Dan O. Popa

Open-loop versus closed-loop control of MEMS devices: Choices and issues

From a controls point of view, micro electromechanical systems (MEMS) can be driven in an open-loop and closed-loop fashion. Commonly, these devices are driven open-loop by applying simple input signals. If these input signals become more complex by being derived from the system dynamics, we call such control techniques pre-shaped open-loop driving. The ultimate step for improving precision and speed of response is the introduction of feedback, e.g. closed-loop control. Unlike macro mechanical systems, where the implementation of the feedback is relatively simple, in the MEMS case the feedback design is quite problematic, due to the limited availability of sensor data, the presence of sensor dynamics and noise, and the typically fast actuator dynamics. Furthermore, a performance comparison between open-loop and closed-loop control strategies has not been properly explored for MEMS devices. The purpose of this paper is to present experimental results obtained using both open- and closed-loop strategies and to address the comparative issues of driving and control for MEMS devices. An optical MEMS switching device is used for this study. Based on these experimental results, as well as computer simulations, we point out advantages and disadvantages of the different control strategies, address the problems that distinguish MEMS driving systems from their macro counterparts, and discuss criteria to choose a suitable control driving strategy.

Micro and Mesoscale Robotic Assembly

Abstract Some of the challenges associated with Microsystems assembly are examined in this paper and illustrated with examples of ongoing research at the authors’ institution. One of the basic challenges in precision assembly is the need for very high accuracy over a large range of motion. This challenge is addressed through a “multiscale” approach, which involves the design of assembly tools and processes at multiple scales, and their integration into coherent system architectures. Parallelism is an important aspect of this architecture, with the goal of enabling high-throughput, fault-tolerant assembly at moderate cost. The modularity of the architecture is also important, given the need to frequently reconfigure microsystem assembly cells for small-batch production. This paper presents several concepts for the development of multiscale robotic tools for the assembly of microsystems. Numerical simulations and experimental results are used to illustrate the relevance of the proposed approaches. Extensions to manipulation at the nanoscale are briefly discussed. At the conclusion are some guidelines for the design of multiscale assembly systems.

Adaptive sampling algorithms for multiple autonomous underwater vehicles

Sampling is a critical problem in the observation of underwater phenomena using single or multiple AUV platforms. The determination of optimal paths and sampling strategies that effectively utilize available resources is critical to these missions. Recent work performed jointly at RPI and AUSI on the development of adaptive sampling algorithms (ASA) utilizes information measures, estimation theory, and potential fields to direct the robots to the locations in space most likely to yield information about the sensed field variable of interest. Typical sensory information can consist of spatial distribution of one or more field variables, such as salinity, dissolved oxygen, temperature, current, etc. In order to test our algorithms we have created the MATCON simulation environment, an underwater experimental platform using solar AUVs, and a land-based experimental testbed using inexpensive wheeled robots.

An analysis of some fundamental problems in adaptive control of force and impedance behavior: theory and experiments

Force control in robotic mechanisms has been extensively researched. There have been several results which have concentrated either on explicit force control or impedance control. Further, many of these works require accurate identification of the stiffness of the environment. In this work the authors examine the role of adaptive controllers for force control with unknown system and environment parameters and clearly integrate several fundamental issues such as explicit force control, impedance control, and impedance control combined with a desired force control. All of these issues are treated using standard model-reference adaptive control (MRAC) approach. Within the same framework, the authors analyze other equally fundamental issues such as environment stiffness identification, stability of the complete system, and parameter convergence. The authors demonstrate the effectiveness of the proposed theory through experiments.

Robotic deployment of sensor networks using potential fields

Deploying large numbers of sensors has been receiving a lot of attention for detection of hazardous biological or chemical substances in public buildings, airports, shallow water harbors, etc. The sensor-carrying robots are in fact agents that facilitate the repositioning of network nodes in order to increase their coverage and accuracy. Wireless network communication is an essential technology in transmitting the sensed and telemetry information between robots, but it has traditionally been addressed separately from mobile robot navigation. In this work we propose to use a potential field framework to control the behavior of the mobile sensor nodes by combining classical robotic team concepts (obstacle avoidance, goal attainment, flight formation, environment mapping and coverage) with traditional sensor network concepts (node energy minimization, optimal data rate and congestion control, routing in ad-hoc networks). Simulation results are used to illustrate the proposed concepts, and an experimental mobile sensor fleet is built at the authors institution.

μ 3 : Multiscale, Deterministic Micro-Nano Assembly System for Construction of On-Wafer Microrobots

One of the major issues enduring with micro-scale mechanics has been to design high fidelity miniature machines capable of performing complex operations. Though achieved in some proportion through conventional in-plane and out-of-plane designs, the efficacy of such micro-electromechanical systems (MEMS) structures is highly limited due to complicate fabrication and inadequate robustness. On the other hand, the use of precise robots to assemble MEMS parts of comparatively simpler design to build 3D micromechanical structures has recently emerged as a viable approach. Such modular assemblies of microscale parts typically utilize minimum energy connectors that are multifunctional, e.g., mechanical, electrical etc. The μ3 is a 3D microassembly station consisting of 19 DOF arranged into 3 micromanipulators, with additional microgrippers and stereo microscope vision. The platform is capable of motion resolutions of 3nm and is small enough to be used inside of a scanning electron microscope (SEM) for nano-manipulation. In this paper we discuss how systematic identification and calibration of the station, combined with appropriate part connector designs can lead to multi-degree of freedom active MEMS robots assembled on a wafer

Dynamic modeling and input shaping of thermal bimorph MEMS actuators

Thermal bimorphs are a popular actuation technology in MEMS (micro-electro-mechanical systems). Their operating principle is based on differential thermal expansion induced by Joule heating. Thermal bimorphs, and other thermal flexure actuators have been used in many applications, from micro-grippers, to micro-optical mirrors. In most cases open-loop control is used to difficulties in fabricating positioning sensors together with actuator. In this paper we present several methods for extracting reduced-order thermal flexure actuator models based on experimental data, physical principles, and FEA simulation. We then use the models to generate optimal driving signals using input shaping techniques. Both simulation and experimental results are included to illustrate the efficacy of our approach. This framework can also be applied to other types of MEMS actuators, including electrostatic comb drives.

A Multiscale Assembly and Packaging System for Manufacturing of Complex Micro-Nano Devices

Reliable manufacturability has always been a major issue in commercialization of complex and heterogeneous microsystems. Though successful for simpler and monolithic microdevices such as accelerometers and pressure sensors of early days, conventional surface micromachining techniques, and in-plane mechanisms do not prove suffice to address the manufacturing of todays wide range of microsystem designs. This has led to the evolution of microassembly as an alternative and enabling technology which can, in principle, build complex systems by assembling heterogeneous microparts of comparatively simpler design; thus reducing the overall footprint of the device and providing high structural rigidity in a cost efficient manner. However, unlike in macroscale assembly systems, microassembly does not enjoy the flexibility of having ready-to-use manipulation systems or standard off-the-shelf components. System specific designs of microparts and mechanisms make the fabrication process expensive and assembly scheme diverse. This warrants for a modular microassembly cell which can execute the assembly process of multiple microsystems by reconfiguring the kinematics setup, end-effectors, feedback system, etc.; thus minimizing the cost of production. In this paper, we present a multiscale assembly and packaging system (MAPS) comprising of 20 degrees of freedom (DoFs) that can be arranged in several reconfigurable micromanipulation modules depending on the specific task. The system has been equipped with multiple custom-designed microgrippers and end-effectors for different applications. Stereo microscopic vision is achieved through four high-resolution cameras. We will demonstrate the construction of two different microsystems using this microassembly cell; the first one is a miniature optical spectrum analyzer called microspectrometer and the second one is a MEMS mobile robot/conveyor called Arripede.

Reconfigurable micro-assembly system for photonics applications

The assembly of parts with dimensions several hundred microns or less is a challenging problem, and has received increasing attention for applications in areas such as telecommunication, automotive, and biotechnology. Current state of the art micro-assembly systems are often specialized devices and software. In this paper we present a reconflgurable assembly system designed to handle micro-parts in such a way that high precision actuation and sensing is used only in the subsystems where it is actually necessary. Aspects related to part gripping, fucturing, sensing, motion and bonding are discussed. Analysis and experiments are presented to show that this architecture can lead to a relatively low cost and flexible assembly solution.

The lateral instability problem in electrostatic comb drive actuators: modeling and feedback control

Comb drives inherently suffer from electromechanical instability called lateral pull-in, side pull-in or, sometimes, lateral instability. Although fabricated to be perfectly symmetrical, the actuator’s comb structure is always unbalanced, causing adjacent finger electrodes to contact each other when voltage–deflection conditions are favorable. Lateral instability decreases the active traveling range of the actuator, and the problem is typically approached by improving the mechanical design of the suspension. In this paper, a novel approach to counteracting the pull-in phenomenon is proposed. It is shown that the pull-in problem can be successfully counteracted by introducing active feedback steering of the lateral motion. In order to do this, however, the actuator must be controllable in the lateral direction, and lateral deflection measurements need to be available. It is shown herein how to accomplish this. The experimentally verified dynamic model of the comb drive is extended with a lateral motion model. The lateral part of the model is verified through experimental results and finite element analysis and is hypothetically extended to accommodate both sensor and actuator functionalities for lateral movement. A set of simulations is performed to illustrate the improved traveling range gained by the controller.