The Use of MBD Modelling Techniques in the Design and...
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The Use of MBD Modelling Techniques in the Design and Development of a Suspension System David J. Fothergill 2nd European HTC Strasbourg September 30th – October 1st, 2008
Transcript of The Use of MBD Modelling Techniques in the Design and...
The Use of MBD Modelling Techniques in the Design and Development of a Suspension System
David J. Fothergill
2nd European HTC
Strasbourg September 30th – October 1st, 2008
Introduction to the ULTra system
Advanced Transport Systems Ltd Urban Light
Transport (ULTra)
Introduction to the ULTra system
Advanced Transport Systems Ltd Urban Light Transport (ULTra)
Personal Rapid Transport (PRT)
Fleet of low power electric vehicles on dedicated guideway network
Recharges during pickup/drop-off stops
Central control responds to passenger requests, allocates vehicles for journeys as required
On the guideway each vehicle is autonomous, steered by sensors in track and on vehicle
Some free-running (unguided) capability
– Marshalling
– Passenger terminus
Payload up to 500 kg including four passengers and luggage
Speeds up to 40 kph (25 mph)
Minimum turning radius 5 m
Gradients up to 20%
Currently being installed at
Heathrow Terminal 5
Introduction to the ULTra system
For more complete information see ATS website :
www.atsltd.co.uk/prt/vehicle/comparative_info.doc
The Application of Mechanical Analysis
Requirements
Tight turning circle
Accommodate large variation in load magnitude and location
High roll stiffness (vehicle sensors alignment)
Occupant comfort comparable to that of a small passenger car
Steering and drive systems were to provide a linear, well damped, response to control inputs.
Durability
– Repeated acceleration, braking and cornering loads
– “Abuse” cases arising from striking obstacles on the track.
Weighing at berthing stations (using on-board sensors)
– minimal impact on the vehicle operation or design
MBD Modelling - Design and Development
of a Suspension System
Combined Parametric Vehicle and MBD Suspension models
Parametric Whole Vehicle Handling Simulation
Suspension Elasto-Kinematics with MotionSolve
– Develop to meet cascaded targets
– Optimise weighing strategy
– Predict forces from quasi-static and dynamic loads
– Estimate response to track inputs
Optimise motor mounts
Minimise joint articulation angles
…
Technical approach
– Target Attribute and Sub-System Models
Target Attribute Vehicle ModelRepresent all vehicle suspension attributes
Optimized to meet functional requirements
Source of subsystem targets
Parameters only
– Mass and inertia
– Bump / rebound: Steer, Castor, Camber…
– Compliance…
– Damping
– Front and rear roll stiffness
Optimise parameters and cascade
– Pass down as targets to MBD sub-system models…..
‘CarSim Version 6.05’, Mechanical Simulation Corporation
Technical approach – Cascade to MBD
Cascade Example…
Parameterized Whole
Vehicle Model
MBS Suspension
Model
Technical approach – MBD Subsystem Model
MBD ModelDetailed suspension model
Optimise MBD Sub-system model– Best achieve the target properties within a physically
realizable design– Export characteristics from MotionSolve to be readable by
parametric model
Then….
Use (whole vehicle) target attribute model to assess the characteristic parameters exported from the MBD model
‘Motion Solve Version 7.0’, Altair Engineering Inc.
Technical approach - Parametric and MBD
SS Cornering - Roll/g
y = 10.105x - 0.0868
y = 4.8378x - 0.0872
-1.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80
Latac (g)
Ro
ll a
ng
le (
deg
)
Taxi Roll/g VW Roll/g
Roll Angle Vs. Lateral Acceleration
Base run Target run
Develop parametric requirements
Cascade parametric requirements to subsystems
Design and optimise subsystems to meet requirements
Pass elasto-kinematic properties back to parametric model and predict vehicle performance
Compare Parametric
Prediction
To Objective
Vehicle Targets
Technical approach - Parametric and MBD
Evaluate cumulative effects of any compromises
– Rarely exactly achieve all targets!
Highlight parameters giving cause for concern
Study to evaluate the magnitude of change required
to reach acceptable values
Generate modified targets as required
Eliminate “non-starter” designs
Show development potential for design candidates
Target Cascade Practical Example
- Stable predictable steering response
Example Vehicle Level Attributes for Stable Predictable Steering Response
No significant resonant amplification in the range of excitation of the control system
Insensitivity to asymmetric weight distribution (e.g. one heavy passenger at the side of a seat) limited steering “drift” for fixed steering angle.
Stability with sudden loss of tyre pressure
Vehicle level model showed that these attributes were largely
dependant upon the following….
Target Cascade Practical Example
- Stable predictable steering response
Dependant Subsystem Attributes…
Roll control– Including effect of antiroll bars and highly non-linear suspension
stiffness
Optimised roll damping– Matched to pitch damping requirements
Minimal roll steer – matched front to rear– Sub-cascaded to quarter suspension bump-steer characteristics
Self aligning torque– Dependant upon caster angle and KPI etc.
Ackerman angles– Dependant on steering system geometry
Target Cascade Practical Example
- Stable predictable steering response
Bump Steer, Castor and Camber
– hyperstudy to optimize bush geometry
Roll Stiffness – manually tune antiroll bar
Ackerman – Hyperstudy to optimize joint
geometry – target points on curve
Target Cascade Practical Example
- Stable predictable steering response
Dependant Subsystem Attributes… export back to attribute model
Develop feasible design that satisfied the packaging and elasto-kinematic target requirements
Export X-Y data from MotionView
Re-run Vehicle model
– Drift test, swept sine & blow out stability,
Animate sine
95% passenger in rear LH Seat
Fixed steering
0.2m drift in 60mAnimate puncture
Target Cascade Practical Examples
- Ride Comfort on Guideway Joints
Example Vehicle Level Attributes for Ride Comfort
Passenger response acceleration to be below ISO
comfort curve from track excitation.
– Ride Frequencies (and front to rear ride ratios) to be
maintained irrespective of load
Target Cascade Practical Examples
- Ride Comfort on Guideway Joints
Dependant Subsystem Attributes…
Longitudinal compliance compatible with values for typical B/C class passenger car
Particular suspension force Vs
travel curves to be achieved
– Parameterized non-linear suspension stiffness curves specifications created with the
objective of keeping the ride stiffness and the ride ratio near
constant with load
Target Cascade Practical Examples
- Ride Comfort on Guideway Joints
Constant Ride Frequency with Load
Linear
Suspension RateFrequency drops
as load increases
Rising
Suspension Rate
Frequency constant
with increasing load
Target Cascade Practical Examples
- Ride Comfort on Guideway Joints
Loop around cascade….
Optimise elasto-kinematics
Feed back to vehicle level model parmetric model
Achieve vehicle level target…
Load Modelling
Calculate forces to be used in durability calculations
Quasi-static – previously well documented (ref 1)
Steady state examples
– Steady cornering
– Acceleration
– Braking
– …
Dynamics
Transient load
– Obstacle on track
Load Modelling – Dynamic Example
Traversing an obstacle on the track
“Cow catcher” type device sweeps large debris aside
Size of obstacles ridden over depends on ride height
Lightly loaded vehicle traverses largest obstacle
– No bump-stop contact but high damper velocity
Vehicle at GVW only traverses smallest obstacle
– Low damper velocity but heavy bump-stop contact
Load Modelling – Dynamic Example
Moving brick model
– Vary brick size and speed
– Vary suspension settings
– Extract forces at suspension attachments
Load Modelling – Dynamic Example
Moving brick modelExample study:
Effect of varying bump-stop clearance
Note damper force direction
changes as wheel drops
(velocity sign change)
Load Modelling – Dynamic Example
Moving brick model
Load Modelling – Dynamic Example
Moving brick model
Damper force velocity curve
needs high definition near to
velocity reversal
Berth Rail Support System
- Weighing Simulation
Functions:
Steady the vehicle at the berth
Accurately position the height relative to the platform
Facilitate weighing
Model:
Investigate Stability and Accuracy
At berth, vehicle is lifted above
unladen condition by two small
auxiliary wheels, riding on a pair of
short rails.
Conclusions
Combined MBD and Parametric models:
– Efficient approach for the analysis of the dynamics of a vehicle suspension system.
MotionSolve [V7] sufficiently robust to solve whole vehicle model dynamics, but care required defining components with velocity dependencies (i.e. dampers).
Hyperstudy wizard provides a simple way of optimizing complex mechanical systems .
Expression builder is indispensable in the creation of complex output functions
– but it would be useful if MotionView incorporated a reliable method for the calculation of body rotations in a Cartesian axis set, as an alternative to the Eulerian, co-ordinate system.
Report wizard is a very efficient tool for repeat plotting tasks.
V9 MotionView (V7 was used for the work presented in this paper)Incorporating impacts between bodies, will obviate the need for contact approximations involving non-linear springs (e.g. the jockey wheel rail contact). This should also enhance model solution stability.
