1. CAE Simulation of a Power Liftgate Subsystem for an SUV
Vishwabharathi M. Santosh K Swamy Umesh Nayaka
Senior Engineer, CAE-Closures
Closures Lead Engineer, CAE-Closures Senior Group Leader, CAE-Closures
Leader
GMTCI GMTCI GMTCI
Bangalore- 560066 Bangalore-560066 Bangalore-560066
Bangalore
Abbreviations:
FEA - Finite Element Analysis
DESVAR-Design Variables
DESOBJ-Design Objective
DCONSTR- Design Constraints
Abstracts
This paper presents the work done on the t typical Liftgate subsystem of a sport utility vehicle (SUV) which has two variants; a
manual operated liftgate and a power actuated liftgate, with the liftglass assembly. In addition to the usual loads the manually
manua
operated liftgate experiences, the power liftgate has the power loading exerted by the struts, which are usually higher in magnitude.
iences,
This loading is a combination of gas strut and actuator loads. A load matrix is developed specific to each vehicle & type of power
s
system used. The matrix will include combinations of normal operation, reversal, & obstacle detection conditions. Also included will
he
be variations on temperature & grade. There can be more than 30 load cases contained in the load matrix. Some of the loading loadin
conditions may be conflict with respect to others. It is the responsibility of the analyst to evaluate each loading condition using FE
s FEA
judgment.
The work highlights the optimization of power liftgate subsystem for weight reduction. The optimized sub-system not only met the
sub
performance requirements for the power loadcase It also ensured that mass is below the baseline mass of the liftgate. A worst
loadcase.
power load case is considered for the optimizations using technique available in HyperWorks® OptiStruct. There is approximately
e OptiS
1.5 kg of weight reduction compared to baseline design.
Introduction
The liftgate subsystem that is used for the study is a typical system with a power liftgate and a manual
iftgate
liftglass assembly.
a. Power Liftgate Systems: Activation of the system completely opens the liftgate
Systems
until it comes to a stop. Reactivation of the system closes the liftgate in the
latched position.
b. Manual Liftglass Assembly: The Liftglass is Unlatched, pulled open, allowed to
,
auto-rise, and then slammed to close.
rise,
The liftgate subsystem in an SUV is typically manual. In this vehicle, power liftgate has been made
optional. Same subsystem assembl is used for both manual and power liftgate variants. The liftgate
ame assembly
should meet the performance targets for both cases and should not be over designed for manual system.
should
The loading is a combination of gas strut and actuator loads, in which a load matrix is developed specific
to each vehicle & type of power system used. The matrix will include combinations of normal operation,
reversal, & obstacle detection conditions. Variations on temperature & grade are included. There can be
acle V
more than 30 load cases contained in the load matrix. Some loading conditions may be in conflict with
respect to others. The worst load case is considered for further study. The grade (also called slope,
consider
incline, gradient, pitch or rise) refers to the amount of inclination of the road surface to the horizontal. This
)
grade is included in the gravity card.
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Simulation Driven Innovation

2. This work highlights the optimization of the liftgate subsystem to meet the performance targets as well as
mass reduction of the power liftgate. Inner panel, reinforcements and plates (figure1a) are considered as
figure1a)
design variables. Displacement and stresses are considered as design constraints. The existing liftgate is
acement
of Aluminum and satisfies most of the static and dynamic performance targets of the manual liftgate. In
target
order to satisfy the performance target of the power liftgate (where the loading magnitude is 3 times
®
higher than that in the manual case) the optimization techniques available in HyperWorks OptiStruct
technique
were used to get the effective thickness for reinforcements and plates to meet the required criterion
ctive
without increasing the weight of the existing design.
Analysis Phase
Geometry and FE Model
A OptiStruct finite element (FE) model was employed ,which consists of fully trimmed liftgate with inner
truct
panel, outer panel, strut bracket, strut plates, hinge reinforcements ,hinge assembly, glass assembly,
hinge assembly
liftgate interface components such as latch–striker assembly, contact wedge and slam bumpers
latch striker
(figure 1a) and liftglass interface components such as latch–striker assembly, contact and slam bumpers.
latch striker bumpers
Outer and inner panels are connected with adhesive. Hemming is done along the edge of the outer and
inner panels. Hinge reinforcement is connected with inner panel through spot welds. Hinge assembly is
bolted to inner panel and hinge reinforcement. Hinge pivot is modeled in such a way that liftgate rotates
freely about hinge axis. Glass hinge pivot is in-line with the liftgate hinge and the glass hinge is modeled
in such a way that liftglass rotates freely about the common hinge axis.
at
Figure 1a: Liftgate assembly
a: Figure 1b: Reinforcements and plates
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Simulation Driven Innovation

3. Process Flow:
Steps for Topology and gauge optimization are shown in figure 2.
Figure 2: Process flow chart for Analysis Phase
Definition of Optimization Parameters:
Topology and Gauge optimization technique is selected in order to get the optimal design and optimal
thicknesses to meet the performance target and for significant weight reduction. In the topology
optimization, design variable is inner panel thickness, which is defined by DESVAR card and related to
PSHELL properties. The objective function of the present work is to minimize the volume of inner panel
which can be defined by DESOBJ card. Deflections and volume responses is created through response
responses
card. Ball stud deflection is design constraints which can be defined by DCONSTR card. Thickness of the
inner panel is varied from 10 mm to base thickness. Base thickness is taken as the thickness of the
manual liftgate. Problem definition for topology optimization is described in the table 1.
1
For gauge optimization, thickness of reinforcement and plates is the design variable which is defined by
DESVAR card and is related to individual PSHELL properties can be defined by design variable property
relationship card. The objective function of the present work is to minimize the weight of reinforcements
and plates which can be defined by DESOBJ card. Deflections and stress responses are created through
response card. Displacement and stress are the design constraints which can be defined by DCONSTR
ment
card. Thickness of the reinforcement and plates is varied from base thickness of 2 mm and 3 mm
respectively. Base thickness is taken from the manual liftgate. Problem definition for gauge optimization is
described in the table 2 for each load case.
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Simulation Driven Innovation

4. Design Objective
Design Gage
Responses
Variables Range Constraints Function
10 mm to Minimize
Power Inner panel Displacement Displacement
Base the
Loading Thickness and Volume < 1mm
Thickness volume
Table 1: Problem defination for Topology optimization
Design Objective
Design Gage
Responses
Variables Range Constraints Function
Base Displacement
Power Reinforcements Displacement Minimize
Thickness < 1mm
Loading and Plates and Volume Volume
to 2mm Stress < YS
Combined
Base Displacement
(Power Reinforcements Displacement Minimize
Thickness < 1mm
load + and Plates and Volume Volume
to 3mm Stress < YS
Gravity )
Table 2: Problem defination for Gauge optimization
Results & Discussions:
Topology and gauge optimization is performed separately on Inner panel and reinforcement, plates
respectively. This is done because increasing thickness favors gravity loading, so no significant reduction
in ball stud displacement. This study tells which area on the inner panel need to be stiffened. Figure3
areas eed
shows the inner panel mass distribution and critical area that need to be improved. Based on this study
the changes are done on the reinforcement that is merging the 2 reinforcement into one shown in figure4
which meets the performance targets on power liftgate. For mass reduction the scope was to perform
further gauge optimization on the reinforcement and the hinge and strut plates. The optimized thickness is
shown in the table 3. Hence gauge optimization for power load and combined load (powerload + Gravity)
conditions is performed and effective thickness values are obtained. There is almost 1.5 kgs of weight
o
reduction in the reinforcement and plates without deviating from both the manual and power liftagate
performance.
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Simulation Driven Innovation

5. Figure 3: Mass Distribution and area need to be stiffen Figure4: Changes done on reinforcements
Current
Base
Components Optimized
Design
Gauge
Reinforcement 3.0 mm 2.1 mm
Plates 4.5 mm 3.5 mm
Table 3: Thickness comparsion for base design and current optimized design
Conclusions & Remarks:
Using topology optimization technique on inner panel, it was possible to increase stiffness to meet the
performance targets of power liftgate without affecting the performance of the manual liftgate.
Using this technique one component is eliminated from the total assembly, by which the manufacturing
y,
cost is saved.
Using gauge optimization technique, approximately 1.5 kgs of weight is reduced in the liftgate without
deviating from both the Manual liftgate and power liftgate performance.
ACKNOWLEDGEMENTS
The author would like to thank Mr. Elliot Deavasagayam (EGM - Body & Exterior Engineering, GMTCI),
Mr. Edward J Sizen (Director - Vehicle Systems) and Dr. Rao M Chalasani (GM India Director,
Director
GM India Advanced Vehicle Development), for giving us the opportunity to work on this challenging
project and to present this paper in Altair user conference. We would like to thank our colleagues for their
valuable support.
REFERENCES
[1] HyperStudy 10.0 Manual.
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Simulation Driven Innovation