Laser-based vibration analysis of motor vehicles

Transcription

1 Laser-based vibration analysis of motor vehicles Krzysztof Pietrusewicz, PhD Assistant Professor at the Institute of Control Engineering, Szczecin University of Technology University Centre of Mechatronics, Szczecin University of Technology 1. Introduction 2. Laser Doppler Effect vibrometry 3. Measurements in transportation PSV400 3D scanning vibrometer 4. A simple example solution 5. University Centre of Mechatronics 6. Conclusions

2 2. Laser Doppler Effect vibrometry Laser vibrometry uses the well-known Doppler Effect to measure vibrations, If light is scattered from the moving (vibrating) object its frequency is changed slightly, Within a Polytec vibrometer a high precision interferometer detects the minute frequency shifts of the back-scattered laser light, To achieve this, the interferometer splits the light into two parts, The so-called reference beam is pointed directly to the foto-detector, while the measurement beam is incident on the test object, where the light is scattered by the moving (vibrating) surface, Depending on the velocity and displacement the back-scattered light is changed in frequency and phase, The characteristics of the motion (VIBRATIONS) are completely contained in the back-scattered light.

3 3. PSV400 3D scanning vibrometer in transportation Key features of PSV-400 scanning vibrometers Rapid, full field non-contact vibration measurement, Fast, precise, non-contact 3-dimensional measuring technique, Easy and intuitive operation, Intuitive presentation of the measurement results in 3D animation, Short setup time, Digital data acquisition, Complete acquisition of the optically accessible 3-dimensional vibration vectors, Predefined or interactively created scan grids, Use of either predefined (after importing geometry data) or interactively created scan mesh, User-defined datasets enable extended and user-defined evaluation procedures, Simple calibration of the sensor heads position in the moving object s coordinate system, Focus position of each sample point can be individually set, Simultaneous measurement using 3 linear independently oriented sensor heads, Mountable distance sensor (optional) allows acquisition of geometry data, Incorporates proven OFV-5000 Vibrometer Controller and OFV-505 Sensor Head technology, High spatial resolution, Export of data in UFF- and other formats for processing in Modal Analysis Systems.

4 Performing a typical measurement involves these simple steps: Free positioning of the sensor heads in front of the object to be measured, Train the space coordinate system, Define the sample points on the object, Set the parameters for analog and digital acquisition of measurement values, Start scan, Evaluate and/or export the data (in the best way: as a tables of values, as a graphic files, or as an animations).

5 The PSV-3D for car body structural dynamics testing real-world test Introduction Test with the use of accelerometers are usually time-consuming, Accelerometers have to be carefully mounted on the measured surface, For high spatial resolution a lot of accelerometers have to be used, It is impossible due to limited channels of DAQ front-ends, The main drawback: the change in mass of a car body, introduced by the mounted accelerometers

6 4. Example Structural Dynamics Test Setup A Skoda Fabia Sedan has been used for the test. The test object has been excited with the use of a modal hammer. The car was staying at the hard surface. During this simplified test, we performed measurements on the selected sections of a car: front left door part, rear left fender.

7 Workflow In order to provide an insight on how a measurement with the PSV-3D is performed, a simplified workflow is outlined below: Physically setup the heads on their tripods to cover the desired scan area, 2-D alignment to establish the connection between position on the video image and scan angles, 3-D alignment definition of the coordinate system by pointing the sensor head with the distance sensor to 3 known points on the car, Measurement grid definition, Geometry scan to get the 3D coordinates of the measurement points, Focus setting, A/D setting of the measurement system, Perform measurement points are automatically scanned, Data visualization in the PSV software to check deflection shapes, Data export.

8 Results Front left door part Displacement spectrum Velocity spectrum Acceleration spectrum

9 Results Front left door part displacement 4Hz 73Hz 136Hz

10 Results Rear left fender Displacement spectrum Velocity spectrum Acceleration spectrum

11 Results Rear left fender displacement 4Hz 63Hz 126Hz

12 Experiment time The required time for the entire test was up to 6 hours. It is composed out of the time for the initial setup, and the setup and measurement time for each segment of a car body. The total time depends on the selected measurement segment. If the surface is well accessible, has easy geometric shape and good backscattering properties, the total setup time will be shorter than for hidden, complicated surfaces with bad backscattering properties. The time saving as compared to accelerometers is considerable. It gets even more important as soon as the number of simultaneously measurable accelerometers is exceeded, as in the case accelerometers would have to be displaced during the measurement process. For higher density grids, the time saving highly increases, as for the PSV-3D only measurement time increases, the setup time remains nearly constant.

13 Experiment Summary The PSV-3D scanning vibrometer proves to be very well suited for data acquisition for structural dynamics test on car bodies. The main advantages as compared to conventional accelerometers are: Significant decrease of test time, ease of setup, Higher point density (test resolution) allows more precise results, No error checking of cabling, Other applications possibilities: No alteration of structure (no change of mass and stiffness). Brake disks measurement triggered by a microphone, Optimization of the air conditioning systems, Insight into aerodynamically excited structural vibrations of a car body, Tire measurements, Turbines and turbine blades measurements.

14 5. University Centre of Mechatronics Joint initiative of: Faculty of Mechanical Engineering (among others Institute of Mechanical Technology), Szczecin University of Technology Faculty of Electrical Engineering, (among others Institute of Control Engineering), Szczecin University of Technology Type of activity: Research centre, Centre of excellence. Expected time of starting: This year (November)

15 Thank You for Your attention Team of the University Centre of Mechatronics, Szczecin University of Technology, Szczecin, Poland Poznan, Pavillion 3A, stage 74