a noncontact laser Doppler vibrometry (LDV) [2,3]. An LDV measures the velocity signal of an object using the Doppler frequency shift due to the interference between the incident and scattered light reflected from the surface of a vibrating object. LDVs are considered to be a technology that could rapidly and correctly measure the vibration of a desired posi- tion. To estimate the mode shapes considering the measurement of the entire surface, the vibration of multiple points can be easily measured using a pair of electric motors combined with the use of an LDV, an instrument which is known as a laser scanning vibrometer (LSV). For measuring true vibration that occurs in an object, three sets of LSV are needed. This is because the vibration of the object can occur along three distinct directions [4]. 3D vibration measure- ments using LSVs, called a 3D scanning laser Doppler vibrometer (3D-SLDV) are increasingly needed for various industrial applica- tions because of their accurate and high-speed measurements. 3D-SLDV is used to measure true vibration in many applications. For example, in [5] it was used to measure the vibration of power tool, in [6] bridge vibration measurements, in [7] the measurement of 2D dynamic stress distributions, and in [8] structural health monitoring using ultrasonic surface waves. A new algorithm for 3D vibration measurements is introduced in [9] using just one LSV set rather than three sets of LSVs, which has a notable disadvantage due to the high cost. This method has the same effect in regard to the use of the three sets of LSVs since a vibration analysis in the frequency domain is independent of the time at which the vibration becomes uniform. For 3D vibration measure- ments of an object, it is required to measure the vibration of points on the object at three different locations. The proposed method in [9] is a good approach for obtaining the 3D vibration components. From the measured vibration signals at three arbitrary posi- tions, and the angles between the local coordinates and the LSV locations, the vibration components of the in-plane and out-of- plane directions are calculated. However, the accuracy of vibration in the in-plane and out-of-plane components can be degraded depending on the measurement angles. In addition to the accuracy problem, the noise contained in the LSV signal can be amplified depending on the measurement angles. Hence, it is necessary to introduce an analysis methodology for the angles so that the accu- racy of the vibration measurement can be increased, and the noise http://dx.doi.org/10.1016/j.measurement.2016.09.003 0263-2241/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: khpark@gist.ac.kr (K. Park). Measurement 94 (2016) 883–892 Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement