Alexander H. Slocum

Characteristics of Beam-Based Flexure Modules

The beam flexure is an important constraint element in flexure mechanism design. Nonlinearities arising from the force equilibrium conditions in a beam significantly affect its properties as a constraint element. Consequently, beam-based flexure mechanisms suffer from performance tradeoffs in terms of motion range, accuracy and stiffness, while benefiting from elastic averaging. This paper presents simple yet accurate approximations that capture the effects of load-stiffening and elastokinematic nonlinearities in beams. A general analytical framework is developed that enables a designer to parametrically predict the performance characteristics such as mobility, over-constraint, stiffness variation, and error motions, of beam-based flexure mechanisms without resorting to tedious numerical or computational methods. To illustrate their effectiveness, these approximations and analysis approach are used in deriving the force-displacement relationships of several important beam-based flexure constraint modules, and the results are validated using finite element analysis. Effects of variations in shape and geometry are also analytically quantified.

Constraint-based design of parallel kinematic XY flexure mechanisms

This paper presents parallel kinematic XY flexure mechanism designs based on systematic constraint patterns that allow large ranges of motion without causing over-constraint or significant error motions. Key performance characteristics of XY mechanisms such as mobility, cross-axis coupling, parasitic errors, actuator isolation, drive stiffness, lost motion, and geometric sensitivity, are discussed. The standard double parallelogram flexure module is used as a constraint building-block and its nonlinear force-displacement characteristics are employed in analytically predicting the performance characteristics of two proposed XY flexure mechanism designs. Fundamental performance tradeoffs, including those resulting from the nonlinear load-stiffening and elastokinematic effects, in flexure mechanisms are highlighted. Comparisons between closed-form linear and nonlinear analyses are presented to emphasize the inadequacy of the former. It is shown that geometric symmetry in the constraint arrangement relaxes some of the design tradeoffs, resulting in improved performance. The nonlinear analytical predictions are validated by means of computational finite element analysis and experimental measurements.

Fabrication of composite microstructures by capillarity-driven wetting of aligned carbon nanotubes with polymers

The interaction, or wetting, of long aligned carbon nanotube (CNT) forests with off-the-shelf (no solvent added) commercial thermoset polymers is investigated experimentally. A technique for creating vertically aligned CNT composite microstructures of various shapes is presented. The effective wetting of the forests, as evidenced by a lack of voids, by three polymers with widely varying viscosities supports the feasibility of using CNT forests in large-scale hybrid advanced composite architectures. Among various routes identified for the polymer to penetrate the forest, capillarity-driven wetting along the CNT axis is the preferred route. Aligned CNT microstructures are useful in many applications including test structures for direct mechanical and multifunctional property characterization of the aligned CNT?polymer composite materials.

A centrally-clamped parallel-beam bistable MEMS mechanism

This paper presents a monolithic mechanically-bistable mechanism that does not rely on residual stress for its bistability. The bistable mechanism comprises two centrally-clamped parallel beams that have a curved shape but no residual stress after fabrication. Modal analysis and FEA simulation of the beams are used to predict and design the bistable behavior, and they agree well. Micro-scale mechanisms are fabricated by DRIE and their test results agree well with the theoretical and numerical predictions.

A bulk-micromachined bistable relay with U-shaped thermal actuators

This paper reports a deep-reactive ion etching (DRIE)-through-etched laterally bistable MEMS relay for power applications, with a primary emphasis on the design and modeling of its U-shaped transient thermal actuators, and a secondary emphasis on the design and fabrication of its contact element. In this relay, a contact crossbar is carried by a curved-beam bistable mechanism , which is toggled by transient U-shaped thermal actuators with their hot beam adiabaticly heated by electrical pulses. Each U-shaped thermal actuator comprises uniform-thickness hot and cold beams with a gap between them so they bend differently. This paper develops both a basic model and a complete model for the actuator that are verified by Finite Element Analysis and serve as effective design tools. The DRIE process creates nonideal etched surfaces, which pose challenges for good relay contacts. Both contact design and process development are discussed to help alleviate this problem. The fabricated relay exhibits a minimum total on-state resistance of 60 m/spl Omega/, and a maximum current carrying capacity of 3 A. It switches with a 1 ms actuation pulse, and a maximum 5 Hz repetition rate.

A high-current electrothermal bistable MEMS relay

This paper reports the design, fabrication and testing of a thermally-actuated bistable MEMS relay. Mechanical bistability ensures zero actuation power in both the on and off states, and permits actuation with a transient thermal actuator. In the off state, this relay stands off more than 200 V. In the on state, it exhibits a minimum total resistance of 60 n/spl Omega/ and a maximum current carrying capacity of 3 A. It switches with a maximum 5 Hz rate.

Design of three-groove kinematic couplings

Abstract Kinemtic couplings are statically determinant structures that are often used in precision fixturing applications because of their high repeatability. The simplicity of their design also makes the design of accurate interchangeable couplings a realistic task. This paper discusses three-groove kinematic coupling design methodologies and then describes the theory required to calculate stresses at the contact interfaces and error motions at any point on the couplings. The theory presented is incorporated into a kinematic coupling design spreadsheet (written in Microsoft Excel) that can run on a personal computer.

A Patient-Mounted, Telerobotic Tool for CT-Guided Percutaneous Interventions

This paper describes Robopsy, an economical, patient-mounted, telerobotic, needle guidance and insertion system, that enables faster, more accurate targeting during CT-guided biopsies and other percutaneous interventions. The current state of the art imaging technology facilitates precise location of sites within the body; however, there is no mechanical equivalent to then facilitate precise targeting. The lightweight, disposable actuator unit, which affixes directly to the patient, is composed primarily of inexpensive, injection molded, radiolucent, plastic parts that snap together, whereas the four micromotors and control electronics are retained and reused. By attaching to a patient, via an adhesive pad and optional strap points, the device moves passively with patient motion and is thus inherently safe. The device’s mechanism tilts the needle to a two degree-of-freedom compound angle, toward the patient’s head or feet (in and out of the scanner bore) and left or right with respect to the CT slice, via two motor-actuated concentric, crossed, and partially nested hoops. A carriage rides in the hoops and interfaces with the needle via a two degree-of-freedom friction drive that both grips the needle and inserts it. This is accomplished by two rubber rollers, one passive and one driven, that grip the needle via a rack and pinion drive. Gripping is doctor controlled; thus when not actively being manipulated, the needle is released and allowed to oscillate within a defined region so as to minimize tissue laceration due to the patient breathing. Compared to many other small robots intended for medical applications, Robopsy is an order of magnitude less costly and lighter while offering appropriate functionality to improve patient care and procedural efficiency. This demonstrates the feasibility of developing cost-effective disposable medical robots, which could enable their more widespread application.

Precision passive mechanical alignment of wafers

A passive mechanical wafer alignment technique, capable of micron and better alignment accuracy, was developed, fabricated and tested. This technique is based on the principle of elastic averaging: It uses mating pyramid (convex) and groove (concave) elements, which have been previously patterned on the wafers, to passively align wafers to each other as they are stacked. The concave and convex elements were micro machined on 4-in (100) silicon wafers using wet anisotropic (KOH) etching and deep reactive ion etching. Submicron repeatability and accuracy on the order of one micron were shown through testing. Repeatability and accuracy were also measured as a function of the number of engaged elements. Submicrometer repeatability was achieved with as little as eight mating elements. Potential applications of this technique are precision alignment for bonding of multiwafer MEMS devices and three-dimensional (3-D) interconnect integrated circuits (ICs), as well as one-step alignment for simultaneous bonding of multiple wafer stacks. Future work will focus on minimizing the size of the elements.

Optimal design techniques for kinematic couplings

Abstract Kinematic couplings are well known to the precision engineering community as simple devices that provide rigid, repeatable connection between two objects through usually six local contact areas. They serve many applications that require 1) separation and repeatable engagement, and/or 2) minimum influence that an imprecise or unstable foundation has on the stability of a precision component. Typically, the coupling design process starts by arranging or adapting one of two classic configurations, the three-vee coupling or the tetrahedron-vee-flat coupling, to suit the geometry of the application. It is often sufficient to analyze only the contact stresses and perhaps the coupling stiffness when the configuration remains fairly conventional (i.e., planar) and the application is not particularly demanding. Otherwise, effort spent optimizing the configuration through additional analysis and/or testing is well worthwhile. This paper proposes several optimization criteria and presents analysis techniques for optimizing kinematic coupling designs. The general modeling approach uses [6 × 6] transformation matrices to reflect contact stiffness matrices to a common coordinate system where they are added together as a parallel combination, for example. This method has wider applications particularly for flexure systems, which will be the subject of a future article. In addition, the reader may find the kinematic coupling designs presented in this paper useful for future applications.