汽车座椅外文文献.docx

comparison of seat system resonant frequency testing methodsa seat system developed without an accurate structural dynamics model has a higher probability of squeaks, rattles, excessive seat back motion, and poor ride characteristics. if these issues are not addressed during development testing and are allowed to go into production, engineering changes are more costly and difficult to implement. because todays seat systems are more complex, engineers must use the latest technology to determine the seat system response characteristics.modal analysis is the process of developing a dynamic model of a structure or a mechanical system which will be used for problem solving and trouble shooting, simulation, prediction，and optimization. the dynamic model is a set of modal parameters consisting of natural frequencies, damping factors, and mode shapes. these parameters are based on the structure or system. experimental modal analysis can use either time based, or frequency domain based measurements to calculate the modal parameters. this method provides the most thorough definition of the dynamic response characteristics of the isolated seating system.resonant impact analysis is used to determine the approximate dynamic response of a seating system. this method provides frequency response functions which describe the natural frequencies of the system. resonant impact analysis provides information quickly, but does not define the dynamic response characteristics as completely as modal analysis.multi-axis shaker table testing is another tool used to determine resonant frequencies in the seat system. the shaker table is able to input sine sweep and random inputs into the seating system. the amplitude of the sine sweep or random input can be controlled in acceleration or displacement control. the shaker table is also capable of simulating road conditions of a customers proving grounds in the laboratory. these roads generate loads in vehicle components such as seats. controlled laboratory tests allow duplication of complex multi-channel time histories of a test specimen. the shaker table can reproduce road inputs in six degrees of freedom: vertical, lateral, longitudinal, pitch, roll, and yaw motions.experimentala correlation study of seat resonant frequencies involved the comparison of seat resonant frequency data acquired by: resonant impact analysis, modal analysis, and shaker table testing using a six-axis simulation reproducing both sinusoidal sweeps and simulated road data. all seat were installed in the oem design position and rigidly attached to either the shaker table or modal bedplate for testing.modal analysismodal analysis was one method used to characterize the dynamic properties of the seats. this involved collecting frequency domain measurements, more specifically frequency response functions, to describe the dynamic characteristics. an h1 estimator was used to calculate the frequency response functions of the seat systems. the seat structures were excited with two electrodynamic shakers, one mounted laterally at the top of the seat back and one mounted fore/aft at the bottom of the seat back. the response was measured over a frequency range of 0 to 50 hz with 200 spectral lines. twenty averages were taken for each frf measurement.the excitation signal was an 80% burst random function. burst random excitation was chosen to excite the entire frequency range of interest uniformly and allow the system response to die out prior to the end of the measurement. a burst random signal is for fft analysis, which assumes a periodic signal, because it ensures that the signal levels are zero at the beginning and at the end of the measurement.in addition to the excitation technique, it was important to test the seat systems in a representative environment. modal analysis can be performed in a free-free environment where the seat system is completely suspended, or in a variety of clamped positions. the samples tested for this paper were attached to a rigid bedplate in design position to simulate the boundary conditions present when the seat systems are installed in a vehicle. the rigid bedplate is exceptionally stiff representing the optimal vehicle floor pan.once the measurements were taken and the frequency response functions calculated, the modal parameters were estimated. the least squares complex exponential method was used for estimating the frequency and damping characteristics, while the least squares frequency domain method was used to estimate the mode shapes. this method is accurate for systems with typical damping values below 5%, as seen in the seat systems that have been tested. tables 1 through 4 detail the modal analysis results for the samples tested. the mode shape is a description of seat back response, unless the base is specifically mentioned. the frequency and damping of each mode shape are also provided. finally, a mode participation is assigned.this number is the percentage of the total response of the modal due to the particular mode.table 1: bucket seat 1 modal analysis test resultsmode shapefrequency(hz)dampingmode participationback rolling about/axis9.784.45%2.4%lateral11.763.85%19.4%lateral12.344.51%40.4%fore/aft12.476.66%22.5%lateral torsion with fore/aft14.692.10%1.3%lateral bending24.501.49%3.6%lateral bending25.802.05%6.6%torsion35.594.54%3.8%table 2: bucket seat 2 modal analysis test resultsmode shapefrequency(hz)dampingmode participationfore/aft14.222.16%10.0%fore/aft16.945.11%26.4%lateral19.382.21%54.9%vertical torsion40.725.25%8.7%table 3: bucket seat 3 modal analysis test resultsmode shapefrequency(hz)dampingmode participationfore/aft10.134.27%33.1%fore/aft10.803.14%11.2%fore/aft with some lateral11.763.36%32.7%lateral12.492.68%11.2%torsion29.225.33%11.8%table 4: bucket seat 4 modal analysis test resultsmode shapefrequency(hz)dampingmode participationfore/aft6.335.13%2.5%fore/aft12.154.19%17.1%lateral with some fore/aft14.221.47%2.6%lateral17.602.94%50.9%torsion31.643.66%11.6%base vert back lat bending36.574.05%15.4%resonant impact testing when a complete modal mode is not required, resonance impact analysis is used to determine the location of seat system resonances. this method involved recording the frequency response function of the seat back when excited by an impact from a modal hammer. the hammer had a force transducer at the tip to record the input force, while accelerometers were attached to the seat back to measure the response. the seat systems were impacted at the top left corner of the seat back and the responses, lateral and fore/aft, were collected at top center of the seat back. the response was measured from 0 to 200 hz with 800 spectral lines. this resulted in a 25 hz resolution. there was a 10% exponential window placed on the output and twenty averages were used to generate the frequency response function. the impact testing results are detailed in table 5. when morn than one value is given for a particular resonance, there are resonances at more than one frequency.table 5: resonant impact resultssamplelateralfore/aftfreq (hz)coherencefreq(hz)coherencebucker seat 111.75, 14.250.990, 0.95411.75, 14.00, 15.000.985, 0.989, 0.994bucker seat 216.25 19.750.992, 0.99616.25, 20.000.997, 0.997bucker seat 312.500.99411.750.992bucker seat 411.50, 14.25, 17.250.982, 0.989, 0.98111.50, 14.250.985, 0.992test result analysisthe frequency response functions generated by resonance impact testing provide the fore/aft and lateral resonant frequencies of the seating system. due to the low level of excitation provided by the impact hammer, this method of testing does not always excite every resonance effectively. when looking at bucket seat 3 there was a significant fore/aft resonance observed in the 10-11hz range from both modal and shaker table analysis that resonance impact testing could not differentiate from the 11.75 hz resonance.the extent to which a particular seat is non-linear can cause impact testing resonant frequencies to be slightly different than resonant frequencies calculated from testing that provides excitation levels. this characteristic was observed in bucker seat 2. the modal analysis indicated the main fore/aft resonance to be 16.94 hz and the main lateral resonance to be at 19.38 hz while resonant impact analysis placed these modes at 16.25 hz and 19.75 hz respectively.the resonant impact testing method is of value when an approximate seat system frequency response needs to be determined quickly. when multiple design iterations are being compared, impact analysis proves to be an excellent tool for ranking the frequency response characteristics of the designs. the methods limitations in providing adequate excitation for all modes and a detailed model are offset by the speed with which the measurements can be taken.when a full modal analysis is performed on a seating system significant information about the overall structural dynamic properties can be ascertained in addition to the frequencies at which the seat resonates. the analysis can be used to model the structure and provide data to indicate what structural modifications may be made to improve the frequency response characteristics of the seat system. modal analysis is much more complicated than resonant impact testing and is used only when structural modifications are needed, or fea models need to be correlated.modal analysis details the relative importance of various seat modes in the dynamic model most effectively. impact analysis on bucket seat 4 revealed resonances at 11.5 hz, 14.25 hz, and 17.25 hz, but provided limited information to the importance of each resonance. the modal analysis of bucket seat 4 illustrated the importance of each mode. a lateral mode at 17.60 hz was 50.9% of the response, a fore/aft mode at 12.15 hz was 17.1% for the response, and lateral mode14.22 hz was 2.6% of the response.while modal analysis provides detailed information about a seats frequency response characteristics, it generates this information by uniformly exciting the seat system with a broad band burst random signal. this is an excellent method for exciting all the modes in the seat, but is not representative of the input spectrum that a vehicle provides. it is important to have the main seat resonances at frequencies which the vehicle does not excite, so their impact can be minimized. bucket seat 4 had a main lateral mode at 17.60 hz and a minor lateral mode at 14.22 hz, but the simulation road load testing only excited the minor lateral resonance.shaker table testing is tool used by the oems and suppliers to subject specimens to squeak, rattle and durability testing. a seat may be subjected to thousands of simulated miles or used to input sine sweep inputs to assist in determining the source of each squeak and rattle issue. unoccupied seat shake often causes squeak and rattle issues in the seating system. when the resonant frequency of a seat is below 16 hz the probability of in-vehicle squeaks and rattles increases.the first method of testing was subjecting the seats to sinusoidal vertical, and longitudinal inputs. the inputs were similar to those used for current testing specifications. acceleration input levels are very important when performing sine sweep tests. if the acceleration levels are too high it will not accurately represent acceleration levels similar to those seen on the road. tables 6 through 8 detail the sine sweep testing. the vertical input to the shaker table was from 2 to 20 hz. the data for each input was analyzed to determine correlation between vertical input and lateral output, vertical input and fore/aft output, lateral input and lateral output, fore/aft input and fore/aft output. bucket seat 1,3 and 4 had similar resonant frequency characteristics in the fore/aft direction which was not revealed during the sine sweep testing. the resonant frequency was typically lower when analyzing the data from lateral input lateral output and fore/aft input and fore/aft output. during the sine sweep evaluation the start and stop frequency and acceleration amplitudes are recorded. these values are compared to the vehicle asd plots.table 6: vertical sine sweep resultssample lateral (hz)fore/aft(hz)bucket seat 113.0013.50bucket seat 218.5018.00bucket seat 311.5010.00bucket seat 413.509.00table 7: lateral sine sweep resultssample lateral (hz)fore/aft(hz)bucket seat 113.5013.00bucket seat 218.5016.50bucket seat 310.0011.00bucket seat 411.5011.50table 8: longitudinal sine sweep resultssample lateral (hz)fore/aft(hz)bucket seat 113.5013.50bucket seat 216.5011.50bucket seat 311.009.50bucket seat 413.008.00the second method