Automotive RKE Fob Battery Terminal AnalysisFEA Evaluation Study Using ANSYS Mechanical APDL

Premchand GunachandranMechanical Engineering Graduate Study

University of Wisconsin- Milwaukeegunacha2@uwm.edu

Drew PedenMechanical Engineering Undergraduate Study

University of Wisconsin- Milwaukeedpeden@uwm.edu

Abstract— The automotive RKE fob1 battery terminal analysis will be evaluated using the ANSYS mechanical APDL2

simulation software tool. There will be two components soldered to the key fob printed circuit board called positive and negative terminals. The components were designed using Creo PTC3 CAD modeling software and will be imported as IGES format4 into ANSYS. The terminals are intended to be always in contact with a coin type battery5 to get power for the normal operating signal from RKE fob. The purpose of this analysis is to observe and evaluate the stress distribution in the terminal models by applying the enforced displacement caused by the placement of a coin battery into RKE fob. The terminal von Mises and principal stresses will be evaluated for possible improvement in design.

Keywords— Fob; Terminal; Positive; Negative; displacement; von Mises; Principal Stress

I. REMOTE KEYLESS ENTRY FOB DESIGN OVERVIEW Remote keyless entry fobs are widely used electro-

mechanical security products, see the below “Fig.1”, used, for example, to lock and unlock the car doors and trunk by pressing the required buttons on the fob.

Fig. 1. Remote keyless entry of a fob

These mechanical button actuators trigger the electronic embedded hardware switches to generate the software codes to enable the radio frequency6 (RF) signal thus authentication of fob signal matches with RF hub and body control module of the car.

Once the signal codes have been authenticated between the fob RF signal and RF hub, the car door will be opened. Once the driver inserts the key into the ignition module to start the car the low frequency (LF) signal from key fob has to challenge and authenticate with the LF antennas within the car which have a range of around two meters radius in coordination with body control module of car. Once the RF & LF coil transponder7 signal codes are authenticated successfully, the car can be started. Otherwise the car can’t be started without challenging the RF & LF Signal.

II. REMOTE KEYLESS ENTRY FOB SIGNAL FAILURE PROBLEM

A. ManufacturingThe production assembly plant found rejections through the

end of line tester because of RF signal failure issues. Manufacturing has requested that Product Design reviews the

RKE fob design to control the RF signal failure rates. The RF component is an electrical component soldered into a printed circuit board (PCB). The entire PCB is enclosed within the mechanical plastic housing components called upper and battery cover housings. “Fig. 2” shows the LF Transponder coil soldered into PCB. The housings sandwiches the PCB by snapping together.

Fig. 2. Printed circuit board with terminal

B. Product Engineering DesignThe design team cross checked the various failure modes

for the fob signal failure problem. One of the potential failure modes is considered to be the fob battery (cr2032). The Panasonic coin battery is not supplying sufficient power for fob button activation. Thus there will be no RF signal from the fob which leads to a dysfunctional product. Tolerance stack up analysis calculation shows that there is 0.98 mm of interference between the PCB terminal and battery when the fob is in a fully assembled condition. Thus the coin battery deflects the terminals contacts by 0.98 mm inside the assembled fob.

III. FEA PROJECT OBJECTIVE AND SCOPE

To evaluate the failure of the RF signal when the fob button is activated during normal operating conditions.

The reason for this failure is because of coin battery doesn’t provide the power to the PCB electronic system inside the fob. It is evident that in assembled condition the coin battery provides aggressive enforced displacement over the terminals. The product engineering design team arrived to the root cause for this failure by analyzing the several failure modes.

A. ObjectiveTo evaluate the structural strength of the battery terminals

(positive terminal & negative terminal) by deflection.

B. Project Scope To determine whether positive terminal will withstand deflection of 1.09mm is applied on it by displacing the coin battery.

To determine whether negative terminal will withstand deflection of 1.57mm is applied on it by displacing the coin battery.

IV. FEA EXCLUSIONS AND INPUTS REQUIRED Terminal geometry design and its engineering has been

done by design team previously using creo 3d cad modeling techniques as per design requirement.

A. ExclusionsNo physical tests has been carried out.

B. Inputs Required CAD models see “Fig.3” (positive

terminal.igs, negative terminal.igs) formats translated from Creo CAD software.

Fig. 3. Printed circuit board with terminals and coin battery

Fob PCB Battery terminal design functional study.

Material properties as per design requirement.

V. MATERIAL PROPERTIES As per the automotive quality PCB terminal design

requirement stainless steel “301 ¾ hard” material properties have been considered, see the “Table I” for more details.

TABLE I. TERMINAL MATERIAL PROPERTIES

Component Name

Stainless Steel 301 ¾ Hard

Young’sModulus

(Mpa)

Poisson’sRatio

Yield Strength(Mpa)

Ultimate Strength

(Mpa)

Percentage of

Elongation (%)

Terminals Positive and Negative

190000 0.29 931 1207 12

VI. UNITS FOLLOWED See “Table II” for units followed for this FEA project.

TABLE II. UNITS FOLLOWED FOR TERMINAL FEA

Dimensions UnitsForce N

Dimensions UnitsLength mmStress Mpa

VII. BOUNDARY CONDITIONS AND LOADS CALCULATIONS

The terminals are soldered to pcb surface thus the terminals are part of pcb assembly. The soldering location of terminals in pcb done by pcb manufacturer.

A. Boundary ConditionRefer to “Fig.4” for the positive and negative terminals

components boundary condition detail. The terminals pcb mounting surface has been fixed constraint in all degrees of freedom direction.

Fig. 4. Terminal componets boundary condition

B. Loading Condition Calculations Refer to “Fig.5” for the terminals positive and negative components load condition details.

Enforced displacement (1.57mm) is applied on the positive terminal surfaces (highlighted in red color) in radial x-direction.

Enforced displacement (1.09mm) is applied on the negative terminal surfaces (highlighted in red color) in z direction.

Fig. 5. Terminal componets loads condition

Positive Terminal

Negative Terminal

Coin Battery

Negative terminal pcb mounting surface highlighted in green is fixed in all degrees of freedom

Positive terminal enforced displacement is applied on highlighted surface in red color radially x direction

Positive terminal pcb mounting surface highlighted in green is fixed in all degrees of freedom

VIII.METHODOLOGY APPROACH AND SOFTWARE USED The terminals thickness is 0.3 mm as per the design and it

is flexible as per design requirement because of coin battery contact. Terminal is light weight component and small in size geometry.

Geometry cleanup has been done on the model de-featuring small fillets without compromising the interested region.

Second order tetrahedral elements are used to build the finite element model.

Material properties are used as mentioned in “Table I”

Loads and boundary conditions are applied as mentioned in “Fig. 4” and “Fig. 5”.

Finite element analysis software used for terminal evaluation is ANSYS mechanical APDL for activities pre-processing, analysis and post-processing.

IX. ASSUMPTIONS The terminals are under compression inside the assembled

fob because the coin battery displaces the terminal contact areas thus stack up of assembled fob components pushes the coin battery against the terminals which causes the terminal deflection.

Only components of interest are considered based on the loading conditions.

Assumed that bonded contacts are defined between the terminals and PCB

Linear material properties are considered for SS 301 ¾ hard material.

No temperature effects considered for analysis.

Yield stress of the material is considered as assessment criteria.

X. MESH SENSITIVITY STUDY Refer to “Fig. 6” for mesh sensitivity study details.

Regarding the positive terminal, the behavior of the mesh sensitivity8 graph is slightly different than for the negative terminal. The max fine mesh element size is 0.1 mm for positive terminal converged around 7900 MPa.

Fig. 6. Mesh sensitivity study graph

10-8.43 = 1.57mm

Center of coin battery

8.43

1

Negative terminal enforced displacement is applied on highlighted surface in red color z direction

Gradually the element size was incremented to 0.5 mm max coarse mesh. It can be observed that the higher the element size the von Mises stress is increasing abruptly. The reason for the positive terminal mesh sensitivity plot may be the imported geometry into ANSYS mechanical APDL.

With respect to the negative terminal the mesh sensitivity graph is somewhat non-linear approaching the fine mesh element size of 0.1 mm, however it was decided to use the 0.1 mm element size results for reported observations. Meshing type used is tetrahedral elements are used to build the finite element model for both terminals.

XI. FEA RESULTS EVALUATION AND VERIFICATION The positive and negative terminals displacement, von

Mises stress, and principal stress plots will be discussed below in detail.

A. Positive Terminal Finite Element Analysis Results Refer to “Fig. 7” for the positive terminal resultant displacement (mm) loading condition. The maximum displacement is 1.57 mm.

Fig. 7. Postive terminal resultant displacment loading condition

Refer to “Fig. 8” for the positive terminal von Mises stress (MPa) loading condition at two critical locations (A and B) as per design functionality. Lighter blue region shows the stresses above the yield strength of material (931 MPa).

Fig. 8. Postive terminal von Mises stress loading condition

Refer the “Fig. 9” for the positive terminal maximum principal stress (Mpa) loading condition at two critical locations (G and H) as per design functionality. Lighter blue region shows the stresses above the ultimate strength of material

A

B

(1207 Mpa). Locally permanent deformation occurs in the highlighted boxes.

Fig. 9. Postive terminal maximum principal stress loading condition

B. Negative Terminal Finite Element Analysis Results

Refer the “Fig.10” for the negative terminal resultant displacement (mm) loading condition. The maximum displacement is 1.09 mm.

Fig. 10. Negative terminal resultant displacment loading condition

Refer the “Fig.11” for the negative terminal von Mises stress (MPa) loading condition at two critical locations (C and D) as per design functionality. Lighter blue region shows the stresses above the yield strength of material (931 MPa).

Fig. 11. Negative terminal von Mises stress loading condition

G

HC

D

Refer the “Fig.12” for the negative terminal maximum principal stress (MPa) loading condition at two critical locations (E and F) as per design functionality. Lighter blue region shows the stresses above the ultimate strength of material (1207 MPa). Locally permanent deformation occurs in the highlighted boxes.

Fig. 12. Negative terminal maximum principal stress loading condition

XII. CONCLUSION AND SUMMARY OBSERVATION

Regarding the positive terminal for a single cycle of loading by displacing the coin battery against the terminal, the positive terminal locally crosses the ultimate strength of the material shown in “Fig. 8” for von Mises stress and in “Fig. 9” for max principal stress. There may occur some permanent deformation or fracture at those highlighted areas of design functional areas.

Regarding the negative terminal for a single cycle of loading by displacing the coin battery against the terminal, the positive terminal locally crosses the ultimate strength of material shown in “Fig. 11” for von Mises stress and in “Fig. 12” for max principal stress. There may occur some permanent deformation or fracture at those highlighted areas of design functional areas.

Based on these observations it is suggested that the design of the terminals be modified at the critical functional areas as per the fit, form, and function. Alternatively, a change in the hardness of the terminals should be considered; a lower hardness might result in an acceptable deformation but not fracture.

ACKNOWLEDGMENT The FEA project team would like to thank Strattec

Security Corporation mechatronic product engineering group for permitting to use the terminal design geometry model for performing the academic FEA project successfully.

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