Associate Professor Yuen Yong

This article describes a monolithic nanopositioner constructed from in-plane bending actuators which provide greater deflection than previously reported extension actuators, at the expense of stiffness and resonance frequency. The proposed actuators are demonstrated by constructing an XY nanopositioning stage with a serial kinematic design. Analytical modeling and finite-element-analysis accurately predicts the experimental performance of the nanopositioner. A 10µm range is achieved in the X and Y axes with an applied voltage of +/-200 V. The first resonance mode occurs at 250 Hz in the Z axis. The stage is demonstrated for atomic force microscopy imaging.

Soft pneumatic actuators are usually fabricated using molding and casting techniques with silicone rubbers, which requires intensive manual labor and limits repeatability and design flexibility for complex geometries. This article presents the design and direct 3D-printing of novel omnidirectional soft pneumatic actuators using stereolithography (SLA) with an elastic resin and fused deposition modeling (FDM) with a thermoplastic polyurethane (TPU). The actuator is modeled and optimized for bending performance using the finite element method along with a hyperelastic material model that is based on experimental uniaxial tensile data. The designs inspired by fast pneumatic network actuators (PneuNets) allow for multimodal actuation including bending, extension and contraction motions under positive, negative or differential pressures. The predicted results from the finite element method are compared with the experimental results for a range of actuation configurations. These novel omnidirectional actuators have significant potential in applications such as pipe inspection and biomedical devices.

Pressure control plays a major role in the overall performance of fluid-driven soft robots. Due to the increasing demand for higher speed actuation and precision, a need exists for a practical design methodology that converts actuator performance specifications to a set of minimum pneumatic specifications, such as receiver volume and pressure, and valve conductance. This article presents a systematic parameter selection approach for pneumatic soft robotic systems by taking into consideration the desired closed-loop pressure responses. The two controllers under evaluation here are the PI controller with anti-windup and the on-off controller with hysteresis. Simulations are developed within Simscape Fluids to evaluate the effect of physical components and controller parameters on the actuator performance. The proposed parameter selection procedures are then applied on three soft actuators and their closed-loop pressure responses are experimentally evaluated. The measured pressure responses are in close agreement with the simulations and satisfy the rise time specifications.

The article describes the design and modeling of a five-axis monolithic nanopositioning stage constructed from a bimorph piezoelectric sheet. Six-axis motion is also possible but requires 16 amplifier channels rather than 8. The nanopositioner is ultra low profile with a thickness of 1 mm. Analytical modeling and finite-element-analysis accurately predict the experimental performance. The stage was conservatively driven with 33% of the maximum voltage, which resulted in an X and Y travel range of 6.22 µm and 5.27 µm respectively; a Z travel range of 26.5 µm; and a rotational motion of 600 µrad and 884 µrad about the X and Y axis respectively. The first resonance frequency occurs at 883 Hz in the Z axis. Experimental atomic force microscopy is performed using the proposed device as a sample scanner.

The quality (Q) factor is an important parameter of the resonance of the microcantilever as it determines both imaging bandwidth and force sensitivity. The ability to control the Q factor of multiple modes is believed to be of great benefit for atomic force microscopy techniques involving multiple eigenmodes. In this paper, we propose a novel cantilever design employing multiple piezoelectric transducers which are used for separated actuation and sensing, leading to guaranteed collocation of the first eight eigenmodes up to 3 MHz. The design minimizes the feedthrough usually observed with these systems by incorporating a guard trace on the cantilever chip. As a result, a multimode Q controller is demonstrated to be able to modify the quality factor of the first two eigenmodes over up to four orders of magnitude without sacrificing robust stability.

Recent development in high-speed nanotechnology applications, such as scanning probe microscopy and nanofabrication, has increased interest on the advancement of high-bandwidth flexure-guided nanopositioning systems. These systems are capable of providing motions with sub-nanometer resolution over a positioning bandwidth of a few kilohertz or more. High-speed nanopositioning devices are commonly driven by compact and stiff piezoelectric stack actuators. However, these actuators are highly sensitive to tensile and lateral forces. During high-speed operations, excessive inertia force due to the effective mass of nanopositioning system could potentially damage the actuator. To protect the piezoelectric actuator, preload is often applied to compensate for these inertial forces. This article surveys key challenges in existing preload techniques in the context of high-speed nanopositioning designs, and explores how these challenges can be overcome.

Piezoelectric stack actuators are the actuator of choice for many ultra-high precision systems owning to its fast responses and high pushing force capabilities. These actuators are constructed by bonding multiple piezoelectric layers together. An inevitable drawback of these actuators is that there are highly intolerant to tensile and shear forces. During high-speed operations, inertial forces due to effective mass of the system cause the actuators to experience excessive tensile forces. To avoid damage to the actuators, preload must be applied to compensate for these forces. In many nanopositioning systems, flexures are used to provide preload to the piezoelectric stack actuators. However, for high-speed systems with stiff flexures, displacing the flexures and sliding the actuators in place to preload them is a difficult task. One may reduce the stiffness of the flexures to make the preload process more feasible; however, this reduces the mechanical bandwidth of the system. This paper presents a novel preload mechanism that tackles the limitations mentioned above. The preload stage, which is connected in parallel mechanically to a high-speed vertical nanopositioner, allows the piezoelectric stack actuator to be installed and preloaded easily without significantly trading of the stiffness and speed of the nanopositioning system. The proposed vertical nanopositioner has a travel range of 10.6¿µ¿m. Its first resonant mode appears at about 24¿kHz along the actuation direction.

This article presents a high-speed single- and dual-stage vertical positioners for applications in optical systems. Each positioner employs a unique end-constraint method with orthogonal flexures to preload a piezoelectric stack actuator. This end-constraint method also significantly increases the first mechanical resonance frequency. The single-stage positioner has a displacement range of 7.6 µm and a first resonance frequency of 46.8 kHz. The dual-stage design consists of a long-range slow-stage and a short-range fast-stage. An inertial counterbalance technique was implemented on the fast-stage to cancel inertial forces resulting from high-speed motion. The dual-stage positioner has a combined travel range of approximately 10 µm and a first evident resonance frequency of 130 kHz.

© 2016 Elsevier B.V.This article presents a novel vertical positioning stage with a dual-sensor arrangement suitable for scanning probe microscopy. The stage has a travel range of 8.4¿µm and a first resonance frequency of 24¿kHz in the direction of travel. The sensor arrangement consists of an integrated piezoelectric force sensor and laminated piezoresistive strain sensor. The piezoelectric force sensor exhibits extremely low noise and introduces a zero into the dynamics which allows the use of integral force feedback. This control method provides excellent damping performance and guaranteed stability. The piezoresistive sensor is used for tracking control with an analog PI controller which is shown to be an approximate inverse of the damped system. The resulting closed-loop system has a bandwidth is 11.4¿kHz and 6s-resolution of 3.6¿nm, which is ideal for nanopositioning and atomic force microscopy (AFM) applications. The proposed vertical stage is used to replace the vertical axis of a commercial AFM. Scans are performed in constant-force contact mode with a tip velocity of 0.2¿mm/s, 1¿mm/s and 2¿mm/s. The recorded images contain negligible artefacts due to insufficient vertical bandwidth.

A key hurdle to achieve video-rate atomic force microscopy (AFM) in constant-force contact mode is the inadequate bandwidth of the vertical feedback control loop. This paper describes techniques used to increase the vertical tracking bandwidth of a nanopositioner to a level that is sufficient for video-rate AFM. These techniques involve the combination of: a high-speed XYZ nanopositioner; a passive damping technique that cancels the inertial forces of the Z actuator which in turns eliminates the low 20-kHz vertical resonant mode of the nanopositioner; an active control technique that is used to augment damping to high vertical resonant modes at 60 kHz and above. The implementation of these techniques allows a tenfold increase in the vertical tracking bandwidth, from 2.3 (without damping) to 28.1 kHz. This allows high-quality, video-rate AFM images to be captured at 10 frames/s without noticeable artifacts associated with vibrations and insufficient vertical tracking bandwidth.

This paper explores the possibility of piezoelectric actuators with integrated high-voltage power electronics. Such devices dramatically simplify the application of piezoelectric actuators since the power electronics are already optimized for the voltage range, capacitance, and power dissipation of the actuator. The foremost consideration is the thermal impedance of the actuator and heat dissipation. Analytical and finite-element methods are described for predicting the thermal impedance of a piezoelectric bender. The predictions are compared experimentally using thermal imaging on a piezoelectric bender with laminated miniature power electronics.

A flexure-guided serial-kinematic XYZ nanopositioner for high-speed Atomic Force Microscopy is presented in this paper. Two aspects influencing the performance of serial-kinematic nanopositioners are studied in this work. First, mass reduction by using tapered flexures is proposed to increased the natural frequency of the nanopositioner. 25% increase in the natural frequency is achieved due to reduced mass with tapered flexures. Second, a study of possible sensor positioning in a serial-kinematic nanopositioner is presented. An arrangement of sensors for exact estimation of cross-coupling is incorporated in the proposed design. A feedforward control strategy based on phaser approach is presented to mitigate the dynamics and nonlinearity in the system. Limitations in design approach and control strategy are discussed in the Conclusion.

We report the design of a two-degree-of-freedom microelectromechanical systems nanopositioner for on-chip atomic force microscopy (AFM). The device is fabricated using a silicon-on-insulator-based process to function as the scanning stage of a miniaturized AFM. It is a highly resonant system with its lateral resonance frequency at ~850 Hz. The incorporated electrostatic actuators achieve a travel range of 16 ~{&L~}m in each direction. Lateral displacements of the scan table are measured using a pair of electrothermal position sensors. These sensors are used, together with a positive position feedback controller, in a feedback loop, to damp the highly resonant dynamics of the stage. The feedback controlled nanopositioner is used, successfully, to generate high-quality AFM images at scan rates as fast as 100 Hz. © 2013 IEEE.

In this brief, the transient performance of the signal transformation approach (STA) is considerably enhanced by initializing the state vector of the compensator to appropriate values. For triangular reference tracking, it is shown that the proposed method is identical to the impulsive state multiplication (ISM) approach. Through simulations and experiments, we also show that the proposed method can be equally applied to improve the STA for arbitrarily shaped desired signals, where ISM is not applicable. Tracking efficacy of the proposed method compared with that of an ordinary feedback loop with a similar noise rejection performance is also demonstrated. © 2013 IEEE.

Raster scanning is common in atomic force microscopy (AFM). The nonsmooth raster waveform contains high-frequency content that can excite mechanical resonances of an AFM nanopositioner during a fast scan, causing severe distortions in the resulting image. The mainstream approach to avoid scan-induced vibrations in video-rate AFM is to employ a high-bandwidth nanopositioner with the first lateral resonance frequency above 20 kHz. In this paper, video-rate scanning on a nanopositioner with 11.3-kHz resonance frequency is reported using a smooth Lissajous scan pattern. The Lissajous trajectory is constructed by tracking two sinusoidal waveforms on the lateral axes of the nanopositioner. By combining an analog integral resonant controller (IRC) with an internal model controller, 1-and 2-kHz single tone set-points were successfully tracked. High-quality time lapsed AFM images of a calibration grating recorded at 9 and 18 frames/s without noticeable image distortions are reported. © 2013 IEEE.

We report on a novel application of internal model control for accurate tracking of a high speed spiral trajectory in scanning probe microscopy. With a closed-loop bandwidth of only 1 kHz, we achieved tracking errors as low as 0.31% of the scan diameter and an ultra-video frame rate of 37.5 frames per second (FPS) for a high pitch spiral trajectory generated by amplitude modulation of 3 kHz sinusoids.

This paper presents the combination of active and passive damping control techniques to increase the vertical tracking bandwidth of a fast nanopositioner for video-speed atomic force microscopy (AFM). The passive damping technique utilizes an inertial counterbalance arrangement to mechanically cancel the low 20-kHz vertical resonant mode of the nanopositioner; and the active control technique employs an integral resonant control technique to augment damping to resonant modes at 60 kHz and above. The combined damping techniques increases the vertical tracking bandwidth from 2.3 kHz to 28.1 kHz. This allow high-quality AFM images to be recorded at video speed, i.e. up to 1000 Hz line rate without noticeable image artifacts.

This article identifies the design considerations for a two degree of freedom (DoF) miniature robotic leg utilizing piezoelectric bimorph actuators with a specific focus on the resonance modes of the system. An analytical model was developed using three independent lumped mass models with superposition for tuning the resonance frequencies and optimizing the performance of the leg. The model was verified both experimentally and using finite element analysis (FEA).

This paper presents the position control of a three-DOF (degree-of-freedom) micro-motion stage. This stage provides micro scale planar motion along the x and y axis and rotational motion along the z axis. It uses a 3 RRR (three revolute-revolute-revolute) flexure hinge based compliant mechanism driven by three piezoelectric stack actuators to achieve the micro motion along x, y and z axis. The micro-motion stage with operating frequency up to 1Hz is controlled to follow a prescribed circular trajectory. An analytical Jacobian of the stage is derived to relate the input displacements of the piezo-actuators to the output displacements of the stage. Closed-loop positioning control is achieved using the standard PI controller. The errors related to the position control along the x and y axis are presented.