Study On Vibration Isolation And Optimization Of Commercial Vehicle Cab Suspension System

Recently, the domestic commercial vehicle went through a very fast development accompanied by the yearly increasing production and sales record. Meanwhile, there are more requirements on the performances of the commercial vehicles especially on the comfortableness. Because of the complicated traffic environments and the long driving time, drives are more easily to have fatigue, which is very bad for their health and even worse directly cause the traffic accident. Therefore, how to enhance the comfortableness of the commercial vehicles has become a hot issue to the automotive industry. The vibration on commercial vehicles mainly comes from road and the power train, as a result of which, most of the current research lies on the improvement and design optimization of the suspension and power train, while less effort has been put on the study of cab suspension system. Cab suspension system can effectively absorb the vibration generated from the road and power train, thus it is a better way to downsize the vibration in the cab if properly improved and optimized.This paper starts with study on the vibration modal, transfer characteristics, and the vibration isolation of the cab suspension. Vibration models of cab suspension and whole vehicle were built through ADAMS. Finite element and multi-body dynamics were collaborated into the analysis of the vibration modal and transfer characteristics of the cab suspension system, the response and the isolation feature of cab suspension based on the whole vehicle, and the influence of variable parameters to the vibration response. To increase the isolation ability of the suspension, improvements and optimization design were proposed and an encouraging result was obtained.The modal of the cab suspension system is the key point in the vibration. Unsuitable distribution of the modal frequencies and modal shapes will result a bad isolation or even worse could strengthen the vibration on the cab. This paper studied the vibration modal though the ADAMS model of the suspension system. The cab was assumed to be a rigid body for simplification. Because of the bulky volume, it is hard to measure the moment of inertia of the cab using traditional method. Thus, a method of frequency recognition was used in the measurement, which was verified its validity by square box modal test afterward. During modeling, the influence of rubber bushing was taken into account through obtaining the static stiffness of the bushing by finite element method. The overall stiffness of the suspension system was also simulated and tested through the whole stiffness of cab suspension test, for the accuracy of the modal analysis of the suspension system. The undamped and damped model of the suspension were analyzed and verified by modal analyzing and bench test respectively. Using complex modal analysis, the inconsistency of frequency for damped and undamped system was studied. Dynamic analysis was implemented in the analysis of the impacts from excitation and damping to the complex vibration mode.In order to study the response under medium and low frequency excitation, the cab suspension rigid-elastic coupling model was established. Based on the finite element method, the BIW(body-in-white) model was built. The vibration modal of the flexible bodies was analyzed and the results were compared with those from the test-rig for its accuracy. The finite element model was finally introduced into ADAMS and to replace the former built rigid body model. Therefore, a rigid-elastic coupling cab suspension system model was built. Burst random method was implemented to excite the cab suspension system to study the its vibration response. The relationship between the acceleration power spectral density curve peak frequency and modal on the response points were analyzed and verified by the associated bench test. Through the design of experiments method, the sensitivity of vibration response to the parameters of the cab suspension system was analyzed. The results show that the damping and stiffness are the two main factors for the vibration response.The vibration of the cab and the whole vehicle are closely related. The cab suspension system model based on the whole vehicle will help analyzing the vibration of the cab excited from the road and power train and the isolating ability of the cab suspension system. The multi-body dynamic model of the vehicle was built based on the cab suspension system. This model includes the cab and its suspension, the power train and its mount, the frame, the suspension, the vehicle-bridge, the steering system and the tires, etc. During modeling, the excitation characteristics of the 6-cylinder engine and the vibration modal of the power train mount were analyzed. Finite element model of the frame was built to analysis its modal. The modeling method of leaf spring was studied. Timoshenko beam method was used to build up the model of a multi-leaf spring. Virtual shackle was used to solve the contact problems between the leaf spring and the vehicle-bridge of the tandem SuspensionUnder stable speed and stable engine revolution, the vibration analysis was carried out on the vehicle-based cab suspension system. According to the national standard <Mechanical vibration-Road surface profiles-Reporting of measured data> (GB/T7031-2005), standard B level road model was built. Simulations were carried out fro this road based on different vehicle speed. The changing pattern of the acceleration RMS(Root Mean Square)value for the vehicle-bridge, power train mount, the driver's seat, and the seat slide were analyzed under different vehicle speed. The acceleration RMS values were also compared with the number from the test to verify the validity of the model. The frequency response of the acceleration on the seat slide was studied to analyze the relationship between the peak frequency and the vibration modal. The relation of the vibration isolation ability of the cab suspension and the vehicle speed was also analyzed, in which the result shows the vibration isolation ability increases as the vehicle speed. Under stable engine revolution, the changing pattern of the acceleration RMS value of the acceleration for the suspension of the power train and the seat slide were analyzed under different engine revolution. Still the acceleration RMS values were compared with the number from the test. The study of the acceleration power spectral density curve shows that the excitation of the cab comes from the third order of the engine excitations. To study the impacts of the vehicle parameter to the cab vibration, the stiffness and damping of the suspension, stiffness and damping of the cab suspension, and the stiffness of the power train mount were changed according to these two working conditions, to find the changing pattern of the acceleration RMS value of the seat slide in terms of the changing of above parameters. The result shows that, the increasing of the stiffness of suspension, the stiffness and damping of cab suspension and the stiffness of the power train will enhance the vibration of the cab, while the increasing of the damping of the suspension will reduce the vibration of the cab. Design of experiments method was taken to quantitatively analyze the impacts of changing these parameters.According to the vibration analyzing results of the vehicle, improving the stiffness and damping of cab suspension is an effective way of enhance the comfortableness. Therefore, when doing the optimization of the cab suspension system, the stiffness and damping of the cab suspension as well as the position were taken as the optimization variables, while the sum of cab suspension vibration transmissibility was the optimization target. The genetic algorithm was used as the optimization method. The optimization results show that there only limited effectiveness on coil spring. So the improvement plans were proposed to replace the coil spring with an air spring and setting transverse damper. According to the plans, the original cab suspension was modified. The piecewise function method was used in fitting the characteristic of the air spring. This method can intuitionally reflect the stiffness characteristic of the air spring within its working area, thus is good for the design and optimization toward different working stage. Meanwhile, the damping characteristic curve of the cab suspension was also designed. Based on the optimization of the modified suspension, there was a great enhancement on the vibration isolation. The lateral and vertical vibration of the seat sliding decreased a lot. Though the ride comfort analysis, the weighted acceleration RMS value inside the cab was reduced to 29.18%~34.87%. The result shows the optimization and the modification were effective and correct.