Andrew J. Fleming

Sensorless vibration suppression and scan compensation for piezoelectric tube nanopositioners

Piezoelectric tube scanners are employed in highresolution positioning applications such as scanning probe microscopy and nano-fabrication. Much research has proceeded with the aim of reducing hysteresis and vibration, the foremost problems associated with piezoelectric tube scanners. In this paper, two simple techniques are proposed for simultaneously reducing hysteresis and vibration. Experimental results demonstrate significant reduction in hysteresis due to the use of a charge amplifier. Previous problems involved with the implementation of such ampliers are resolved to provide DC accurate performance with zero voltage drift. Secondly, piezoelectric shunt damping, a technique previously resident in the field of smart structures, is applied to damp tube vibration. By attaching an LCR impedance to a single tube electrode, the first mechanical mode is reduced in magnitude by more than 20 dB.

Integral resonant control of collocated smart structures

This paper introduces integral resonant control, IRC, a simple, robust and well-performing technique for vibration control in smart structures with collocated sensors and actuators. By adding a direct feed-through to a collocated system, the transfer function can be modified from containing resonant poles followed by interlaced zeros, to zeros followed by interlaced poles. It is shown that this modification permits the direct application of integral feedback and results in good performance and stability margins. By slightly increasing the controller complexity from first to second order, low-frequency gain can be curtailed, alleviating problems due to unnecessarily high controller gain below the first mode. Experimental application to a piezoelectric laminate cantilever beam demonstrates up to 24 dB modal amplitude reduction over the first eight modes.

High-Performance Control of Piezoelectric Tube Scanners

In this paper, a piezoelectric tube of the type typically used in scanning tunneling microscopes (STMs) and atomic force microscopes (AFMs) is considered. Actuation of this piezoelectric tube is hampered by the presence of a lightly damped low-frequency resonant mode. The resonant mode is identified and damped using a positive velocity and position feedback (PVPF) controller, a control technique proposed in this paper. Input signals are then shaped such that the closed-loop system tracks a raster pattern. Normally, piezoelectric tubes are actuated using voltage amplifiers. Nonlinearity in the form of hysteresis is observed when actuating the piezoelectric tubes at high amplitudes using voltage amplifiers. It has been known for some time that hysteresis in piezoelectric actuators can be largely compensated by actuating them using charge amplifiers. In this paper, high-amplitude actuation of a piezoelectric tube is achieved using a charge amplifier.

A New Method for Robust Damping and Tracking Control of Scanning Probe Microscope Positioning Stages

This paper demonstrates a simple second-order controller that eliminates scan-induced oscillation and provides integral tracking action. The controller can be retrofitted to any scanning probe microscope with position sensors by implementing a simple digital controller or operational amplifier circuit. The controller is demonstrated to improve the tracking bandwidth of an NT-MDT scanning probe microscope from 15 Hz (with an integral controller) to 490 Hz while simultaneously improving gain-margin from 2 to 7 dB. The penalty on sensor induced positioning noise is minimal. A unique benefit of the proposed control scheme is the performance and stability robustness with respect to variations in resonance frequency. This is demonstrated experimentally by a change in resonance frequency from 934 to 140 Hz. This change does not compromise stability or significantly degrade performance. For the scanning probe microscope considered in this paper, the noise is marginally increased from 0.30 to 0.39 nm rms. Open and closed-loop experimental images of a calibration standard are reported at speeds of 1, 10, and 31 lines per second (with a scanner resonance frequency of 290 Hz). Compared with traditional integral controllers, the proposed controller provides a bandwidth improvement of greater than 10 times. This allows faster imaging and less tracking lag at low speeds.

Nanopositioning System With Force Feedback for High-Performance Tracking and Vibration Control

In this study, the actuator load force of a nanopositioning stage is utilized as a feedback variable to achieve both tracking and damping. The transfer function from the applied actuator voltage to the measured load force exhibits a zero-pole ordering that greatly simplifies the design and implementation of a tracking and damping controller. Exceptional tracking and damping performance can be achieved with a simple integral controller. Other outstanding characteristics include guaranteed stability and insensitivity to changes in resonance frequency. Experimental results on a high-speed nanopositioner demonstrate an increase in the closed-loop bandwidth from 210 Hz (with an integral controller) to 2.07 kHz (with a force-feedback control). Gain margin is simultaneously improved from 5 dB to infinity.

A grounded-load charge amplifier for reducing hysteresis in piezoelectric tube scanners

In this paper, a charge amplifier adapted for piezoelectric tube scanners is presented. Previous problems involved with the implementation of such amplifiers are resolved to provide dc accurate performance with zero voltage drift. In our experiment, hysteresis was reduced by 89% when compared to a voltage amplifier.

Model Predictive Control Applied to Constraint Handling in Active Noise and Vibration Control

The difficulties imposed by actuator limitations in a range of active vibration and noise control problems are well recognized. This paper proposes and examines a new approach of employing model predictive control (MPC). MPC permits limitations on allowable control action to be explicitly included in the computation of an optimal control action. Such techniques have been widely and successfully applied in many other areas. However, due to the relatively high computational requirements of MPC, existing applications have been limited to systems with slow dynamics. This paper illustrates that MPC can be implemented on inexpensive hardware at high sampling rates using traditional online quadratic programming methods for nontrivial models and with significant control performance dividends.

Multiple mode current flowing passive piezoelectric shunt controller

Abstract A method for multiple mode piezoelectric shunt damping will be presented in this paper. The proposed “current flowing” shunt controller has a number of benefits compared to previous shunt damping schemes; it is simpler to implement and requires small number of passive circuit elements. The passive control strategy is validated through experimentation on two piezoelectric laminated structures.

Optimization and implementation of multimode piezoelectric shunt damping systems

Piezoelectric transducer (PZT) patches can be attached to a structure in order to reduce vibration. The PZT patches essentially convert vibrational mechanical energy into electrical energy. The electrical energy can be dissipated via an electrical impedance. Currently, impedance designs require experimental tuning of resistive circuit elements to provide optimal performance. A systematic method is presented for determining the resistance values by minimizing the H/sub 2/ norm of the damped system. After the design process, shunt circuits are normally implemented using discrete resistors, virtual inductors and Riordian gyrators. The difficulty in constructing the shunt circuits and achieving reasonable performance has been an ongoing and unaddressed problem in shunt damping. A new approach to implementing piezoelectric shunt circuits is presented. A synthetic impedance, consisting of a voltage controlled current source and a digital signal processor system, is used to synthesize the terminal impedance of a shunt network. A two-mode shunt circuit is designed and implemented for an experimental simply supported beam. The second and third structural modes of the beam are reduced in magnitude by 22 and 18 dB.

Charge drives for scanning probe microscope positioning stages

Due to hysteresis exhibited by piezoelectric actuators, positioning stages in scanning probe microscopes require sensor-based closed-loop control. Although closed-loop control is effective at eliminating non-linearity at scan speeds below 10 Hz, it also severely limits bandwidth and contributes sensor-induced noise. The need for high-gain feedback is reduced or eliminated if the piezoelectric actuators are driven with charge rather than voltage. Charge drives can reduce hysteresis to less than 1% of the scan range. This results in a corresponding increase in bandwidth and reduction of sensor induced noise. In this work we review the design of charge drives and compare them to voltage amplifiers for driving lateral SPM scanners. The first experimental images using charge drive are presented.