Anthony G. Fowler

A 2-DOF Electrostatically Actuated MEMS Nanopositioner for On-Chip AFM

A new 2-DOF microelectromechanical systems (MEMS)-based parallel kinematic nanopositioner with electrostatic actuation is presented. The device has been designed, fabricated, and implemented using the silicon-on-insulator-based MEMSCAP SOIMUMPs process. Experimental characterization shows that in-plane displacements in excess of 15 μm are achievable and that the first resonant mode along each axis is located at approximately 820 Hz. The nanopositioners use in a practical application is demonstrated, with the device being used as the scanning stage during an atomic force microscope scan.

A Feedback Controlled MEMS Nanopositioner for On-Chip High-Speed AFM

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 μm 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.

Design and Analysis of Nonuniformly Shaped Heaters for Improved MEMS-Based Electrothermal Displacement Sensing

Conventional heaters used in microelectromechanical systems (MEMS) electrothermal displacement sensors typically feature a uniform cross section, which results in a nonuniform temperature profile. In this paper, electrothermal sensors with a shaped beam profile are introduced, with simulation results showing that a much flatter temperature distribution is achieved across the length of the heater. The proposed sensor design is implemented as the displacement sensor for a MEMS nanopositioner together with a more conventional electrothermal sensor design for comparative purposes. Experimental testing indicates that the shaped profile significantly improves upon the conventional sensor design in a number of areas, including sensitivity, linearity, and noise performance.

Control of a Novel 2-DoF MEMS Nanopositioner With Electrothermal Actuation and Sensing

This paper presents the full characterization, modeling, and control of a 2-degrees-of-freedom microelectromechanical systems (MEMS) nanopositioner with fully integrated electrothermal actuators and sensors. Made from nickel Z-shaped beams, the actuators are able to move the devices stage in positive and negative directions (contrary to classical V-shaped electrothermal actuators) and along two axes (x and y). The integrated electrothermal sensors are based on polysilicon resistors, which are heated via Joule heating due to an applied electrical bias voltage. The stage displacement is effectively measured by variations in their resistance, which is dependent on the position of the stage. The characterization tests carried out show that the MEMS nanopositioner can achieve a range of displacement in excess of ±5 μm for each of the x and y axes, with a response time better than 300 ms. A control scheme based on the combination of feedforward and internal model control-feedback is constructed to enhance the general performance of the MEMS device, and in particular to reject the cross-coupling between the two axes and to enhance the accuracy and the response time. The experimental results demonstrate the efficiency of the proposed scheme and demonstrate the suitability of the designed device for nanopositioning applications.

Force-Controlled MEMS Rotary Microgripper

This paper presents a force-controlled microelectromechanical systems rotary microgripper with integrated electrothermal sensors. The proposed microgripper achieves a large displacement (85 μm) at low driving voltages (≤80 V). Closed-loop force control is implemented to ensure the safety of the operation where the controller gain is experimentally tuned so that the desired response is achieved. One of the main contributions of this work is the implementation of a null-displacement feedback control force-sensing technique, where the controller counteracts the input disturbance (contact force) and an integrated electrothermal displacement sensor provides a feedback signal to close the control loop. In this manner, the contact force is measured without moving the structure. Finally, the effectiveness of the controller and the performance of the proposed microgripper are verified by a set of experiments. The results demonstrate the satisfactory performance of the proposed force-controlled microgripper in a practical application.

High-stroke silicon-on-insulator MEMS nanopositioner: Control design for non-raster scan atomic force microscopy

A 2-degree of freedom microelectromechanical systems nanopositioner designed for on-chip atomic force microscopy (AFM) is presented. The device is fabricated using a silicon-on-insulator-based process and is designed as a parallel kinematic mechanism. It contains a central scan table and two sets of electrostatic comb actuators along each orthogonal axis, which provides displacement ranges greater than ±10 μm. The first in-plane resonance modes are located at 1274 Hz and 1286 Hz for the X and Y axes, respectively. To measure lateral displacements of the stage, electrothermal position sensors are incorporated in the design. To facilitate high-speed scans, the highly resonant dynamics of the system are controlled using damping loops in conjunction with internal model controllers that enable accurate tracking of fast sinusoidal set-points. To cancel the effect of sensor drift on controlled displacements, washout controllers are used in the damping loops. The feedback controlled nanopositioner is successfully used to perform several AFM scans in contact mode via a Lissajous scan method with a large scan area of 20 μm × 20 μm. The maximum scan rate demonstrated is 1 kHz.

An Omnidirectional MEMS Ultrasonic Energy Harvester for Implanted Devices

This paper presents the design and characterization of a microelectromechanical systems (MEMS)-based energy harvester with target applications, including implanted biomedical sensors and actuators. The harvester is designed to utilize ultrasonic waves from an external transmitter for mechanical excitation, with electrostatic transducers being used to convert the vibrations of a central mass structure into electrical energy. The device features a novel 3-degrees of freedom design, which enables energy to be produced by the harvester in any orientation. The harvester is fabricated using a conventional silicon-on-insulator MEMS process, with experimental testing showing that the system is able to generate 24.7, 19.8, and 14.5nW of electrical power, respectively, via the devices x-, y- and z-axis resonance modes over a 15-s period.

Internal Model Control for Spiral Trajectory Tracking With MEMS AFM Scanners

We demonstrate the application of internal model control for accurate tracking of spiral scan trajectories, where the reference signals are orthogonal sinusoids whose amplitudes linearly vary with time. The plant is a 2-D microelectromechanical system nanopositioner equipped with in situ differential electrothermal sensors and electrostatic actuators. This device is used as the scanner stage in an atomic-force microscope. Additional internal model components are included in the controllers to compensate for the residual tracking errors due to plant nonlinearities. In a large scan range with a diameter of 16 μm, we achieved tracking of 1430-Hz spiral sinusoids, a frequency beyond the undamped fundamental resonance of the plant at 1340 Hz. This leads to a video-rate scan speed of 18 frames/s.

A 3-DoF MEMS ultrasonic energy harvester

The harvesting of electrical energy from vibrational motion to power electronic devices remains a major focus of microelectromechanical systems (MEMS) research. Conversion mechanisms such as piezoelectric, electromagnetic or electrostatic transducers have been used for this purpose. Most MEMS energy harvesters to date use ambient vibrations as their source of kinetic energy. In contrast, this paper presents the design and characterization of a 3-degree of freedom MEMS energy harvester that utilizes electrostatic transducers to convert ultrasonic waves into electrical energy. The devices ability to harvest energy is demonstrated via the charging of a storage capacitor, with experimental results showing that the device is able to harvest 12.6nW of average electrical power.

On-Chip Dynamic Mode Atomic Force Microscopy: A Silicon-on-Insulator MEMS Approach

The atomic force microscope (AFM) is an invaluable scientific tool; however, its conventional implementation as a relatively costly macroscale system is a barrier to its more widespread use. A microelectromechanical systems (MEMS) approach to AFM design has the potential to significantly reduce the cost and complexity of the AFM, expanding its utility beyond current applications. This paper presents an on-chip AFM based on a silicon-on-insulator MEMS fabrication process. The device features integrated xy electrostatic actuators and electrothermal sensors as well as an AlN piezoelectric layer for out-of-plane actuation and integrated deflection sensing of a microcantilever. The three-degree-of-freedom design allows the probe scanner to obtain topographic tapping-mode AFM images with an imaging range of up to