ENSC 494 Project Report PolyMUMPs Flexure .ENSC 494 Project Report PolyMUMPs Flexure Design Garet

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  • ENSC 494 Project Report

    PolyMUMPs Flexure Design

    Garet Kim Jessica McAlister

    Lydia Tse

    August 8, 2003

  • Abstract .............................................................................................................................. iii 1. Introduction.................................................................................................................... 1

    1.1 Background on Flexure Hinges .......................................................................... 1 1.2 Advantages of Flexure Hinges............................................................................ 1 1.3 Limitations of Flexure Hinges ............................................................................ 2 1.4 PolyMUMPs Technology Background............................................................... 2 1.5 Project Goal ........................................................................................................ 3

    2. Design Evolution ........................................................................................................... 3 2.1 Original Design................................................................................................... 3 2.2 Intermediate Design ............................................................................................ 4 2.3 Final Design ........................................................................................................ 6

    3. Design Implementation.................................................................................................. 7 3.1 List of Components............................................................................................. 7 3.2 Design Variations................................................................................................ 7 3.3 Pulling Mechanisms............................................................................................ 8 3.4 Motion Predictions............................................................................................ 10

    3.4.1 Basic Flexures........................................................................................... 10 3.4.2 Cascaded Flexures .................................................................................... 11 3.4.3 Buckling Systems...................................................................................... 12

    3.5 Design Issues .................................................................................................... 14 4. Future Research ........................................................................................................... 15 5. Conclusion ................................................................................................................... 15 6. References.................................................................................................................... 16 Appendix A: List of Components ..................................................................................... 17 Appendix B: List of Variations........................................................................................ 19

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  • Abstract Most mechanical designs rely on rotation of some sort in order to function. Many of these designs make use of pin-joints to satisfy rotational requirements. The drawback of pin-joints is that backlash can affect the performance of the system. When designing on a micro scale, the backlash problem becomes a more significant issue. Flexure hinges are a relatively new strategy for providing zero backlash rotation. Flexure hinges on a micro scale are not yet well understood and this paper outlines a project designed to further the research in micro scale flexure hinges. This paper also outlines designs that use the unfavorable buckling phenomenon in a productive manner. All designs were formed using a PolyMUMPs process.

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  • 1. Introduction

    1.1 Background on Flexure Hinges A Flexure hinge, as defined by Smith, is a mechanism consisting of a series of rigid bodies connected by compliant elements that is designed to produce a geometrically well-defined motion upon application of a force (2000). Elastic deflections have been used in mechanical design for more than 300 years, but flexure hinges are a relatively new field, especially on a micro scale. Flexure hinges are generally divided into two subsets: leaf hinges and notch hinges. A leaf hinge has historically been created by clamping a thin plate between two rigid bodies. The result is a thinned out piece of material connecting two rigid bodies formed of the same material. Figure 1a below show a leaf hinge. Notch Hinges, on the other hand, are created by machining symmetrical elliptical patterns from each side of a solid body creating a thin path between two rigid bodies. Figure 1b below shows a circular notch hinge. (Smith 2000)

    Figure 1: Flexure hinges, a) leaf hinge, b) circular notch hinge

    1.2 Advantages of Flexure Hinges Pin-joints, particularly on a micro scale, require space between the pin and the cog to allow for rotation. When micromachining, this space must be significant to ensure that the components do not fuse together. Flexure hinges are useful in mechanical design because they are a zero-backlash alternative to pin-joints. Other advantages of flexure hinges are (Lobontiu 2003): Flexure Hinges 1) do not suffer from friction losses

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  • 2) do not require lubrication 3) do not suffer from hysteresis 4) are compact 5) are capable of functioning in small-scale applications 6) are easy to fabricate 7) require little to no maintenance

    1.3 Limitations of Flexure Hinges Flexure hinges are not exempt from drawbacks. Some of the limitations include (Lobontiu 2003): Flexure Hinges: 1) provide low level of rotation 2) do not provide pure rotation (deformation of a flexure is complex) 3) do not have a fixed rotation centre 4) are usually sensitive to temperature variations 5) cannot tolerate large loads

    1.4 PolyMUMPs Technology Background PolyMUMPs technology is a three-layer polysilicon surface micromachining process. Figure 2 below shows a cross sectional view of the three-layer process.

    Figure 2: Cross sectional view of PolyMUMPs process

    The Nitride layer shown above simply acts as an electrical isolation layer. Poly0 is a 500nm LPCVD polysilicon film patterned by photolithography. The 1st oxide is simply a sacrificial layer to free the first mechanical layer. This sacrificial layer also allows for a dimple mask. Poly1 is the first structural layer. The 2nd Oxide layer provides a

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  • mechanical and electrical connection between Poly1 and Poly2. Poly2 is the second structural layer. Finally the metal layer allows for probing, bonding, electrical routing and mirrored surfaces. (Koester 2003)

    1.5 Project Goal Our project objectives is as follows:

    A flexure-spring based extender stage is to be designed. Instead of using hinges or pin-joints, a folding/extending stage is to be designed using flexure joints. By combining this flexure folding system with a linear stepper motor, a MEMS zero-backlash slider can be created.

    The limitations of the PolyMUMPs technology forced us to modify the project objectives throughout the course of the semester. In the end, we aimed to create a test bench type environment to aid in the collection of data regarding flexure hinges of varying dimensions. We hope that this initial research into flexure hinges will lead to more meaningful designs in future applications.

    2. Design Evolution

    2.1 Original Design Our original plan was to implement the slider shown below in Figure 3. The slider functions with the application of a small force (push) on one side, resulting in a larger range of motion on the opposite end. Unfortunately, implementation of this approach using PolyMUMPs technology was impossible because we had essentially only one layer to manipulate, meaning that all components would slide as one rigid body unless anchored, which would obstruct sliding. The slider design could be implemented using pin-joints, but this would introduce backlash, which defeats the purpose of the project.

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  • Small Push

    Big Slide

    Figure 3: Slider Design Approach

    2.2 Intermediate Design

    Notch and leaf hinges were discussed in the introduction, but we decided to use a new family of flexures, namely corner-filleted flexure hinges (Lobontiu 2003), which are essentially a combination of the leaf and notch flexure hinges. The flexure hinge we used in our designs is shown below in Figure 4. The figure also includes the variations in neck length we used. Leaf hinges distributes deflection over the length of the entire hinge causing lower stress and higher deflection to beam length. Notch hinges, on the other hand, are more immune to parasitic forces. (Smith 2000) The decision to use the corner-filleted flexure hinge was in an effort to combine both of these attributes. The down-side is that very little literature documents the behavior of this type of hinge making it extremely difficult to reliably predict motion.

    1 um,5 um,9 um,11 um,13 um

    1 um,5 um,9 um,11 um,13 um

    Figure 4: Corner-filleted flexure hinge (showing variations of neck length)

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  • We came up with an alternative to the slider design which involves cascading flexure-hinged beams. The intermediate design is shown below in Figure 5. We can apply a small force at the left end, and achieve a larger range of motion on the far right end. This gives us the same effect as the slider would, only in an opposite orientation. The beams in the figure are connected with a spr