International Conference on Emerging Research in Electronics, Computer Science and Technology – 2015 International Conference on “Emerging Research in Electronics, Computer Science and Technology (ICERECT-2015)”
MEMS BASED OPTICAL IMAGE STABILIZATION S.B.Rudraswamya, Anushree A Bannadabhavib, AshwinKumarb,JayanthBapub, SwathiPai Bb a Assistant Professor, Dept. of ECE, SJCE, Mysore 570006 b Dept. of ECE, SJCE, Mysore 570006 Email:
[email protected] Abstract: The thought of taking a perfect image gives way to a plethora of ideas which work towards improving the image quality using Image Stabilization. Currently, the methods used for Optical Image Stabilization(OIS) use Motion sensors and detect vibration and correct for any jitter using VCM (Voice coil motors).The concept of the proposed work is to simulate a MEMS gyroscope and monitor the system to detect change in rotation (or any vibrations). These vibrations in turn are used to trigger a voltage pulse which is conditioned and fed into structures called comb drives which are micro structures inturn actuate on application of optimal voltages. This comb drive arrangement is made to move a lens or a sensor (depending on which type of OIS is suitable for the application) and adjust the lens such that, even if the camera shakes, the image obtained is not blurred because the setup inside the camera which captures the image also shakes, but in the opposite direction.Hence, a clear image can be obtained in a novel method using MEMS gyroscope coupled with a MEMS comb-drive actuator set-up. Keywords:MEMS,OIS,COMSOL,gyroscope,comb-drive
1.
INTRODUCTION
Image stabilization (IS) is a family of techniques used to reduce blurring associated with the motion of a camera or other imaging device during exposure. Generally, it compensates for pan and tilt (angular movement, equivalent to yaw and pitch) of the imaging device, although electronic image stabilization can also be used to compensate for rotation. It is used in imagestabilizedbinoculars, still and video cameras, astronomical telescopes, and also smart phones, mainly the high-end. With still cameras, camera shake is particularly problematic at slow shutter speeds or with long focal length (telephoto or zoom) lenses. With video cameras, camera shake causes visible frame-to-frame jitter in the recorded video. In astronomy, the problem of lens-shake is added to by variations in the atmosphere over time, which will cause the apparent positions of objects to change[1]. The thought of taking a perfect image gives way to a plethora of ideas which work towards improving the image quality using IS. Currently, the methods used for OIS use motion sensors and detect vibration and correct for any jitter using Voice Coil Motor (VCM).The concept of the proposed work is to detect change in rotation (or any vibrations) using MEMS gyroscope coupled with a MEMS comb-drive actuator set-up.
2.
which captures the image also shakes, but in the opposite direction. Hence a clear image can be obtained in a novel method using MEMS gyroscope coupled with a MEMS comb-drive actuator set-up. The block diagram of the stabilizer is as shown in Fig 1. 2.1 Block diagram Block 1 – The first block in the block diagram detects vibration and jitters using a gyroscope and sends it to the next block, which is for processing that parameter. The gyroscope can be fabricated along with a CMOS circuit to convert the analog output of the gyroscope to digital data. Block 2 – This block is responsible for processing the obtained inputs such as data from the gyroscope, and then converting it into electricalvoltages and increasing the voltage levels for the operational need of the comb-drive actuator. Block 3 – The MEMS comb drive actuator obtains these signals and moves the movable comb drive in order to compensate for the jitter or shake of the lens, to stabilize the image.
METHODOLOGY
The concept of our work is to simulate a MEMS gyroscope and monitor the system to detect change in rotation (or any vibrations). These vibrations in turn are used to trigger a voltage pulse which is conditioned and fed into structures called comb drives which are micro structures which actuate on application of optimal voltages. This comb drive arrangement is made to move a lens or a sensor (depending on which type of OIS is suitable for the application) and adjust the lens such that, even if the camera shakes, the image obtained is not blurred because the setup inside the camera 978-1-4673-9563-2/15/$31.00©2015 IEEE
Fig1. Block diagram of OIS system
2.2 Sensing MEMS tuning-fork style gyroscopes are small silicon devices that detect rotation. Although, they are currently not as sensitive as traditional rotating ring gyroscopes, because of their small size and low cost, they are used in many different electronic devices, including the iPhone 4 and the Wii
International Conference on Emerging Research in Electronics, Computer Science and Technology – 2015
Motion Plus accessory[2]. The basic mechanical structure of a MEMS gyroscope is shownin Fig 2. Table1. Dimensions of the teeth of the comb drive actuator
Dimensions R1 R2 R3 R4 R5 R6 R7 R8
Height in µm 12 5 5 12 17 45 15 2
Width in µm 91.5 7 3 86.5 45 15 25 150
Fig 2. MEMS tuning fork gyroscope
3.2 Mathematical Equations 2.3 Actuation The comb drive actuator we designed consists of interdigitated finger structure, where some comb fingers are fixed and the others are connected to complaint suspension. Applying a voltage difference between the comb structures will result in a deflection of the movable combs due to the developed electrostatic force which provides the actuation in the direction of the length of the comb fingers. 3.
DATA
The software implementation was done using COMSOL Multiphysics.The sensing device gyroscope and the actuator comb-drive were designed using COMSOL and their working was observed.
3.1 Design of comb drive actuator Rectangular comb drives find use in a variety of MEMS applications. The following model shown in Fig 3, is an electrostatically actuated comb drive. The model includes just a few of the teeth. It simulates only the comb drive and its attachment using double-folded beam springs. The upper half of the comb is fixed, as is the end of the beam spring. The system applies an electric potential to the beam spring and the lower comb; the upper comb is grounded[4]. Dimensions of the teeth of the comb drive actuator are mentioned in Table.1.
Applying voltage difference between the comb structures will result in a deflection of the movable comb structure by electrostatic forces which causes change in area between the combs i.e. (y + y•)as the overlapping area changes, the capacitance between the fixed and movable combs changes. The change in capacitance can be expressed as:
ܥൌ
ଶఢ ሺ௬ା௬ሻ ௗ
(1)
where, n is the number of combs, İ• is the dielectric constant in air, h is the height of the comb fingers, y• is the initial comb finger overlap, y is the comb displacement and d is the gap spacing between the comb fingers.The lateral electrostatic force in the ydirection can be expressed as: . ଵ డ ଶ ఢ ଶ ܨ ൌ ܸ ൌ ܸ (2) ଶ డ௬
ௗ
where, V is the applied voltage between the movable and fixed combs. As shown in Eq.1 and 2 the number of combs and the thickness of combs are directly proportional to the electrostatic force so, if the number of combs is increased there is increase in capacitance as well as the electrostatic force. Large deflection comb drive actuators at low driving voltages should employ large number of comb fingers. If the distance between the comb fingers is decreased then also the electrostatic force and the capacitance between the fixed and movable combs gets increased 3.3 Design of MEMS tuning fork gyroscope The structure is fixed at the anchors shown in Fig.2, while the rest of the structure, including the large proof masses is free to move. Since the gyroscope uses the Coriolis effect to detect rotation, the device must be in constant motion to function as the Coriolis force only acts on moving bodies. This motion is accomplished by vibrating the structure at one of its natural frequencies to achieve the motion[3]. This vibration is referred to as the Drive Mode.
Fig3. Dimensions of the teeth of the comb drive actuator
978-1-4673-9563-2/15/$31.00©2015 IEEE
When the structure begins to rotate, the Coriolis force acting on the moving proof masses changes the direction of the vibration from horizontal to vertical. This vertical vibration
International Conference on Emerging Research in Electronics, Computer Science and Technology – 2015
corresponds to a higher natural frequency of the structure than the horizontal drive mode vibration and is referred to as Sense Mode. 4.
RESULTS
Comb drive actuator and gyroscope were modeled in COMSOL Multiphysics and the results are presented in the sub-sections. 4.1 Comb drive actuator Because electrostatic forces attract the combs to each other, any geometric change has an impact on the electric field between them. To account for this effect the model uses an arbitrary Lagrangian-Eulerian(ALE) method implemented in COMSOL. In this model the displacements are relatively large. Therefore, Plane Stress application mode’s support for large deformations was used. Input voltage was varied from 0V to 600V. Electric potential field for an actuation voltage of 600V is as shown in Fig 4. The displacements inside the comb drive at 600V are shown in Fig 5.The total electric energy for different voltages are mentioned in Table.2.
Fig4. Electric potential field for an actuation voltage of 600V
Fig5. Displacements inside the comb drive at 600V
4.2 MEMS gyroscope The gyroscope we simulated is a tuning fork gyroscope. Tuning fork gyroscopes contain a pair of masses that are driven to oscillate with equal amplitude but in opposite directions. When rotated, the Coriolis force creates an orthogonal vibration that can be sensed by a variety of mechanisms.The gyroscope in drive mode is shown in Fig 6.
Fig6. Drive mode frequency
Table 2. Total electric energy obtained for different voltages.
Vin 0 50 100 150 200 250 300 350 400 450 500 550 600
Total electric energy(J) 0 6.5414e-7 2.642e-6 6.05262e-5 1.10725e-5 1.79898e-5 2.70955e-5 3.86268e-5 5.28579e-5 7.03188e-5 9.17323e-5 1.1607e-4 1.4267e-4
978-1-4673-9563-2/15/$31.00©2015 IEEE
5.
CONCLUSION
Simulation of 2D comb drive actuator and MEMS gyroscope was done using COMSOLMultiphysics. The gyroscope is made to operate in drive mode and when a rotating frame is applied to this, due to Coriolis force, the structure changes its direction of vibration from horizontal to vertical. This is how the working of the tuning fork gyroscope was understood through simulation. The behaviour of comb drive as an actuator was studied. The displacements at different actuation voltages were observed. We experimented with different parameters like the dimensions of the comb drive teeth and the distance between them, actuation voltage. We found that these parameters influenced the working of comb drive.
International Conference on Emerging Research in Electronics, Computer Science and Technology – 2015
REFERENCES [1] T. Shimizu, S. Nagata, S. Tsuneta, T. Tarbell, C. Edwards, R. Shine, C. Hoffmann, E. Thomas, S. Sour, R. Rehse, "Image Stabilization System for Hinode (Solar-B) Solar Optical Telescope", Book chapter, The Hinode Mission, pp 167-178. [2] Amanda Bristow, Travis Barton, Stephen Nary,"MEMS Tuning-Fork Gyroscope" , unpublished. [3] T.Madhuranath, R.Praharsha, Dr.K.SrinivasaRao,"Design and simulation of MEMS piezoelectric gyroscope Using COMSOL Multiphysics" , Proceedings of COMSOL conference, Bangalore, 2013. [4] Shefali Gupta, TanuPahwa, Rakesh Narwal, B.Prasad, Dinesh Kumar, "Optimizing the performance of MEMS Electrostatic Comb Drive Actuator with Different Flexure Springs" ,Proceedings of COMSOL conference, Bangalore, 2012.
978-1-4673-9563-2/15/$31.00©2015 IEEE
