شبيه سازي و اصلاح ارتعاشي پلتفورم كمباين برداشت غلات با استفاده از آناليز مودال عملياتي ﴿OMA﴾

چكيده انگليسي :

Simulation and Modification of Vibrational Model of Corn Harvest Combine Platform Using Operational Modal Analysis OMA Reza Ebrahimi reza ebrahimi@me iut ac ir 25th August 2012 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language FarsiSupervisor Mohsen Esfahanian mesf1964@cc iut ac irAbstractVibration in cutting platform of corn harvest combine causes increase in grain loss reduction of vehicle lifetimeand driver s comfort and also affects the machine working precision In order to determine linear vibrationbehavior of a mechanical structure a model which relates acting forces on the structure with the resultingresponses of it plays a significant role For a great number of mechanical structures only response data aremeasurable while the actual loading conditions are not known Operational Modal Analysis OMA is a processof determining dynamic characteristics of a structure based on the output responses only i e input excitationinformation is not required The use of such processes allows the identification of modal models of structureswhich are excited by unknown ambient force and vibration In this thesis controlled vibration tests areconducted on cutting platform of a John Deer 955 combine During the test the combine is excited by its engineand the resulting vibrations of different working sections such as the trashing mechanism elevator and cutterbar The acceleration signals are recorded with a 6 channel data acquisition device associated with PULSE LabShop measurement software The PULSE system is regulated for sampling interval of 1 953 ms Therefore about 16384 data points are acquired corresponding to a measurement time of 32 s Measurements are taken forconstant values of the engine speed and the cutting height Next the frequency domain decomposition FDD technique is used by the MEscope software to estimate the modal parameters of vibrating structure in realoperational conditions FDD technique is a non parametric technique for operational modal analysis OMA ofstructures that estimates the modal parameters directly from signal processing calculations Then a finiteelement model is constructed representing the vibration behavior of the cutting platform For this purpose thebody of platform knife spiraled auger reel and a total of 56 bolts 13 knife clips and 28 guards are modeled inABAQUS These parts are assembled by 140 coaxial constrains and 100 face to face constrains and meshedautomatically with tetrahedron elements Then the finite element model is successfully updated by comparingthe estimated results with the measured data from operational modal analysis Also a resonance condition isfound around the natural frequency of the fifth mode shape 50 Hz Since the scaling of estimated operationalmode shape depends on the unknown level of the ambient excitation an extra effort is required in order tocorrectly merge the different parts of the mode shapes Therefore the mass change strategy is used to estimatethe scaling factor The FRF matrix is then calculated by scaled mode shapes Finally the modification techniqueis introduced on the structure by change of mass matrix in order to achieve better harvest performance and haveeasier and more comfortable operation The structural modification methods often prove to be the best andsometimes the only strategy for the redesign of complex structures As a result of this modification naturalfrequency of fifth mode shape 50 Hz of the combine cutting platform was shifted to 48 Hz Thus the resonancecondition and vibration of cutting platform is reduced Keyword cutting platform FDD technique operational modal analysis mass modification

ابراهيمي، رضا

شبيه سازي و اصلاح ارتعاشي پلتفورم كمباين برداشت غلات با استفاده از آناليز مودال عملياتي ﴿OMA﴾