Leon Clark

Experimental Investigation of Robust Motion Tracking Control for a 2-DOF Flexure-Based Mechanism

The design, parameter identification and robust motion tracking control of a two degree of freedom (2-DOF) flexure-based micro/nanomechanism are presented in this paper. In the presented compliant mechanism, the cross-axis coupling ratio is below 1% indicating excellent decoupling performance. Despite this, during motion tracking the cross coupling effect cannot be ignored. To enhance the accuracy of micro/nanomanipulation, a laser interferometry-based sensing and measurement system is established. Nonlinearities such as creep/drift and hysteresis are present in this system, which are compensated with closed-loop control. Open-loop tracking results for a 1-DOF trajectory, with and without cross-axis coupling compensation are also presented. Robust motion tracking control is extended to support 2-DOF motion trajectories. This controller is implemented to track the desired trajectories over one and two axes of motion. Robust motion control demonstrates high precision and accurate motion tracking of the 2-DOF flexure-based mechanism. The cross-axis coupling is treated as a known disturbance and the performance of tracking 1-DOF trajectory, with and without cross-axis coupling compensation, is presented. Circular motion trajectories with radii of 10 μm, 1 μm, and 250 nm are also tracked. The experimental results presented in this paper demonstrate effective compensation of the cross-axis coupling with high precision motion tracking. The resultant 2-DOF closed-loop position tracking error in the X and Y axes are within ±20 nm during dynamic motion, and ±8 nm in the steady state.

Laser-Based Sensing, Measurement, and Misalignment Control of Coupled Linear and Angular Motion for Ultrahigh Precision Movement

This paper presents a novel methodology for position and orientation (pose) measurement of stages used for micro/nano positioning which produce coupled motions with three planar degrees of freedom (DOF). In the proposed methodology, counter-rotation of the entire mechanism prevents the misalignment of the measurement beams within a laser-interferometry-based sensing and measurement technique. To detect such a misalignment, a sensing strategy constructed around a position sensitive diode has been developed. A feedforward-feedback compound controller has been established to provide the necessary counter-rotation input to reduce the misalignment error. Experimental validation has been conducted through the measurement of the workspace of a three-DOF planar micro/nano positioning stage. Experimental results demonstrate the capability of the technique to provide combined linear/angular measurement.

A vision-based measurement algorithm for micro/nano manipulation

This paper presents a vision-based measurement methodology which utilises a micrometre calibration slide and confocal microscope. The technique is suitable for measurement of both angular and linear displacements. The algorithm presented in this paper overcomes the shortcomings of limited CCD pixel count and image size, out of focus areas at the boundaries of the image, and vertical motion of the slide out of the focal plane. Many parameters can be changed within the setup, allowing the resolution and range of measurement to be modified for a given application. Experimental verification has been performed in an angular measurement configuration, demonstrating noise-limited linear and angular resolutions of 0.04 μm and 0.29 μrad, respectively.

Experimental System Identification, Feed-Forward Control, and Hysteresis Compensation of a 2-DOF Mechanism

Most of the micro/nano manipulation mechanisms and systems are commonly based on flexure-based monolithic structures, and are generally driven by piezoelectric actuators. In the presented work, experimental system identification, 1-DOF trajectory tracking with feed-forward control, and hysteresis compensation are investigated. An experimental research facility with laser interferometry-based sensing and measurement technique is established. System identification experiments were performed on a 2-DOF flexure-based mechanism to investigate its dynamics. The system identification procedure, experimental design, data acquisition, analysis and validation of the identified system are presented in details. A linear sine swept signal is applied to the system as an input and the corresponding response of the system is measured with laser interferometry-based sensing and measurement technique. The experimental results are used to evaluate the transfer function and the first natural frequency of the system in the X and Y axes. Experimental validation data is used to verify the accuracy of the identified model. Further, a feed-forward controller is established to track a 1-DOF smooth multiple-frequency trajectory. For hysteresis compensation, inverse PI Prandtl-Ishlinskii model is derived from classical PI model. The parameters of the inverse PI model is estimated and validated with the experimental data. Finally, inverse PI model is directly adopted as a feed-forward controller for hysteresis compensation of piezoelectric actuators.

Development of a Passive Compliant Mechanism for Measurement of Micro/Nanoscale Planar 3-DOF Motions

This paper presents the design, optimization, and computational and experimental performance evaluations of a passively actuated, monolithic, compliant mechanism. The mechanism is designed to be mounted on or built into any precision positioning stage, which produces three degree-of-freedom (3-DOF) planar motions. It transforms such movements into linear motions, which can then be measured using laser interferometry-based sensing and measurement techniques commonly used for translational axes. This methodology reduces the introduction of geometric errors into sensor measurements, and bypasses the need for increased complexity sensing systems. A computational technique is employed to optimize the mechanisms performance, in particular, to ensure the kinematic relationships match a set of desired relationships. Computational analysis is then employed to predict the performance of the mechanism throughout the workspace of a coupled positioning stage, and the errors are shown to vary linearly with the input position. This allows the errors to be corrected through calibration. A prototype is manufactured and experimentally tested, confirming the ability of the proposed mechanism to permit measurements of 3-DOF motions.

Development of Piezo-Driven Compliant Bridge Mechanisms: General Analytical Equations and Optimization of Displacement Amplification

Compliant bridge mechanisms are frequently utilized to scale micrometer order motions of piezoelectric actuators to levels suitable for desired applications. Analytical equations have previously been specifically developed for two configurations of bridge mechanisms: parallel and rhombic type. Based on elastic beam theory, a kinematic analysis of compliant bridge mechanisms in general configurations is presented. General equations of input displacement, output displacement, displacement amplification, input stiffness, output stiffness and stress are presented. Using the established equations, a piezo-driven compliant bridge mechanism has been optimized to maximize displacement amplification. The presented equations were verified using both computational finite element analysis and through experimentation. Finally, comparison with previous studies further validates the versatility and accuracy of the proposed models. The formulations of the new analytical method are simplified and efficient, which help to achieve sufficient estimation and optimization of compliant bridge mechanisms for nano-positioning systems.

Improved uniform degree of multi-layer interlaminar bonding strength for composite laminate:

Due to their high strength-to-weight and stiffness-to-weight ratios, composite materials are widely used in the aerospace industry. As a key factor for the placement process, the compaction force directly affects the product performance by changing the interlaminar bonding strength. Besides the magnitude of the interlaminar bonding strength, the uniformity degree of the bonding strength for each contact interface also affects the overall structural rigidity of laminates. This study is aimed at analyzing and optimizing the compaction force when using a rubber roller to produce a composite product with uniform interlaminar bonding strength. The amount in which the compaction force from a rubber roller on the current layer influences the bonding strength of the layers underneath is investigated, and methods for optimizing the interlaminar bonding strength are developed and experimentally demonstrated.

The bounds on tracking performance utilising a laser-based linear and angular sensing and measurement methodology for micro/nano manipulation

This paper presents an analysis of the tracking performance of a planar three degrees of freedom (DOF) flexure-based mechanism for micro/nano manipulation, utilising a tracking methodology for the measurement of coupled linear and angular motions. The methodology permits trajectories over a workspace with large angular range through the reduction of geometric errors. However, when combining this methodology with feedback control systems, the accuracy of performed manipulations can only be stated within the bounds of the uncertainties in measurement. The dominant sources of error and uncertainty within each sensing subsystem are therefore identified, which leads to a formulation of the measurement uncertainty in the final system outputs, in addition to methods of reducing their magnitude. Specific attention is paid to the analysis of the vision-based subsystem utilised for the measurement of angular displacement. Furthermore, a feedback control scheme is employed to minimise tracking errors, and the coupling of certain measurement errors is shown to have a detrimental effect on the controller operation. The combination of controller tracking errors and measurement uncertainty provides the bounds on the final tracking performance.

Design and optimization of a compact, large amplification XY flexure-mechanism

This article presents the design methodology, optimization, and computational verification of a compact, high-gain planar XY flexure mechanism. The presented mechanism consists of a decoupled XY mechanism, and a modified Scott-Russel mechanism, which maintain linear XY motion, and amplify the input piezoelectric actuator (PEA) displacement respectively. Each mechanism offers the potential for use in isolation. When operated together, the combined mechanism is capable of nanometer scale precision and millimeter scale range in a vacuum-compatible design. The presented mechanism could therefore be used in micro-assembly operations including those performed in electron microscopes. The design methodology and mechanism models are presented, with the selected design computationally optimized and analyzed. By using a broad design space, and minimizing assumptions, the optimized mechanism produces workspace of approximately 930 × 940 μm2 from 15 μm of input displacement.

Development of a dexterous haptic micro/nanomanipulator utilizing a hybrid parallel-serial flexure mechanism

Flexible new micro/nanomanipulation tools are required to intuitively perform micro/nano operations without the need for expensive, bespoke tools. In this paper, the design and analysis of a dexterous 4 degree of freedom (DOF) hybrid serial-parallel micro/nanomanipulator is presented. The manipulator is controlled through a haptic device, enabling a trained user to control the systems motion, and sense the contact force applied to the target. The hybrid mechanism consists of a planar 3-DOF flexure-mechanism coupled to a 1-DOF flexure-mechanism. Both of the subsystems are monolithically constructed, and incorporate piezoelectric actuators (PEAs). The total mechanism is capable of motions up to 39.16, 36.18, and 38.9 micrometers in the X, Y, and Z axes, respectively, and 2.309 milliradians of rotation about the Z axis. A combination feedforward-feedback scaled haptic control scheme is developed. Using this, and with appropriate choice of end-effector, a user can perform a range of high-precision micromanipulation and assembly tasks. The mechanical design and analysis of the 4-DOF mechanism, and haptic control scheme are presented. The performance of the system is experimentally investigated, and the haptic scheme verified.