103033525 Mid-term

1. LONG STROKE OUT-OF-PLANE ACTUATOR USING COMBINATION OF ELECTROSTATIC AND PNEUMATIC FORCES MEMS 2014 Tetsuo Kan1, Akihiro Isozaki1, Hidetoshi Takahashi1, Kiyoshi Matsumoto2, and Isao Shimoyama1, 2 1Department of Mechano-Informatics, Graduate School of Information Science and Technology, The University of Tokyo, Tokyo, Japan 2IRT Research Initiative, The University of Tokyo, Tokyo, Japan Presenter: Chung-Wei Huang 2015.04.28
2. OUTLINE Introduction • electrostatic actuation • pull-in effect • pneumatic actuation Design Actuation procedure Fabrication& Experimental setup Experimental results Conclusion
3. INTRODUCTION-1 • Advantages: 1.Low power consumption 2.Fast response • Disadvantage: Pull-in effect→ limit stroke length  Electrostatic actuation
4. INTRODUCTION-2  Electrostatic actuation http://www.memsolver.com/Help/tochtml/combdr.htm In-plane Out-of-plane moving parallel to the substrate moving vertical to the substrate e.g. comb actuators e.g. vertical comb Jin-Chern Chiou et al, 2007 Tetsuo Kan et al, 2013
5. INTRODUCTION-3  Pull-in effect k x y w g deflection constantspring F:forcespring V )-(g A 2 1 F:forceticelectrosta s 2 2e     δ k kδ   Fs Fe deflection force Increasing voltage
6. INTRODUCTION-4  Pneumatic actuation Microvalve ActuatorMicropump G. B. Lee et al, 2006 Armin Werber et al, 2008Fundamentals and Applications of Microfluidics
7. DESIGN  Spiral structure  Electrostatic force +pneumatic force
8. ACTUATION PROCEDURE  Instability 1.Electrostatic actuation →pull-in effect 2.Pneumatic actuation → fluttering
9. FABRICATION & EXPERIMENTAL SETUP  Fabrication  Experimental setup
10. Altering pressure Altering voltage EXPERIMENTAL RESULTS-1  Displacement measurement 103 𝜇𝑚
11. EXPERIMENTAL RESULTS-2  Dependency on the spiral dimensions s1:Spiral without a disk s2:Disk diameter 60 𝜇𝑚 s3:Disk diameter 80 𝜇𝑚 s4:Disk diameter 90 𝜇𝑚
12. CONCLUSTION • This work presents a new idea using two types of actuation to drive the structure simultaneously and get a large stroke length. →maximum displacement: 103 μm. • The fabrication of the actuator using SOI process and verify the results of the concept of the new driving type. • Application: optical systems.
13. THANK YOU.
14. 2 2 22 V )-(g A 2 1 V C 2 1 )CV 2 1 ( Work F             xxx 2 )-2k(g A   δ →curve gk k 3 1 )-(g AV 0 )-(g AVFnet 3 2 3 2           →find max →relationship between gap and deflection kδ 2 2net V )-(g A 2 1 F  
15. EXPERIMENTAL RESULTS-3  Mechanical resonance Experiment Simulation

Editor's Notes

Good morning. I’m .
The topic I want to talk today is a long stroke out-of-plane actuator using combination of electrostatic and pneumatic forces.
This paper is from the university of Tokyo, and published in MEMS 2014.
I will give some introduction about the two actuations.
And, introduce the design, actuation procedure.
Also, some unstable situations that we have to consider.
Finally, I will show the results and give the conclusion.
Lots of actuations in MEMS system use electrostatic way to actuate the device.
The advantages of electrostatic actuation are low power consumption, fast response, simple structure and easy to fabricate.
However, it has an important drawback called pull-in effect.
Pull-in effect limits the stroke length of the actuator.
But there still some applications need long stroke length actuator such as inductors and optics system, so develop a long stroke actuator is an important work.
Electrostatic actuation can divided into two categories: in-plane and out-of-plane.
In-plane type is moving parallel to the substrate.
Like most of comb actuators use as accelerometer and resonator.

Out-of-plane type is moving vertical to the substrate.
In micromirror system we need an actuator that can cause large rotation angle and vertical displacement.
Out-of-plane type can achieve the need of long-stroke length and increase the dynamic range.
So in order to have a long stroke length comparable to the size of the actuator itself, they use the out-of-plane type and try to combine pneumatic force with electrostatic force.
So, what is pull-in effect?

The left is a schematic diagram of a parallel plate capacitor.
The spring comes from mechanical structure.
As we increase the voltage applied across the capacitor , the gap between two plate decrease.
The decrease in gap tends to pull the plates closer.
Through the calculation we can know that the deflection cannot over one third of the initial gap, or the system becomes unstable and collapsed.
The reason for this is that the restoring force of the spring increases linearly with deflection, but the electrostatic force goes as square.

Now we look at the right graph, the orange line is electrostatic force and the blue line is spring force.
Through the graph, we can obviously see that when voltage goes up, the electrostatic force will be larger than the spring force and pull-in happens.

In actuator, pull-in effect is disadvantage. But in other application, pull-in can be useful, for example switches.
Switches use pull-in as an on-off function. If you are interested in this, you can find reference on the Internet.
Pueumatic actuation mostly use in microfluidics devices such as microvalve, micropump and micromixer.
Also, some people use pneumatic combine with thermal actuation to actuate the micromirror.

Different from these application, this paper use a new way to actuate the structure.
Their design is an open-type and do not have PDMS as a membrane.
Here is the design.
They design a structure has a 5-turn spiral.
The spiral formed on a suspended Si membrane.
A counter electrode is placed above the spiral with the gap of several hundreds of micrometer.
The spiral and the counter electrode are connected by tunable voltage.
When there is a voltage difference between the spiral and the counter electrode, the spiral is elevated toward the counter side to form a helix shape.
The pneumatic force is applied from the backside of the spiral by supplying the air.
The pressure difference between the upper and bottom sides of the spiral actuate the structure.
Simultaneously apply these two forces to make an out-of-plane actuator.
However, to have a long stroke length may suffer from some instabilities.

In the red zone of the figure is the unstable area due to pull-in effect.
The actuator can normally operate before pull-in voltage, but over the pull in voltage it may cause the spiral sticks to the counter electrode.
Like the top right figure.

In the blue zone of the figure is the unstable area due to fluttering.
Fluttering of the structure is seen when the excessive pressure is applied.
In the fluttering pressure zone, the structure vibrates so that the stroke saturates there.
The fluttering is thought to be attributed to the turbulence caused by the leak air from gaps between the turn beams.

So the operation is done in the white zone of the figure.
They add the two displacements caused by these two force to get the longest stroke.
The left side is the fabrication.
The fabricated structure is a 5-turn spiral on a suspended Si membrane.
The diameter is 150 micrometer and the beam width of the spiral is 8 micrometer.
We use SOI (silicon on insulator) wafer as the starting material.
Second, use RIE to define the spiral structure.
Finally, use RIE and HF etching to release the main structure.

The right side is the experimental setup.
The device is mounted on a jig, which has an air supply channel and an air chamber.
Air pressure is supplied from Nitrogen, and the pressure is monitored by a digital pressure sensor.
An ITO coated glass is used as a material of the counter electrode so that the laser light can go through it.
This is the first experimental result.

In order to verify each actuation force can enlarge the displacement,
first, they alter pressure and apply no voltage.
In the left figure we can see that in pressure 2 kilo pascal has the longest stroke-70 micrometer, and over the pressure the spiral begin fluttering.

Second, they change voltage and keep pressure at constant.
The lowest line shows the displacement only caused by electrostatic force is 20 micrometer, at 330V- is the pull in voltage.
Over the pull in voltage, the spiral begin to be unstable.

When we combine the two forces, the pressure is 2 kilo pascal and voltage is at 330V, we can get the longest stroke-103 micrometer.
This largest stroke length cannot be obtained by applying either electrostatic force or pneumatic force.

And, the figure next to the title shows the spiral’s displacement variation, when we give the voltage and pressure.
The second experiment is to see whether the number of turn beams can change the result.

They prepared four different shapes of spiral.
The parameter on the shape is the diameter of the disk located on center of the spiral.
Shows in the left figure.

And the right graph, the angular position is 0 radian at the root of the outermost beam, and increases as the spiral goes into the inner direction.
The displacement is measured along the angular position.
And the measurement is in the condition of 2 kPa and 330 V.

There are found no significant difference in maximum displacement of these four shapes.
It can be said that the area of the spiral does not play a dominant role in increasing the stroke length.
Almost all displacement is caused by the outer two spiral beams, first and second turn beams from the outer side.
第一共振頻前的那一個peak，本篇paper中並沒有多做任何解釋只有說此應該為一個假的共振頻，並非真正量測到的頻率。

一般來說我們測量到的假的頻率通常有二，一個是環境couple的影響第二個是我們不想要的Spurious mode。而此次的第一個peak的頻率差不多是第一共振頻的一半，個人猜想他可能因為差干涉的量測儀器所造成的一種半頻的假mode。

103033525 Mid-term

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