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2036 |
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Flexure-based Meso-level Stages Driven by Linear Piezo Motors |
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Equipment, Machines & Instruments: Analysis & Modeling |
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| Content |
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The emerging of meso-level machining creates increasing demand on developing linear stages with travels ranging from zero to a few millimeters. The short travel creates a problem for most conventional stages which tend to be based on ball screws and rolling element bearings. For short travels, the hydrodynamic lubrication layer necessary to prevent fretting of the rolling elements cannot always be formed, which can significantly reduce the life of the ball screw and bearings. Flexure based stages, on the other hand, don’t use rolling element bearings, thereby avoiding this issue. However, commercially available flexure stages tend to be driven by piezo-stack actuators, whose limited strain tends to limit the travel of such stages to well below one millimeter.
This paper presents a new type of linear stages that combine the advantages of conventional stages with those of flexure-based system. The moving part of the stage is supported by flexures and is driven directly by linear piezo motors whose working principle is based on resonance of piezo-electric elements that create very small steps on the order of nanometers at very high frequencies. This oscillation is then transferred to a friction plate, thereby generating linear motion. As such, the travel of these stages is limited by the range of the flexures but not by the motors.
As is well known, flexural bearings create extremely straight motion and if designed properly, provide tremendous stiffness in all directions except the direction of motion. They furthermore exhibit neither friction nor stiction, which is a crucial feature in ultra-precision motion control where the reversal of an axis’ motion typically causes a tracking error due to the change from kinetic to static friction.
The fact that the motors are based on friction has a very interesting side effect: Once the slide has reached its target position, no servo control is required to hold this position. Hence, the motors can be turned off, which not only completely avoids any kind of servo noise but also limits the amount of heat generated in the system. This, however, is only true for external forces that do not exceed the frictional force between the motor and the friction plate. There is a difficulty, though. Linear piezo motors tend to exhibit a significant non-linearity in the motor velocity response versus the command voltage. The resulting dead band where the motor creates no velocity despite a non-zero command voltage needs to be accounted for in the control system in order to minimize servo errors during a move. Also of concern is the motor mount. The motors must be preloaded against the friction plate in order to generate a thrust force. In order to maintain preload and to account for changing distances between the motor and friction plate, a compliant mount is needed. However, to avoid backlash, the mount also needs to be very rigid. This paper presents a mount that satisfies both conditions, thereby creating an ideal drive system for flexure-based, meso-level stages.
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