An improved closed-loop controller has been built for a three-axis piezoelectric positioning stage. The stage can be any of a number of commercially available or custom-made units that are used for precise three-axis positioning of optics in astronomical instruments and could be used for precise positioning in diverse fields of endeavor that include adaptive optics, fabrication of semiconductors, and nanotechnology.
In a typical application, the stage is used to move an optic through a small distance with a required resolution of the order of a nanometer or a fraction of a nanometer. Typically, the piezoelectric actuator for each axis can be made to expand through a maximum stroke ≤12 μm by applying a potential ≤120 V to it. To provide position feedback for closed-loop control of the potential applied to the piezoelectric actuator for each axis, the expansion of the actuator is sensed by means of a strain gauge bonded to the side of the actuator. The resistance of the strain gauge changes from about 700 to 701 Ω as the actuator expands through its maximum stroke. The strain gauge is part of a Wheatstone bridge, so that the small change in resistance from a nominal value can be converted to a Wheatstone-bridge output voltage. To close the control loop, the Wheatstone-bridge output voltage is amplified and compared with a voltage representing an actuator set point specified by an external control computer or other external source. The difference between these voltages constitutes a servo error signal, which is amplified for application to the affected piezoelectric actuator.
The improved controller supplants a prior controller that resided in a 19-in. (≈48-cm) rack. The most expensive part of the improved controller consists of servo-controller circuitry on a 4.25-by-7.5-in. (≈11-by-19-cm) printed-circuit board, denoted the main board. The strain gauges are connected into Wheatstone-bridge circuits that include relatively inexpensive, interchangeable resistor bridge circuits (see figure) on a 2.5-by-3.5-in. (approximately 6-by-9-cm) satellite printed-circuit board. The satellite board can readily be replaced by another with different circuitry tailored for a different actuator/strain-gauge combination.
The three Wheatstone bridges are driven by a precision voltage reference on the satellite board, powered by a cable from the main board. The voltages representing the actuator set points are generated by three 18-bit, self- calibrating, digital-to-analog converters on the main board. The amplification of the Wheatstone-bridge output voltages is effected by two-stage, low-noise instrumentation amplifiers on the main board.
The servo error signal for each axis is further amplified and filtered. The filter circuitry can be built to have either 2-Hz bandwidth (resulting in spatial resolution of 0.1 nm) or 200-Hz bandwidth (resulting in spatial resolution of 1 nm). The amplified, filtered signal is fed to a final high-voltage amplifier, the output of which is applied to the piezoelectric actuator. By means of a CMOS input on a control connector, some of the servo-controller circuitry can be bypassed, causing the actuators to operate in a rapid-motion, open-loop control mode.
One of the greatest advantages of improved controller over the prior controller arises from a low-noise design. The dominant component of noise is now Johnson noise from the strain gauges; the input-referred noise from all other components is lower by design. The lower-noise design makes it possible to refine spatial resolution from a prior limit of 1 nm to the present limit of 0.1 nm.
This work was done by Shanti Rao and Dean Palmer of Caltech for NASA’s Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-44806, volume and number of this NASA Tech Briefs issue, and the page number.