Click to edit Master subtitle style

Oliver GazeleyFitzwilliam CollegeUniversity of Cambridge

Vehicle Dynamics Concepts for a Solar Racing Car

SIMPACK User Meeting 2011 – Salzburg, Austria

1

Contents

•Introduction and Motivation•Aims of Project•Structure of SIMPACK model•Validation•Aerodynamic Modelling•Steady-state cornering•Pitch Stability•Future Work•Conclusions

2

“Cambridge University Eco Racing (CUER) designs, builds and races solar-powered cars. Our racing cars showcase cutting-edge sustainable engineering and demonstrate the incredible potential of electric vehicle technologies. By designing a car to run on solar power alone, we are forced to make a step change in vehicle efficiency, resulting in new technologies for a low-carbon future.”

Introduction

3

Introduction

•CUER will be entering a team into the World Solar Challenge in 2011

•3000km from Darwin to Adelaide

•Need to know how the car handles• Stability• Safety

•Need to consider aerodynamic effects

•There is speculation that the battery failed in 2009's challenge due to excessive vibration. Need to investigate vibration issues

World Solar Challenge

October 2011

4

Motivation

A road train on the Stuart Highway Le Mans Car Flip (1999)

•The motivation of this project is to better understand how the car will behave in different scenarios, and thus to inform design decisions for future cars

•The issue of aerodynamic stability is important, as the car is essentially a light aerofoil and will regularly be passed by road trains in Australia, causing aerodynamic disturbances.

5

Aims of the Project

Existing handling model

Existing ride model

Combine into single

comprehensive model

Validate new model

Existing Field-Test

Data

Surveying data for

track input

Model Aero Forces

Data from Aero Team

Parametric Studies

6

SIMPACK model

Basic architecture of model:

Additionally:

•Simple driver model used to control front steer angle to follow desired path

•Control system used to regulate driving speed by applying torque to the front wheel

•Mass of driver, batteries and other instrumentation added to the chassis

•Pacejka similarity tyre model used for simplicity

•Chassis joined to ground using 6 d.o.f. track joint

•Friction model in parallel with spring and damper, applying hysteresis loop

7

SIMPACK model

8

•Screenshots showing the SIMPACK model. Red items are testing equipment, batteries and laptop, green cylinder represents the driver.

•Composite shell is not shown in the model

Validation•Previous projects recorded lots of data for straight line and step tests, including suspension deflections and accelerometers at 5 different locations

•The SIMPACK model can be validated against this data, provided the original road or step input is known

•The data can be compared deterministically or spectrally

9

Validation – Front Step Data•Suspension parameters were tuned to give good deterministic match between measured (green) and simulated (pink) front suspension deflections for a 33mm step

Deflection (m)

Time (s)

Front suspension parameters: k = 30000N/m, c = 600Ns/m, hysteresis loop magnitude = 48N

10

Validation – Rear Step Data•Suspension parameters were tuned to give good deterministic match between measured (green) and simulated (pink) front suspension deflections for a 66mm step

Deflection (m)

Time (s)

Rear suspension parameters: k = 13000N/m, c = 1500Ns/m, hysteresis loop magnitude = 400N

11

Validation – Straight Line Data•GPS data exists from testing performed in previous projects. This data was used to find the location on the track where the tests occurred

•Once located, the track was marked out and surveyed for 100m to define the track excitations to be input to the SIMPACK model for straight line validation

•The vertical height of points were measured every 20cm along the 100m track

12

Validation – Straight Line Data

•This was input to the SIMPACK model as a road-related excitation

•The simulated accelerations seen at above the front wheel and above the rear wheels were compared to the accelerations seen in practice to confirm that the behaviour of the car was suitably modelled by SIMPACK

13

Validation – Straight Line Data

Front accelerations comparison

Rear right accelerations comparison

14

Aerodynamic Modelling•Data acquired from computational fluid dynamics for lift, drag and centre of pressure for different yaw and pitch angles of the vehicle

•Force element added to act at the origin of the chassis of the car, providing a lift force, drag force, pitch moment, yaw moment and roll moment as calculated using expressions in SIMPACK:

Pitch angle = Φ, Yaw angle = δLift, L = f(δ,Φ), Drag, D = f(δ,Φ)Centre of pressure (x,y,z) = f(δ,Φ)

Pitch moment = L.x – D.zYaw moment = -D.yRoll moment = -L.y

15

Steady-state cornering•The model was used to simulate steady-state cornering at a speed of 20m/s for a range of circle radii

•From this, a handling diagram was produced up to the point of instability

0.0000 0.0020 0.0040 0.0060 0.0080 0.0100 0.0120 0.0140 0.01600.0000

1.0000

2.0000

3.0000

4.0000

5.0000

6.0000

7.0000

delta - L/R

u^2/

R

In this graph, u is the velocity of the vehicle, R is the radius of the turn, delta is the steer angle and L is the distance between front and rear wheels

•It can be seen that at large radii or small velocities that the vehicle has the tendency to understeer.

•At higher speeds, or tighter turns, the vehicle begins to oversteer until it reaches instability.

16

Pitch Stability•Pitch Stability was investigated by applying an upward impulse to the front of the car while it is in steady-state and observing the resulting motion

•It was found that the car was stable as the centre of pressure moves to the rear of the car and causes a restoring moment when the car pitches up

This graph shows the behaviour of pitch angle for three different magnitudes of impulse.

It can be seen that the oscillations decay in a stable manner.

17

Future Work

•Improve friction model for suspension by modelling stick/slip behaviour

•Incorporate chassis bending into model

•Add a realistic braking model and investigate handling while braking or more complicated manoeuvres

18

Conclusions•SIMPACK was used to combine and enhance ride and handling models from previous projects

•Steering and driving torque control systems added

•Aerodynamic response added using a single force element and expressions to calculate required forces and moments from CFD data

•Vehicle response validated using step and straight-line data and tuned to give most realistic behaviour

•Handling and stability issues investigated. Limit of handling stability found and pitch behaviour found to be stable even at large pitch angles

•Behaviour of the car is much better understood before its 3000km journey in October

19

If you have any questions regarding this project, I would be very happy to answer

them.

Please e-mail me at og229@cam.ac.uk

Follow the progress of the CUER team at:

www.cuer.co.uk

20