15th International Conference on Experimental Mechanics
PAPER REF: 2557
(Invited Paper)
COMPACT VIBRATION ISOLATOR FOR PROTECTING MEMS
OSCILLATOR AND SENSITIVE ELECTRONIC DEVICES
Yan Liu, Hejun Du(*), Li King Ho Holden, Neo Mingfeng
School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore
(*)
Email: MHDU@ntu.edu.sg
ABSTRACT
This work proposes two kinds of compact PCB-level vibration isolators (VIs) to protect the
MEMS oscillator and sensitive electronic devices in harsh environment from external
vibration and shock. The proposed VIs were fabricated with electric discharge machining and
studied with experiments and simulations. Apparent vibration isolation effects were observed
with the experiments and simulations.
INTRODUCTION
Vibrations or shock can excite the resonant frequencies in circuit articles such as PCB,
electronic components, MEMS oscillator, etc. The excited resonant frequencies can increase
the phase noise and the drift of oscillators or degrade the precision of RTD. Electrical traces
on the PCBs can also be affected by vibrations or shock negatively. Widely used soft
elastomeric mounts tend to stiffen and gain damping at low temperatures and to soften and
lose damping at elevated temperatures. The variations make them practically impossible to
keep optimized configuration [1,2]. Furthermore, aging may degrade the performance of an
elastomer VI. Many researchers focused heavily on vibration isolation for electronic circuit
boards in the past decade. Veprik and Babitsky suggested that traditional vibration isolation
design uses damped vibration isolators heavily and proposed to focus on the dynamic
properties and responses of the critical internal components of an electronic device [3]. Robert
et al produced a paper on the detail of high frequency vibration isolation of MEMS sensors
for aerospace application [4]. Robin et al published a paper on simplifying printed circuit
board by using a “smeared” approach for sensitivity analysis [5]. His work shows that there is
great interest in simplifying the effect of vibration on electronic circuits for future researches.
In the paper, two small form factor metal isolators for isolating PCBs from vibrations for high
temperature and narrow space were designed, fabricated, and studied to protect MEMS and
electronic devices in harsh environment.
DESIGN AND FABRICATION OF PCB VIBRATION ISOLATORS
Since the devices may work in constant high temperature environments, two major
considerations are taken into account when designing the vibration isolator. First, the
vibration isolator can be operated in the temperature range of -40oC to 125oC. Second, it
should achieve effective vibration isolation while maintaining small physical size. Rubber is
commonly used for vibration isolator. However it will degrade in the required temperature
range. Hence stainless steel 316 (SS316) is chosen to meet the temperature requirement.
ICEM15
1
Porto/Portugal, 22-27 July 2012
Sometimes, external vibrations and shock may transmit through the device housing to the
MEMS and electronic devices on print circuit boards. To isolate the external vibrations and
shock, two kinds of compact mechanical structure, ε-shape and J-shape, were designed,
fabricated, and assembled as shown in Fig. 1. Both structures suspend a PCB and isolate the
external vibration. The thicknesses of the structures are 0.3mm and were fabricated by
electrical Exceltek V850 Wire-cut Electric Discharge Machining (EDM) with a 0.25mm
diameter wire.
Fig. 1 Photo of the VI platforms
FINITE ELEMENT ANALYSES
FEA is performed to the design in order to simulate the performance of vibration isolator
using a low cost and time efficient method. The isolation isolators were modelled with finite
element method. Modal analyses as shown in Fig.2 were performed to find the fundamental
resonant frequencies of the ε-shape and J-shape VIs. To achieve apparent vibration isolation
effects at low frequency, the ε-shape VIs should be designed with large size or thin thickness.
Thus, the following studies focus on the more compact J-shape VIs. This is achieved by
employing four cantilevered beam supports around the four edges of PCB to achieve four
spring supports which will provide vibration isolation effect. Design requires the four ends of
the beam vibration isolator to be rigidly fixed at its base. At its fundamental frequency, PCB
will vibrate in phase with the flexural beams as shown in Fig 2.
Fig. 2 FEM modal analysis of the VI platforms
When external vibration increases beyond its fundamental frequency, vibration isolation
effect will be achieved when PCB vibrates out of phase with the beams. This results in a
cancellation of vibration which then successfully isolates PCB from the influence of external
vibration as shown in Fig 3.
2
15th International Conference on Experimental Mechanics
Fig. 3 Vibration isolation effect
In order to analyze how the length of the beam can affect the vibration isolation effect, 3
samples of different beam length are designed and fabricated as shown in Table 1.
Sample
Length of Beam (mm)
A
8
B
10
C
12
Table 1: Samples of vibration isolator
Modal analysis is performed to investigate on its fundamental frequency and mode shape.
Result is shown in Table 2.
Sample
Fundamental Frequency (Hz)
A
500.86
B
374.95
C
293.75
Table 2: Result of fundamental frequency from FEA
Harmonic Analysis is performed to investigate on its vibration isolation effect. The analysis
result is used to compute for its transmissibility using the formula:
For effective vibration isolation, transmissibility should be less than the value of 1 as it
represents that the displacement of PCB is less than the displacement caused by the external
disturbing vibration.
Using this formula, result of the harmonic analysis is calculated as shown in Table 3 and
plotted in Fig. 4.
Sample
Effective Vibration Isolation Region
A
> 560 Hz
B
> 400 Hz
C
> 280 Hz
Table 3: Result of effective vibration isolation region from FEA
From the result of transmissibility, significant vibration isolation effect can be observed. This
observation clearly shows the effectiveness of vibration isolator design to protect oscillators
embedded on PCB from external mechanical vibration and improve its mechanical vibration
stability.
ICEM15
3
Porto/Poortugal, 22-227 July 2012
Fig. 4 Result of traansmissibility solution from
m FEA
EXPER
RIMENT VALIDATI
V
ONS
Eexperiiments are designed
d
to investigatee how the leength of beeam affects the perform
mance of
vibratioon isolator, and match experimental result with
w FEA reesult to verrify the accu
uracy of
FEA moodel for futuure. Polytecc PSV300 Laser
L
Dropp
pler Vibrom
meter (LDV)) is used to perform
experim
mental modaal analysis where the resonance frequenciess and the coorrespondin
ng mode
shapes can
c be measured and visualized
v
w
while
Bruel and Kjaer Mini
M Shakerr Type 4810
0 is used
to proviide vibratioon for experrimental anaalysis throu
ugh a mountting structuure as shown
n in Fig.
5.
LDV Head
LDV
V controller
DUTT
Functtion Generator
Platform
Shaker
Pow
wer Amplifier
Fig. 5 Setup
S
of expeeriments
mental modaal analysis and
a harmonnic analysis are perform
med to obtaiin the result as
Experim
shown in
i Table 4.
Samplle
damental
Fund
Frequency (Hz)
Effective Vibration
Isolation Reegion
A
4
480.0
> 680 Hzz
B
3
367.2
> 500 Hzz
C
2
274.2
> 380 Hzz
Table 4: Result
R
of funddamental frequuency and efffective vibratio
on isolation reegion from expperiment
4
Editors:
E
J.F. Silva Gomess and Mário A.P. Vaz
115th International Conferrence on Expperimental Mechanics
M
The mode shape att fundamenttal frequenccy obtained in experimeent, as show
wn in Fig. 6,, also
F
solutionn.
matchess with the FEA
Fig. 6 Modde shape of funndamental freequency from experiment.
Based on
o the modde shape obbtained in experiment
e
, the fundaamental freqquency obttained in
experim
ment is of thhe same moode as FEA solution ass shown in Fig.
F 7. Thuus the resultts can be
compareed with each other to verify
v
for acccuracy.
Furtherm
more, as booth the experimental result
r
and th
he FEA sollution matcches well with
w each
other, thhe experimeental result has
h also verrified the acccuracy of FEA
F
modeliing.
Transmissibility
100
0
C
Comparison
betw
ween Experimentt and FEA Transsmissibility for sa
ample 3
10
0
2K
1K
K
3K
1
4K
5K
6K
7K
Frequency
0.1
1
0.01
Finitte Element Analyysis (FEA)
0.001
Exp
periment
0.0001
Fig. 7 Transm
missibility of VI
RESUL
LTS AND CONCLUS
C
SIONS
The beaam design of vibratioon isolator has successsfully achiieved clear vibration isolation
i
effect which
w
is neccessary to maintain
m
freequency stab
bility for addvanced cloocking device when
used in harsh mechanical vibbration environment. The
T result not
n only prooves that su
uccessful
vibratioon isolation can be achhieved withh SS316 wh
hich can peerform undeer high tem
mperature
conditioon, it also demonstrate the possibilities of min
niaturizationn vibration isolator to keep the
complette package of
o advancedd clocking device
d
small.
With veerification on
o the accurracy of FEA
A solution, the
t study onn how differrent parameeters can
affect thhe performaance of vibbration isolaator gives a good undeerstanding tto the best possible
approacch to minim
mize the packaging
p
s
size
while achieving satisfying vibration isolation
i
perform
mance for fuuture advannced clockinng device. From
F
the trrend study, it is propo
osed that
reducingg the widthh of the vibrration isolattor can achieve miniatturization w
while yieldin
ng better
vibratioon isolation performancce. The succcess in thee FEA and experimentt setup of vibration
v
isolator is of great usefulness
u
t future opptimization of
to
o VI.
ICEM155
5
Porto/Portugal, 22-27 July 2012
REFERENCES
Peter A. Engel , Structural analysis of printed circuit board systems, Springer, 2000
A.M. Veprik, vibration protection of critical components of electronic equipment in harsh
environmental conditions, (2003) 259(1), 161-175
A.M. Veprik, vibration protection of sensitive electronic equipment from harsh harmonic
vibration, (2000) 238(1), 19-30.
R. Dean et al, "Vibration Isolation of MEMS Sensors for Aerospace Application,"
International Conference on Advanced Packaging and Systems, v. 4828, pp. 166-170, Mar
2002.
R. A. Amy, "Sensitivity Analysis of Simplified Printed Circuit Board finite element model,"
Microelectronics Reliability, v.49, pp. 791-799. Jul. 2009
6