HEXFLEX: A PLANAR MECHANISM FOR SIX-AXIS MANIPULATION AND ALIGNMENT

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HEXFLEX: A PLANAR MECHANISM FOR SIX-AXIS MANIPULATION AND ALIGNMENT Martin L.

dimensional structures (e.g. heapods) to achieve spatial position control.

The structure of the HeFle TM 1, shown in Fig. 1, is entirel planar and capable of si-ais alignment.

The knowledge gained from this eperiment will then be used to design small-scale versions of the HeFle for MEMS and Microphotonic

1 HEXFLEX: A PLANAR MECHANISM FOR SIX-AXIS MANIPULATION AND ALIGNMENT Martin L. Culpepper, Gordon Anderson and Patrick Petri Massachusetts Institute of Technolog, Cambridge, MA Kewords: Compliant mechanism, photonics alignment, assembl, micro-manipulation, optics 1. INTRODUCTION The need for multi-ais micromanipulation in MEMS and photonics has driven the development of novel designs [ 1 3 ] that use three dimensional structures (e.g. heapods) to achieve spatial position control. Simplicit of design and ease of manufacture/integration into MEMS devices would be improved if such devices were based on planar structures. The structure of the HeFle TM 1, shown in Fig. 1, is entirel planar and capable of si-ais alignment. This paper presents the eperimental validation of the theor used to design a low-cost micro-positioning stage using the HeFle. The knowledge gained from this eperiment will then be used to design small-scale versions of the HeFle for MEMS and Microphotonic alignment/manipulation. Motion stage Tab 1 Grounded FEA Actuator attach Tab 3 Tab 2 Prototpe HeFle components Planar motion Out of plane motion Figure 1: Generic HeFle geometr and deformation response 2. MOTIVATION FOR DEVELOPMENT OF THE HEXFLEX SYSTEM Ideall, one would like to have alignment manipulators integrated into Microsstems so that the can perform initial assembl positioning as well as in-service alignment corrections. Unfortunatel, few small- 1 Patent pending, Culpepper,

2 scale spatial manipulators eist. As a result, si-ais alignment of Microsstems is generall achieved with manipulators that are roughl three orders of magnitude larger than the assembled components [ 2, 3 ]. Although recent efforts have produced novel small-scale versions of common macro-scale manipulators (e.g. MEMS heapods) [ 1 ], friction at segmented joints poses a reliabilit problem for these machines. Some commercial products use MEMS thermal actuators to align components (in three aes) and then fi the alignment with temporar adhesive bonds [ 4 ]. Unfortunatel, the do not present a comprehensive si-ais alignment solution and the dnamic response is limited b the time required to melt/solidif the adhesive. On the other hand, the HeFle provides limited si ais motion without rubbing surfaces. The planar nature of the device and tensegrit-like structure offer improved dnamic response over three dimensional, segmented mechanisms. The subsequent sections of this paper present the concept for the HeFle, provide a brief overview of the means used to model the mechanism, and compare theoretical predictions with eperimental data generated from tests on a low-cost, proof of concept prototpe. 3. HEXFLEX CONCEPT As shown in Fig. 1, si beams act as legs that connect the stage (triangle) to a grounded structure (shown as a ring). Three tabs are provided to interface the actuators with the HeFle stage. In our eample, one actuator drives a tab in-plane (for planar motion) while another actuates the end of a tab in the z direction (for out-of-plane motions). Ideall, planar and non-planar actuation motions would be decoupled with fleural bearings between the actuators and tabs; however for the purpose of constructing a low-cost prototpe we opted to utilize a ball on flat contact between actuator and tab. As will be seen in section 6, this rough design is sufficient to obtain less than 5% error between eperimental data and theor for the stage s centroid displacement. The rough nature of this tab-actuator interface will however be recognized when we eamine parasitic errors. Combinations of in-plane tab actuations ma be used to position the stage in,, and θ z. Out-of-plane tab actuations position the stage in z, θ, and θ. The curious reader ma skip ahead to Fig. 4 to view intuitive eamples of motions that are generated b simple displacements. Planar and non-planar displacements ma be produced simultaneousl. 4. MODELING HEXFLEX MOTIONS The HeFle was modeled using CoMeT TM (Compliant Mechanism Tool) developed b the Precision Sstems Design and Manufacturing Research Lab at MIT. CoMeT is written in Matlab, full utilizing its matri manipulation capabilities. Given user inputs that define the compliant mechanism geometr, material, and constraints, CoMeT develops the global stiffness and compliance matrices via the direct 2

stiffness approach [ 5 ]. The analtic model developed b CoMeT is full parametric and therefore amenable to a wide range of optimization routines within Matlab.

Figure 2 shows screen shots of CoMeT during in-plane and out-ofplane simulations.

3 stiffness approach [ 5 ]. The analtic model developed b CoMeT is full parametric and therefore amenable to a wide range of optimization routines within Matlab. This enables the designer to rapidl converge on an optimum design solution. The compliant mechanism design process is over an order of magnitude faster than design b FEA. Figure 2 shows screen shots of CoMeT during in-plane and out-ofplane simulations. A: Screen shot of in-plane analsis B: Screen shot of out-of-plane analsis Figure 2: CoMeT Compliant mechanism design tool interface/gui 5. EXPERIMENTAL PROTOTYPE The eperimental prototpe and test fiture used to verif concept and theor are shown in Fig. 3. The HeFle structure was designed and optimized using CoMeT. Critical prototpe design characteristics are given in Table 1. Note that one of the advantages of the HeFle concept is the potential to alter the transmission ratio between actuator and stage b changing the geometr and/or compliance of the respective tab. In some cases, near zero or negative transmission ratios ma be obtained. For instance actuating the tabs in the negative z direction can result in stage displacement in the positive z direction if the structure is tuned properl. 3

Probes Target Actuators Base HeFle A: Bottom view B: Top view C: Eploded CAD view Figure 3: Prototpe HeFle in test fiture Table 1: Characteristics of prototpe HeFle mechanism Diameter of circle that

1 mm Transmission ratio achieved via tabs In-plane: /15 and out-of-plane /44 1 1 The main component of the test fiture is a monolithic aluminum base with a thru-hole at its center.

4 Probes Target Actuators Base HeFle A: Bottom view B: Top view C: Eploded CAD view Figure 3: Prototpe HeFle in test fiture Table 1: Characteristics of prototpe HeFle mechanism Diameter of circle that encompasses the HeFle 180 mm HeFle material 6061 Aluminum Tab length / width / thickness 55 mm/5 mm/3.1 mm Transmission ratio achieved via tabs In-plane: /15 and out-of-plane / The main component of the test fiture is a monolithic aluminum base with a thru-hole at its center. Si piezoelectric actuators (40 micron stroke), si capacitance probes, and grounded portions of the HeFle mechanism are rigidl attached to the base. HeFle stage displacement is etracted from capacitance probe measurements of the target position (see Fig. 3, target is rigidl attached to the HeFle stage). 6. EXPERIMENTAL RESULTS Eperimental data for three motion paths is compared to CoMeT predictions in Fig. 4. Note that the maimum linear displacement is less than 5 microns, however one hundred micron displacement has been obtained in preliminar tests with actuators that posses longer strokes. Eperimental results show the maimum error between theoretical and eperimental displacements is less than 5%. The largest parasitic motions of the stage centroid during all tests were 90 nm and 5 µradians. The rough design of the tabactuator interface and the as machined surfaces used to align actuators to the tab are likel causes of the parasitic motions. Manufacturing errors in the HeFle geometr that are not captured in the CoMeT model are another likel source of error. 4

5 A: Centroid θz position [µradians] Test for pure centroid rotation θ z [Tabs 1, 2 and 3 actuated in-plane] Theta z Theor via CoMeT T T3 T2 Tab displacement [ microns ] B: Centroid position [microns] Test for pure motion [Tabs 2 and 3 actuated in-plane] Theor via CoMeT T3 T1 T Tab displacements [ microns ] C: Centroid z position [ microns ] Test for pure z translation [Tabs 1, 2 and 3 actuated out-of-plane] z Theor via CoMeT T3 T1 T2 Tab displacements in z direction [ microns ] Figure 4: Eperimental data & theoretical predictions for HeFle displacements Note: Solid arrows represent direction of positive actuation ( signifies awa from viewer). Centroid displacements/positions are referenced with respect to the global coordinate sstem given in the respective legend sketch. 5

6 7. CONCLUSIONS AND NEXT STEPS Although parasitic errors are large for a precision micromanipulation stage, the accurac of the actuated displacements provides confidence in the CoMeT model. Future efforts will address reducing the parasitic errors b redesigning the tab-actuator interface. The design of HeFle stages for active alignment tasks in Microphotonic and MEMS are presentl underwa. 8. REFERENCES [1] Bernhard, J., Benavides, L., and Allen, J., Planar and Spatial Three-Degree-of-Freedom Micro-stages in Silicon MEMS, Proceedings of the 16th Annual ASPE Meeting, November, [2] Dagalakis, N., and Amatucci, E., Kinematic Modeling of a 6 Degree of Freedom Tri-stage Micro-Positioner, Proceedings of the 16th Annual ASPE Meeting, November, [3] Kota, Sridhar, Eploiting Elasticit in Engineering Design: Snthesis and Applications, Invited Lecture at MIT Fall 2001 Mechanical Engineering Seminars, September 28, [4] US Patent 6,253,011, Micro-aligner for precisel aligning an optical fiber and an associated fabrication method. [5] Kurtis R. Gurle, Direct Stiffness: Beam application, CES4141 Lecture Notes, Summer1998, [6] Petri, P., A Continuum Mechanic Design Aid for Non-planar Compliant Mechanisms, Masters Thesis, MIT, Cambridge, MA, September

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