Kenn R. Oldham

Thin-Film PZT Lateral Actuators With Extended Stroke

Many microelectromechanical system applications require large in-plane actuation forces, with stroke lengths ranging from submicrometer to tens of micrometers in distance. Piezo electric thin films are capable of generating very large actuation forces, but their motion is not easily directed into lateral displacement in microscale devices. A new piezoelectric thin-film actuator that uses a combination of piezoelectric unimorph beams to generate lateral displacement has been developed. The piezoelectric actuators were fabricated using chemical-solution-derived lead zirconate titanate thin films. These actuators have demonstrated forces greater than 7 mN at displacements of nearly 1 mum, with maximum stroke lengths at 20 V greater than 5 mum in a 500-mum-long by 100-mum-wide actuator. Force and displacement capabilities can be manipulated through simple changes to the actuator design, while actuator nonlinearity can produce dramatic gains in work capacity and stroke length for longer actuators.

Targeted vertical cross-sectional imaging with handheld near-infrared dual axes confocal fluorescence endomicroscope

We demonstrate vertical cross-sectional (XZ-plane) images of near-infrared (NIR) fluorescence with a handheld dual axes confocal endomicroscope that reveals specific binding of a Cy5.5-labeled peptide to pre-malignant colonic mucosa. This view is perpendicular to the tissue surface, and is similar to that used by pathologists. The scan head is 10 mm in outer diameter (OD), and integrates a one dimensional (1-D) microelectromechanical systems (MEMS) X-axis scanner and a bulky lead zirconate titanate (PZT) based Z-axis actuator. The microscope images in a raster-scanning pattern with a ±6 degrees (mechanical) scan angle at ~3 kHz in the X-axis (fast) and up to 10 Hz (0–400 μm) in the Z-axis (slow). Vertical cross-sectional fluorescence images are collected with a transverse and axial resolution of 4 and 5 μm, respectively, over a field-of-view of 800 μm (width) × 400 μm (depth). NIR vertical cross-sectional fluorescence images of fresh mouse colonic mucosa demonstrate histology-like imaging performance with this miniature instrument.

Modeling and Optimal Low-Power On–Off Control of Thin-Film Piezoelectric Rotational Actuators

A novel open-loop minimal energy on-off servo system and control strategy are described for ensuring specified displacements from new microscale piezoelectric rotational joints under extremely strict power budgets. The rotational joints are driven by thin-film lead-zirconate-titanate actuators and are targeted for use in autonomous terrestrial microrobots. A lumped-parameter, second-order model of anticipated joint behavior is utilized to estimate the natural frequency and damping ratio of the robot joints, which, in turn, are used to identify necessary sampling rates and switching drive circuit parameters for implementation of on-off control. An identified model of leg joint behavior is then used to both verify lumped-parameter modeling and to optimize on-off input sequences to the rotary joint. The optimization procedure incorporates energy costs from both switching and holding an input voltage on microactuators that behave as a capacitive load, while ensuring that specified final states of a dynamic system are achieved at a specified point in time. Optimization is done via a new application of binary programming. In addition, modest robustness of the system response to parameter variation can be produced during control sequence generation. Optimized input sequences are applied to both macroscale piezoelectric actuators and to prototype thin-film piezoelectric leg joints, and show that specified actuator motions can be achieved with energy consumption of less than 5 μJ per movement.

Vertical cross-sectional imaging of colonic dysplasia in vivo with multi-spectral dual axes confocal endomicroscopy.

Pathologists evaluate histology sectioned perpendicular to the tissue surface, or vertical cross-section. This orientation (XZ-plane) enables evaluation of mucosal differentiation in the basilar-to-luminal direction. Current endomicroscopes use a conventional (single axis) optical design.1 Imaging is limited to horizontal cross-sections (XY-plane) where the micro-anatomy is frequently similar across the field-of-view (FOV). In the dual axes configuration, light is delivered and collected off-axis, and images can be detected over a much larger range of intensities.2 Molecular images collected using fluorescence can improve specificity for disease detection and reveal functional properties about tissue.3 Proper interpretation of these images requires correlation with the micro-anatomy. We aim to demonstrate the simultaneous collection of two fluorescence images in vivo in vertical cross-sections using a dual axes confocal endomicroscope. An overlay of molecular and anatomical images from normal and dysplastic mouse colonic mucosa will be displayed in real time.

Multi-Degree-of-Freedom Thin-Film PZT-Actuated Microrobotic Leg

As a novel approach to future microrobotic locomotion, a multi-degree-of-freedom (m-DoF) microrobotic appendage is presented that generates large range of motion (5° -40°) in multiple axes using thin-film lead zirconate titanate (PZT) actuators. Due to the high driving force of PZT thin films and a robust fabrication process, m-DoF legs that retain acceptable payload capacity ( ~ 2 mg per leg) are achieved. The fabrication process permits thin-film PZT actuator integration with more complex higher aspect ratio silicon structures than previous related processes, using vertical silicon dioxide barrier trenches formed before PZT deposition to provide robust encapsulation of the silicon during later XeF2 release. Planarization of the barrier trenches avoids detrimental effects on piezoelectric performance from the substrate alteration. Once fabricated, kinematic modeling of compact PZT actuator arrays in prototype leg joints is compared to experimental displacement measurements, demonstrating that piezoelectric actuator and assembled robot leg joint performance can be accurately predicted given certain knowledge of PZT properties and residual stress. Resonant frequencies, associated weight bearing, and power consumption are also obtained.

Lateral thin-film piezoelectric actuators for bio-inspired micro-robotic locomotion

Thin-film lead-zirconate-titanate (PZT) actuators are a potential enabling technology for autonomous micro-robots with locomotion abilities rivaling biological systems. Actuators capable of supplying the large forces and extended displacements needed to drive terrestrial micro-robotic locomotion have been designed and tested. These actuators use a combination of upward and downward unimorph bending to generate in-plane robotic joint motion. 500 μm by 100 μm actuators have demonstrated forces greater than 5 mN over almost 1 μm stroke length at just 20 V. These actuators can be leveraged to drive angular displacement of high-aspect ratio silicon flexures. Actuators are currently being integrated into flexural arrays to produce joint angles comparable to insects. Stacks of these silicon joint structures may be used to reinforce load-bearing capacity of the completed micro-robotic legs. Dynamic simulations of hexapedal and many-legged robots less than one centimeter in length utilizing these actuator-joint structures indicate potential payloads ranging from 50 to 200 mg, depending on the joint design, and walking speeds up to approximately 4 cm/s.© 2009 ASME

A three-degree-of-freedom thin-film PZT-actuated microactuator with large out-of-plane displacement

A novel three degree-of-freedom microactuator based on thin-film lead-zirconate-titanate (PZT) is described with its detailed structural model. Its central rectangular-shaped mirror platform, also referred to as the stage, is actuated by four symmetric PZT bending legs such that each leg provides vertical translation for one corner of the stage. It has been developed to support real-time in vivo vertical cross-sectional imaging with a dual axes confocal endomicroscope for early cancer detection, having large displacements in three axes (z, θx, θy) and a relatively high bandwidth in the z-axis direction. Prototype microactuators closely meet the performance requirements for this application; in the out-of-plane (z-axis) direction, it has shown more than 177 μm of displacement and about 84 Hz of structural natural frequency, when two diagonal legs are actuated at 14V. With all four legs, another prototype of the same design with lighter stage mass has achieved more than 430 μm of out-of-plane displacement at 15V and about 200 Hz of bandwidth. The former design has shown approximately 6.4° and 2.9° of stage tilting about the x-axis and y-axis, respectively, at 14V. This paper also presents a modeling technique that uses experimental data to account for the effects of fabrication uncertainties in residual stress and structural dimensions. The presented model predicts the static motion of the stage within an average absolute error of 14.6 μm, which approaches the desired imaging resolution, 5 μm, and also reasonably anticipates the structural dynamic behavior of the stage. The refined model will support development of a future trajectory tracking controller for the system.

Optimal Low-Power Piezoelectric Actuator Control With Charge Recovery for a Microrobotic Leg

This paper describes an efficient control strategy for a piezoelectric microactuator using charge recovery. For piezoelectric actuators, as well as other actuators that behave primarily as capacitive loads, energy consumption can be reduced by minimizing the number of times an actuator is charged and by recovering stored energy when it is turned off. An integer programming-based algorithm is used to drive microrobotic legs powered by piezoelectric actuators to a specified angle in a specified time using minimum energy. Partial charge recovery is incorporated; this allows the use of a more flexible controller than a pure on-off controller, with two or more intermediate voltage levels between the minimum and maximum voltages available to improve positioning accuracy. Simulated and experimental tests show that a prototype piezoelectric robotic leg joint achieved controlled movements with one third of the energy consumed by a pure on-off controller.

Modeling and Simulation of a Parametrically Resonant Micromirror With Duty-Cycled Excitation.

High frequency large scanning angle electrostatically actuated microelectromechanical systems (MEMS) mirrors are used in a variety of applications involving fast optical scanning. A 1-D parametrically resonant torsional micromirror for use in biomedical imaging is analyzed here with respect to operation by duty-cycled square waves. Duty-cycled square wave excitation can have significant advantages for practical mirror regulation and/or control. The mirrors nonlinear dynamics under such excitation is analyzed in a Hills equation form. This form is used to predict stability regions (the voltage-frequency relationship) of parametric resonance behavior over large scanning angles using iterative approximations for nonlinear capacitance behavior of the mirror. Numerical simulations are also performed to obtain the mirrors frequency response over several voltages for various duty cycles. Frequency sweeps, stability results, and duty cycle trends from both analytical and simulation methods are compared with experimental results. Both analytical models and simulations show good agreement with experimental results over the range of duty cycled excitations tested. This paper discusses the implications of changing amplitude and phase with duty cycle for robust open-loop operation and future closed-loop operating strategies.

On-off control for low-power servo control in piezoelectric micro-robotics

Autonomous micro-systems require extremely strict power budgeting to provide useful battery lifetime and functionality. To produce bio-inspired terrestrial micro-robots, this power budget must encompass servo control of a set of robotic leg joints. Piezoelectric actuators can provide high-force actuation necessary for such an application, but driving circuitry for piezoelectric actuator control can consume excessive power. On-off control can reduce actuator power consumption by eliminating analog amplification and minimizing the number of on-off transitions required to perform a given leg joint motion, as compared to conventional PWM controllers. A sample on-off controller designed to require a limited number of on-off transitions from a prototype system is described, and trade-offs to motion accuracy and robustness as power consumption is reduced are discussed. Experimental tests are conducted on a macro-scale piezoelectric test-bed.Copyright