Zhiming Ji

Design of optical triangulation devices

Abstract This paper presents the derivation of the precise relation between the displacement of a light spot on an objects surface and the displacement of its image on the detector in an optical triangulation device, along with applications of the design of triangulation devices. Based on this relation, improved designs of optical triangulation devices, including devices of adjustable configurations, are proposed and discussed.

Measurement of balance in computer posturography: Comparison of methods—A brief review

Some symptoms related to disequilibrium may not be detected by a clinical exam. Therefore, objective study is important in assessing balance. In this paper, methods to measure balance in computer posturography are compared. Center of pressure (COP) displacement, equilibrium score (ES) and postural stability index (PSI), the main measures of assessing balance are described and their merits and disadvantages are discussed. Clinicians should apply that measure which suits the specific strategies in a specific situation. Measuring devices such as Force plate, Balance Master and Equitest are also discussed. Although the Balance Master and Equitest devices are more costly compared to the force plate only, they are more useful for assessing balance relevant to daily life activities that might result in falls.

RAPID FREEZING PROTOTYPING WITH WATER

Abstract Rapid Freezing Prototyping (RFP) with water is a novel solid freeform fabrication technique that can generate three-dimensional ice objects by depositing and rapidly freezing water layer by layer. It provides a means to build a solid part (ice part) with the potential of better performance than other solid freeform fabrication techniques in many aspects. The support, where necessary, is made of brine whose freezing point is lower than pure water. After building the part, the support can be removed by utilizing the melting temperature difference between brine and water. Preliminary experiments have shown that the ice patterns can be used for design visualization and silicone molding. This paper will present the concept and some experimental results of the RFP process as well as its potential applications.

Dynamics Decomposition for Stewart Platforms

This paper shows that special features of the Stewart platform can lead to the decomposition of the moving plate and the legs in the dynamic analysis. Formulas for evaluating, separately, the driving forces needed for the movement of the legs are developed for studying the combined inertia effect of all the leg rotations in a Stewart platform. The proposed formulation is easy to implement for Stewart platforms with nonnegligible leg inertia.

Postural stability index is a more valid measure of stability than equilibrium score.

Researchers, therapists, and physicians often use equilibrium score (ES) from the Sensory Organization Test, a key test in the NeuroCom EquiTest System (a dynamic posturography system) to assess stability. ES reflects the overall coordination of the visual, proprioceptive, and vestibular systems for maintaining standing posture. In our earlier article, we proposed a new measure of anterior-posterior (A-P) postural stability called the Postural Stability Index (PSI), which accounts for more biomechanical aspects than ES. This article showed that PSI provides a clinically important adjunct to ES. In the present article, we show that PSI can provide an acceptable index even if a person falls during the trial, whereas ES assigns a zero score for any fall. We also show that PSI decreases as ankle stiffness increases, which is intuitive, while ES exhibits the opposite behavior. Ankle stiffness is generally recognized as an indicator of postural stability. These results suggest that PSI is a more valid measure of A-P stability than ES.

Study of planar three-degree-of-freedom 2-RRR parallel manipulators

Planar parallel robots are good candidates for microminiaturization into a microdevice. Fully-parallel planar robots have three kinematic chains (legs) connecting the moving platform to the fixed base. Functional workspace of planar fully-parallel robots is often limited because of interference among their mechanical components. Planar parallel robots with two kinematic chains connecting the moving platform to the base can be designed to reduce interference while still maintaining three-degree-of-freedom. This paper presents kinematics and singularity study of planar three-degree-of-freedom 2-RRR parallel manipulators. Forward displacement analysis of this type of robots is governed by a quadratic equation. The inverse problem is decomposed to yield up to four solutions. Singularity analysis reveals up to four singular orientations at each operation point (x,y) of the workspace, in addition to boundary singularities.

Workspace analysis of stewart platforms via vertex space

This article introduces the concept of “vertex space” for the study of the workspace of Stewart platforms. It shows that the vertex space has potential to link the dimensional parameters to the characteristics of the workspace of a platform. A method for constructing vertex spaces is developed. An example is provided to illustrate the influence of dimensional parameters on the vertex space.

Analysis of design parameters in platform manipulators

Multi-loop parallel structures of platform manipulators present challenge to their design analysis. For studying the workspace of the 3-6 class of platforms, we developed the vertex space concept in Ji (1994). This paper extends the vertex spaces concept to other structures for analysing and comparing their workspaces. We classify the vertices of the platform manipulators into types A and B. The platform manipulators are then divided into four groups based on their vertex types. Design parameters in platform manipulators are studied in terms of their functions and effects to vertex spaces and workspace.

Analysis and control of monolithic piezoelectric nano-actuator

This paper describes the analysis and control of a 2-DOF monolithic piezoelectric actuator. This actuator is part of a 6-DOF manipulator capable of linear resolution to 2 nanometer and angular resolution to 1 arc-second. Design of this actuator differs from the existing ones in that it has a monolithic structure which enables a high bandwidth, low drift, and high force realization. A number of nonlinearities exist in the actuator, stemming from the geometry and materials properties. For example, coupling of the actuator elements can be modeled as a soft spring which increases scale factor at high actuation levels. In this work, a combination of feedforward (amplitude-dependent input shaping) and feedback control are applied to reduce the effects of 1) scale factor nonlinearities, 2) hysteresis, and 3) output oscillations. Application of this actuator include: optoelectronics assembly, optical fiber alignment, and semiconductor processing.

Computational method to evaluate ankle postural stiffness with ground reaction forces.

We examined an existing method for evaluating postural sway based on force-plate technology. Through an improved mathematical model of postural dynamics, we propose a new method, which better evaluated postural sway and, in addition, computed ankle moment and ankle postural stiffness directly from the measured ground reaction forces. An example is detailed that demonstrates the utility of this approach. The proposed method does not involve filtering or numerical integration and considers the platform inclination. Results from normal subjects show a linear relation between the ankle moment and the sway angle during quiet standing.