Project #3:Design of a MEMS Vertical

Actuator

Jianwei HengAlvin Tai

ME128 Spring 2005

Introduction

The MEMS vertical actuator described in this project consists of a center mass with a flat surface and is supported by simple flexures arranged symmetrically around the center mass. The device uses electrostatic comb structures to displace the flat surface.

IntroductionDesign constraints

Active Constraints Minimum width of structure 5m Minimum gap 5m Thickness of the device 50m Minimum diameter of center mass 200m E =160 GPa =2.33 gm/cm3

Maximum DC input voltage = 30 V Maximum die area, Ad 10mm2

g

NtVF

20

IntroductionDesign Constraints

Inactive Constraints Minimum displacement under max voltage: 20 m Maximum DC acceleration survival, amax 2,000g

Maximum stress in suspension, max 1.6 GPa Bandwidth of the device 1 kHz (resonance

frequency)

Approach to Problem

Observations To fully maximize mass displacement,

it is desirable to design a system with: Minimum spring constant, k

By using a circular center mass, we can maximize the spring length/constant

Maximum amount of combs, N Maximum overlapping comb distance, t

Approach to Problem

From the force equation, We can easily observe that it is a function of N and t: F = f(N,t)

With our area constraint active, there is a direct effect on one variable when the other is modified (as t is decreased, N can be increased and vice versa). Therefore, a compromise must be realized.

g

NtVF

20

Approach to ProblemRelationship of N and t

Overlap distance, t

Nu

mb

er o

f c

om

bs

, N

If t is designed too large, the combs may deflect. Conversely, if t is too small, the gaps between the combs become significant and limits the number of combs in our system.

t 200 m >> 5 m

t>>5m

t 5m

Approach to Problem1. Used MATLAB to get a preliminary analysis of

the design

Calculated k using Euler-Bernoulli Beam Theory (guided/fixed beam)

Optimized comb overlap, t Calculated displacement

2. Built the model in Solidworks to verify that it

satisfies all constraints

Used static and frequency analysis to check constraints

Approach to ProblemFormation of final design

We initially set the thickness = 50 m as an active constraint, but that resulted either in deflections that are too small, or resonant frequencies that are too small.

Since actuating a maximum of 20 m requires the thickness to be 30 m (to allow for significant comb overlap when fully displaced), we changed the thickness constraint from active to inactive!

Then we redesigned the actuator with thickness = 30 m

Final Design

Final Design of the Actuator

Final Design

Final Design of the Substrate

Final Design

Spring and Center mass Combs design

Final Design

Deflection when 30V force applied

>20um

Min DC displacement underMax DC voltage

SATISFIED

Final Design

Max Stress when 30V force applied

>150MPa<1.6GPa

Max Stress in suspensionMin Deformation stress (Part B)

SATISFIED

Final Design

Max Stress points

Final Design

Max Stress when 30V force applied

Max DC acceleration survival

SATISFIED

<1.6GPa

Final Design

Resonant Frequencies for Modes I to V

Approach to Problem

Although the resonant frequency requirement fails in Mode I, the actuator is unlikely to exhibit this characteristic motion due to the forces applied.

Hence we can rule out Mode I.

Approach to Problem

More importantly, Mode II resonant frequency = 1244.2Hz > 1kHz.

Since the motion in Mode II is the most likely one, we can conclude that the actuator will fulfill the resonant frequency requirement stipulated.

Resonant Frequency Requirement

SATISFIED

Final DesignFEM constraints

Displacement (satisfied) Maximum acceleration shock test (satisfied) Maximum Stress (satisfied) Minimum Deformation Stress, Part B

(satisfied) Resonant Frequency (satisfied)

Concerns Base structure analysis: instability issues

involving comb contact