Half car rig lab instructions PDF

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Lab instructions for the half car rig experiment....

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48601 Mechanical Vibration & Measurement

UTS Faculty of Engineering & IT

Laboratory Session 2 (Project 1): Half Car Rig Vibration Analysis The half car test rig, as shown in Fig. 1, consists of three main components and four degrees of freedom. Two smaller members are supported by springs and represent the unsprung wheel masses; the spring itself represents tyre stiffness. The third larger mass represents the vehicle chassis or sprung mass. It is used to experimentally represent and understand the bounce and roll modes of a half car, i.e. the front or the rear suspension system and associated mass. Additionally there is a hydraulic circuit arranged around the sprung mass which has the effect of increasing the roll stiffness (i.e. like a torsional spring) and damping.

Figure 1: (left) half car test rig, (right) CAD model of test rig (top springs excluded)

The experimental rig is equipped with a hydraulically interconnected suspension (HIS) system offering some advantages over conventional passive independent suspensions [1]. The available HIS is an anti-oppositional interconnections arrangement, as illustrated in Figure 2, primarily stiffening out-of-phase motion (roll mode) relative to in-phase motion (bounce).

Figure 2. Schematic of a general anti-oppositional half-car HIS arrangement.

Procedure: 1) Mathematical Modelling For this project you will be required to develop a lumped mass multi-body model of system and determine its main natural frequencies and mode shapes. Deliverables include:

1. Sketched multi-body model and free body diagrams of the major components 2. Detailed derivation of equations of motion in matrix form 3. Brief summary of any assumptions that you have made in relation to developing the mathematical model Page 1 of 4

48601 Mechanical Vibration & Measurement

UTS Faculty of Engineering & IT

Provided in Table 1 below are the main parameters of the half car test rig, with the exception of suspension damping. NB: roll centre distance is the distance from the centre of gravity for the sprung mass to the suspension system springs. Using these parameters, determine the theoretical natural frequencies vehicle bounce, roll and wheel hop vibration modes. Compare these results to experimental data (see section 2). Table 1: half car test rig parameters Value Units Symbol Unsprung mass ea.

56.1

Kg

mu

Sprung mass

433

kg

ms

Roll inertia

82.7

Kg-m2

I

Tyre stiffness ea.

147.2

kN/m

Kt

Tyre damping ea.

0

Ns/m

Ct

Suspension stiffness

20.5

kN/m

Ks

Roll centre distance

0.75

m

L

Roll torsional stiffness

12.5

kNm/rad Kr

2) Laboratory-base Experimental Testing The physical half car system is located in the Dynamics Lab, CB11.B4.105. During structured lab sessions you will be provided with the opportunity to perform free decay response tests on the rig to experimentally determine natural frequencies and damping modes. Sensors on the rig include four displacement transducers, one for each unsprung mass and one at each suspension support for the sprung mass. The bounce mode is induced by lifting the centre of the sprung mass and then releasing it. The bounce and roll modes are induced together by lifting one end of the sprung mass and then releasing it. The purpose of this lab is to measure the free decay response of the system and use the gathered data to experimentally determine system natural frequencies and damping ratios. 1. Determine frequencies present in bounce and roll tests for both sprung and unsprung masses, 2. Determine the damping ratio for bounce and roll modes, 3. Use the damping ratio to estimate a new damping coefficient for the suspension and reevaluate your modelling results. Note: the roll test is not a pure roll response as the initial displacement of the left sprung mass sensor is not equal to the negative value of the right sensor, therefore the roll and bounce excitation modes must be extracted from the left and right sensors. This can be achieved using kinematic relationships between dependent and independent coordinates in mathematical models.

3) Presentation of results Prepare a brief but detailed laboratory style report detailing the modelling and experimental results. Use a combination of time- and frequency-domain based modelling and experimental analysis techniques to compare and contrast your model with the experimental results obtained for the various different scenarios arranged (i.e. without and with additional mass/inertia and without and with additional, hydraulic cylinder enabled roll stiffness). Investigate the effect of the hydraulic pressure on the natural frequency and damping ratio of both bounce and roll modes and justify your findings using the theoretical concepts of an anti-oppositional HIS. State any assumptions or source of uncertainties. Include any hand calculations and MATLAB code in report appendices for marking.

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48601 Mechanical Vibration & Measurement

UTS Faculty of Engineering & IT

4) Things to pay careful attention to        

The developed model will be a lumped mass representation of the real system We model with independent but measure dependent coords.; relationships required LDVTs require static calibration to convert from measured V to mm displacement LVDTs have a finite range (+/-5 V) and need to be position carefully to avoid clipping Initial positioning of LVDT body is arbitrary and there will, therefore, be a DC offset Since vibration frequencies are low, we cannot use AC coupling to eliminate the DC offset as this would likely also attenuate the vibration signal of interest Bounce damping coefficient can be determined from XsG decaying exponential time waveform using log. decrement The task is to model the system for the three different scenarios and make use of the experimentally obtained data to develop a more accurate model that achieves similar results to those measured; not all of the required physical parameters for all three scenarios are available up front and engineering judgement is therefore required

5) Bounce and pitch frequencies and modes of a real car A) Model a real car as a 2 DoF system having angular motion (pitch mode) and up-and-down linear motion (bounce mode). Find the natural frequencies and the location of vibration centres (nodes) where no oscillations can be sensed in those points. B) Assume the car is travelling on a rough road at the speed of 20 km/h. The road surface varies sinusoidally with an amplitude of 0.04 m and a wavelength of 6 m. Considering the bounce mode only, design a suspension system so that there is a 10% reduction of the displacement transmitted to the chassis. Bounce Pitch

C.G.

l2

l1 Use the following data for your calculations:

Page 3 of 4

48601 Mechanical Vibration & Measurement

UTS Faculty of Engineering & IT

Guide for lab reports: The reports are open ended, and thus there is no set format and/or style for the presentation of data and plots. There is no limit on the number of tables and figures that may be included in the reports either. However, you are urged to consider carefully how you want to present and plot your data in tables and figures to effectively demonstrate your understanding. You should also avoid redundant tables and plots. All tables and figures must have captions. All figures must be illustrative and accompanied by explanation/interpretation. All reports must be typed in using 12-font size with single spacing. The reports should not exceed 10 A4 pages excluding the cover page and appendices . Do not put your key findings and main figures in the appendices.

Reference 1.

Zhang, N., W.A. Smith, and J. Jeyakumaran, Hydraulically interconnected vehicle suspension: background and modelling. Vehicle System Dynamics, 2010. 48(1): p. 17-40.

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