A design is presented for a precision MEMS-based six degrees of freedom (DOFs) manipulator. The purpose of the manipulator is to position a small sample (10 x 20 x 0.2 μm3) in a transmission electron microscope. A parallel kinematic mechanism with slanted leaf-springs is used to convert the motion of six in-plane electrostatic comb-drives into six DOFs at the end-effector. The manipulator design is based on the principles of exact constraint design, resulting in a high actuation compliance (flexibility) combined with a relatively high suspension stiffness. However, due to fabrication limitations overconstrained design has been applied to increase the stiffness in the out-of-plane direction. The result is a relatively large manipulator stroke of 20 μm in all directions combined with a high first vibration mode frequency of 3.8 kHz in relation to the used area of 4.9 x 5.2 mm2. The motion of the manipulator is guided by elastic elements to avoid backlash, friction, hysteresis and wear, resulting in nanometer resolution position control. The fabrication of the slanted leaf-springs is based on the deposition of silicon nitride (SixNy) on a silicon pyramid, which in turn is obtained by selective crystal plane etching by potassium hydroxide (KOH). The design has been analyzed and optimized with a multibody program using flexible beam theory. A previously developed flexible beam element has been used for modeling the typical relatively large deflections and the resulting position-dependent behavior of compliant mechanisms in MEMS. The multibody modeling has been verified by FEM modeling. Presently only parts of the manipulator have been fabricated. Therefore, a scaled-up version of the manipulator has been fabricated to obtain experimental data and to verify the design and modeling.
Kinematic design video link: http://www.youtube.com/watch?v=lwapoNeuuVQ
Flexure design in MEMS video link: http://www.youtube.com/watch?v=RCbEhM7tCNQ
Mode shape analysis by SPACAR vídeo link: http://www.youtube.com/watch?v=O2TKJ8mMUGo
Fig. 1. One of the three legs of the manipulator with the platform.
(a) x-translation (b) y-translation
(c) z-translation (d)-Rx-rotation
(e) Ry-rotation (f) Rz-rotation
Fig. 2. Each of the independent six DOFs of the platform are created by combinations of planar xy-displacements of the three intermediate bodies.
Fig. 3. The MEMS-based 6 DOFs manipulator design. For viewing purposes a section has been cut away. Overall dimensions are 4.9 x 5.2 x 0.5 mm3.
Fig. 4. Three of the three DOFs of the intermediate body are stiff (ideally constrained by the Si-leaf-springs). Two DOFs of the intermediate body are actuated and one DOF is compliant (ideally released).
Fig. 5. The 6 DOFs (2 actuated and 4 compliant) of the platform defined by 1 leg. The slanted leaf-spring releases 3 DOFs. The intersection of the Si-leaf-springs releases 1 DOF. The 3 compliant DOFs near the intermediate body can be regarded as a ball joint, equivalent to the ball joint in Fig. 1. The compliant DOF near the platform can be regarded as the hinge equivalent of Fig. 1.
Fig. 6. Brief overview of the fabrication of the 6 DOFs MEMS-based precision manipulator. Top figure shows a cross-section of the manipulator after KOH etching. Bottom figure shows the cross-section of the manipulator after the total processing.
Table 1: The lowest vibration mode frequencies of the manipulator calculated by CosmosWorks
Vibration mode frequency
Fig. 7. The first vibration mode change with blocked actuators due to platform displacement in the x-, y-, and z-direction.
For more information:
D.M. Brouwer, B.R. de Jong, H.M.J.R. Soemers, Design and modeling of a six DOFs MEMS-based precision manipulator, Journal of Precision Engineering, Vol 34, Issue 2, April 2010, pp. 307-319, doi:10.1016/j.precisioneng.2009.08.001