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2043 |
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Application of Input Shaping® and HyperbitTM Control to Improve the Dynamic Performance of a Six-axis MEMS Nanopositioner |
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Equipment, Machines & Instruments: MEMS, LIGA & Nanotechnology |
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In this paper, we demonstrate how Input Shaping and Hyperbit control may be used to obtain fine resolution motion and minimize vibration errors in all six axes of a six-axis, MEMS nanopositioner. It is difficult to incorporate damping into micro-/meso-scale nanopositioners, therefore the dynamic problems, e.g. ringing/overshoot, that are conventionally addressed by damping must be resolved using control techniques. A second challenge is that a device’s on-chip digital-to-analog converters would need the ability to drive these devices with range to resolution ratios of 1 million or larger. Ideally, one would use a digital-to-analog converter that is capable of providing “high-bit” performance at low-cost.
In this paper, we will first present the dynamic characterization of the nanopositioner, the microHexFlex [1], including the natural frequencies and their corresponding mode shapes. We then demonstrate the effect of Input Shaping and Hyperbit on the nanopositioner’s resolution and settling time. Using these techniques, it is possible to obtain ms settling times with sub-nanometer resolution. The practical implications of this work are that future small-scale precision devices will be able to use these techniques to provide low-cost, multi-axis positioning at high-speeds speed and with fine resolution.
The micro-HexFlex nanopositioner possess a 1 mm footprint and consists of two layers of single crystalline silicon with one layer of silicon dioxide in between. The stage of the micro-HexFlex is supported by axi-symmetric micro-scale flexures. Thermomechanical actuators are used to drive the Micro-HexFlex. In our tests, the thermomechanical actuators were driven via a voltage that was preconditioned using an Input shaping controller. The controller [2] is an implementation of a feed forward technique that acts to remove ringing and overshoot by modifying the input signal to the actuators so as to obtain the best possible performance from the positioner.
We also add Hyperbit DAC technology, a recently developed technique [3] for extending the resolution of digital-to-analog converters (DACs), for instance using a 4 bit DAC to obtain 12 bit functionality. In its simplest implementation, the technique involves pulse-width modulation (PWM) of the least-significant bits (LSBs) of a DAC. Since DAC update rate capabilities are significantly faster than the bandwidths of the devices being driven, this technique allows the idle time-domain capacity of “low-bit” DACs to emulate that of “high-bit” DACs. The improvement in resolution is therefore obtained with simpler DAC equipment/circuitry that is more easily fabricated and integrated with micro- and meso-scale devices. Experimental results indicate reduction in dynamic errors by two orders of magnitude when the positioner was given 100 Hz square wave commands.
1. Chen S, Culpepper ML, “Design of a Six-axis Micro-scale Nanopositioner–Micro-HexFlex,” Prec. Eng., 2006.
2. Singer NC, Seering WP, “Design and comparison of command shaping methods for controlling residual vibration,” IEEE Conf. Robotics Automat., 1989,888-93.
3. Scott C. Jordan, U.S. Patent 6950050, 2005
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