Dibakar Sen

A centro-based characterization of singularities in the workspace of planar closed-loop manipulators

The paper presents a centro-based characterization of singularities in planar multi-DOF mechanisms with arbitrary kinematic structure. A multi-DOF mechanism is treated as an assembly of several single DOF link sets and hence the terms like affected link set, active link set and active centro have been defined. Criteria for end-effector dependent and end-effector independent singularities of a closed-loop manipulator with given output link have been established in the paper in terms of centroes. Based on the relative disposition of the centroes, the singularities have been classified into five types and their physical implications have been explained. End-effector dependent singularities are shown to lie on the coupler curve of ab larger single DOF mechanism. A simple linkage transformation has been proposed for the study of manipulators with physical limits on joint motion under the general framework of close-loop manipulators. Several examples have been worked out to demonstrate the centro-based method of singularity analysis

A discrete state perspective of manipulator workspaces

Abstract The finite resolution of joint drives or sensors imparts a discrete nature to the joints of a manipulator. Because of this an arbitrary point in the workspace cannot be reached wihtout error even in ideal mechanical environment. This paper investigates the effect of this discrete nature of the joints on the accuracy of performance of a manipulator and develops a methods to select the joint states to reach a point with least error. It is shown that the configuration leading to least error cannot, in general, be found from configuration space, especially when there is large variation in the link lengths or joint resolutions or both. The anomaly becomes severe when the gross motion of the end-effector approaches the local resolution of the workspace. The paper also shows how to distinguish two workspaces which may be identical so far as the boundary points are concerned, taking the joint resolutions into account. Finally, the concepts have been extended to define continuous space global and local performance indices for general multi degree of freedom manipulators.

Mesh Processing for Computerized Facial Anthropometry

Understanding of the shape and size of different features of the human body from scanned data is necessary for automated design and evaluation of product ergonomics. In this paper, a computational framework is presented for automatic detection and recognition of important facial feature regions, from scanned head and shoulder polyhedral models. A noise tolerant methodology is proposed using discrete curvature computations, band-pass filtering, and morphological operations for isolation of the primary feature regions of the face, namely, the eyes, nose, and mouth. Spatial disposition of the critical points of these isolated feature regions is analyzed for the recognition of these critical points as the standard landmarks associated with the primary facial features. A number of clinically identified landmarks lie on the facial midline. An efficient algorithm for detection and processing of the midline, using a point sampling technique, is also presented. The results obtained using data of more than 20 subjects are verified through visualization and physical measurements. A color based and triangle skewness based schemes for isolation of geometrically nonprominent features and ear region are also presented. [DOI: 10.1115/1.3330420]

The question of minimax loop-size in planar homogeneous kinematic chains

A homogeneous planar kinematic chain (KC) is a KC having all its constituting smallest independent circuits (fundamental circuits) of the same size. Davies [J. Mech.3, 87–100 (1968)] established, using graph theory, that a planar KC (homogeneous) of mobility M cannot have all fundamental circuits of size greater than M + 4. In the present paper many counterexamples to Daviess statement are presented and it is indicated that the maximum size of the circuits depends not only on the mobility of the mechanism but also on the number of links. The counterexamples given consist of, primarily, single degree of freedom (DOF) planar homogeneous KCs of loop sizes 6, 7 and 8; some zero DOF and tw DOF chains are also included. The causes for the failure of Davies theorem have been analysed in detail to show that it is valid only in the domain of planar KCs with planar graphs.

A vision modeling framework for DHM using geometrically estimated FoV

Digital human modeling (DHM) involves modeling of structure, form and functional capabilities of human users for ergonomics simulation. This paper presents application of geometric procedures for investigating the characteristics of human visual capabilities which are particularly important in the context mentioned above. Using the cone of unrestricted directions through the pupil on a tessellated head model as the geometric interpretation of the clinical field-of-view (FoV), the results obtained are experimentally validated. Estimating the pupil movement for a given gaze direction using Listings Law, FoVs are re-computed. Significant variation of the FoV is observed with the variation in gaze direction. A novel cube-grid representation, which integrated the unit-cube representation of directions and the enhanced slice representation has been introduced for fast and exact point classification for point visibility analysis for a given FoV. Computation of containment frequency of every grid-cell for a given set of FoVs enabled determination of percentile-based FoV contours for estimating the visual performance of a given population. This is a new concept which makes visibility analysis more meaningful from ergonomics point-of-view. The algorithms are fast enough to support interactive analysis of reasonably complex scenes on a typical desktop computer.

Efficient computation of volume fractions for multi-material cell complexes in a grid by slicing

The paper presents a novel slicing based method for computation of volume fractions in multi-material solids given as a B-rep whose faces are triangulated and shared by either one or two materials. Such objects occur naturally in geoscience applications and the said computation is necessary for property estimation problems and iterative forward modeling. Each facet in the model is cut by the planes delineating the given grid structure or grid cells. The method, instead of classifying the points or cells with respect to the solid, exploits the convexity of triangles and the simple axis-oriented disposition of the cutting surfaces to construct a novel intermediate space enumeration representation called slice-representation, from which both the cell containment test and the volume-fraction computation are done easily. Cartesian and cylindrical grids with uniform and non-uniform spacings have been dealt with in this paper. After slicing, each triangle contributes polygonal facets, with potential elliptical edges, to the grid cells through which it passes. The volume fractions of different materials in a grid cell that is in interaction with the material interfaces are obtained by accumulating the volume contributions computed from each facet in the grid cell. The method is fast, accurate, robust and memory efficient. Examples illustrating the method and performance are included in the paper.

FUNCTIONAL SEGMENTATION OF STROKES FOR PRODUCT SKETCH UNDERSTANDING

The paper introduces the concept of functional segmentation for segregating the strokes in a product concept sketch into functionally meaningful groups. Analysis of the distributions of starting points, time and mean pressure associated with the strokes in sketches revealed that shape defining and shape detailing strokes are separable through thresholding of the stroke parameters. The inter-stroke time plot exhibited characteristic spikes which enabled stroke-sets to be conveniently segmented in to temporal groups. The temporal evolution of shape defining elements and the genesis of feature hierarchy showed significant correlation. This algorithmic stroke organization showed significant match with the segmentation done by designers interactively. Spatial grouping algorithms developed using Gestalt principles isolated shapes in a sketch well. A unified multi-parameter grouping and averaging technique is also presented for obtaining single stroke sketches without losing the feature structure. The work thus demonstrates that algorithmic functional segmentation require both geometric and non-geometric data (viz. coordinates, time-stamp and pressure associated with the strokes) and that the role based classification of strokes helps to relate functional composition of the product and temporal evolution of the sketch. The work identifies how the designers emotion, rational thinking and visualization skill influence the product concept sketching process.

On a Novel Geometric Representation of Rotation

This paper presents a novel method of representing rotation and its application to representing the ranges of motion of coupled joints in the human body, using planar maps. The present work focuses on the viability of this representation for situations that relied on maps on a unit sphere. Maps on a unit sphere have been used in diverse applications such as Gauss map, visibility maps, axis-angle and Euler-angle representations of rotation etc. Computations on a spherical surface are difficult and computationally expensive; all the above applications suffer from problems associated with singularities at the poles. There are methods to represent the ranges of motion of such joints using two-dimensional spherical polygons. The present work proposes to use multiple planar domain “cube” instead of a single spherical domain, to achieve the above objective. The parameterization on the planar domains is easy to obtain and convert to spherical coordinates. Further, there is no localized and extreme distortion of the parameter space and it gives robustness to the computations. The representation has been compared with the spherical representation in terms of computational ease and issues related to singularities. Methods have been proposed to represent joint range of motion and coupled degrees of freedom for various joints in digital human models (such as shoulder, wrist and fingers). A novel method has been proposed to represent twist in addition to the existing swing-swivel representation.

Task Dependent Boundary Manikins in Statistical DHM

The primary objective of the paper is to make use of statistical digital human model to better understand the nature of reach probability of points in the taskspace. The concept of task-dependent boundary manikin is introduced to geometrically characterize the extreme individuals in the given population who would accomplish the task. For a given point of interest and task, the map of the acceptable variation in anthropometric parameters is superimposed with the distribution of the same parameters in the given population to identify the extreme individuals. To illustrate the concept, the task space mapping is done for the reach probability of human arms. Unlike the boundary manikins, who are completely defined by the population, the dimensions of these manikins will vary with task, say, a point to be reached, as in the present case. Hence they are referred to here as the task-dependent boundary manikins. Simulations with these manikins would help designers to visualize how differently the extreme individuals would perform the task. Reach probability at the points in a 3D grid in the operational space is computed; for objects overlaid in this grid, approximate probabilities are derived from the grid for rendering them with colors indicating the reach probability. The method may also help in providing a rational basis for selection of personnel for a given task.

Identification of Distinct Events in an Assembly by Automatically Tracking Body Postures

Identification of distinct events in an assembly process plays a vital role in assigning difficulty of assembly to specific events within the process. Literature addresses identification of events in assembly by analyzing videos, which is a manual process. We propose an approach to identify these events automatically using a method based on tracking of body postures and time. We use this method for an assembly exercise designed to introduce progressively more reach difficulty, and was performed in laboratory settings to track all limb data in time. The tracked data was used as an input to identify distinct events in an assembly. We use a technique we call slope method to smoothen the input data. Using this method helps in identifying distinct events involved in an assembly task. The result is validated by identification of events by observation of videos and correlation of these events with those identified automatically using the proposed method.