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The growing demand from the industry in high precision systems introduces new challenges for positioning mechanisms. High accuracy and repeatability up to the sub-micron scale is not uncommon. This is often combined with extreme environments, like high UV light sources, electron beams or vacuum. In this article, we focus on the flexure mechanism for a large stroke planar XY-positioning system, where applications for the flexure mechanism can be found in for example lithography, micromachining or microscopy.
Ger Folkersma, Steven Boer, Dannis Brouwer and Just Herder
Common large-stroke stages include friction or roller bearings to guide the motion. The friction is normally minimized by applying lubricants. In a vacuum environment however, this lubricant slowly evaporates and will contaminate the vacuum and its surroundings. This will result in excessive wear, increased friction and reduces the accuracy of the positioning mechanism itself. This can be solved by using solid lubricants for example , or by avoiding friction with magnetic or air bearings . However, these methods increase the mechanism complexity considerably.
Flexure elements for guiding the motion do not have this problem, since these elements do not need any lubrication and maintenance. Other advantages of flexure hinges are no backlash, diminished friction and high resolution . The mechanism described here combines a large stroke with elastic cross hinges.
Figure 1, Schematic layout of the mechanism. Elastic hinges are indicated by numbers, connecting arms by letters.
Figure 2, SPACAR simulation of stiffness vs. hinge deflection φ, for the two cross hinge variations
Figure 3, Various configurations of cross-hinges. a: ‘2leaf’, b: ‘5leaf’
Figure 4, DOFs of one pair of arms (arms assumed rigid).
Figure 5, Compliances in lower arms
Figure 6, Released DOFs in arm G
Figure 7, Actuators mounted on a upper arm. 1: cross-hinge, 2: upper arm, 3: limit switch, 4: encoder head, 5: motor coil units, 6: encoder scale, 7: motor magnet yokes.
Figure 8, Photo of experimental setup
Figure 9, 3rd mode shape of mechanism, at 115Hz
Figure 10, Measured 3rd natural frequency over workspace
Figure 11, Bode plot of identification results. With Yx and Yy the encoder positions, and Fx and Fy the force supplied to the actuators.