UB Virtual Reality Laboratory

PHYSICALLY-BASED MODELING THROUGH A DYNAMIC ATOMIC UNIT APPROACH FOR HAPTIC RENDERING: Towards Non-linear, Viscoelastic, Anisotropic Behavior

Abstract
The aim of computer haptics is to enable the user to touch, feel and maneuver virtual objects using a haptic interface. As the user "feels" the virtual object by applying force through the interface, complex calculations have to be done in real-time to generate a feedback force appropriate to the material properties of the object being "touched". In this work we propose a method for modeling soft bodies, which incorporate non-linear, viscoelastic, anisotropic behavior that will enable real-time user interaction and still satisfy the high force-feedback frequency requirements. In our work, we restrict the user interaction with virtual objects to palpation.

Project Summary

For simulations involving haptic rendering to be useful, physical deformations and force-feedback must be computed in an accurate and time-efficient manner. In order to meet stringent demands on force-feedback frequency, systems that have used methods such as Finite Element method, Mass-Spring-Damper models, Energy Minimization methods, etc., have had to compromise on the complex nature of soft tissues in order to build systems which involve real-time interactions between the user and the models. We propose a method for modeling soft bodies, which incorporate non-linear, viscoelastic, anisotropic behavior that will enable real-time user interaction and still satisfy the high force-feedback frequency requirements.

Our modeling is limited to the process of simulating palpation. Gould's Medical Dictionary describes the verb to palpate as: application of the hands or fingers to detect characteristics and conditions of local tissues of the underlying organs or tumors. Palpation is an inexpensive and safe assessment tool regarded to be valuable by experts in the field of manual medicine. Continual practice and exposure to various tissue conditions improves the skill of the practioner. Thus a palpation simulator will serve to perfect the art of manual contact techniques. The present work attempts to simulate palpation of soft viscous tissues, and organs with multiple layers of tissues of different characteristics.

The AU method is used to model soft body deformation in real time. The body is divided into uniform 3 dimensional cubes and each cube has physical characteristics encoded in it. The force applied is the input to a single atomic unit. The atomic unit absorbs some of the force and passes the rest onto its neighbors. Each neighbor then does the same until the force falls below a threshold. The total deformation is the accumulation of deformations of all the affected atomic units.

In our system, the input to the atomic unit is the total deformation, a portion of which is "absorbed" by the atomic unit and the rest is passed onto the neighboring units which in turn take up a fraction and this continues till the remaining deformation falls below a threshold. The cubes in the propagation tree compute a force proportional to the amount of deformation taken up by each and these forces are accumulated and fed back to the haptic device. The collection of atomic units representing the body is enclosed within a force-field. When the haptic probe enters the force-field, a force proportional to the distance penetrated by the probe, is fed back to the haptic device.

A single AU cube consists of a spring coupled with a damper connected in parallel (Voigt model of Viscoelasticity). Dynamic behavior of the AU cube is governed by Newton’s equations of motion. Non-linear elastic behavior is simulated by defining a quadratic relation between force and displacement. To simulate hysterisis, the body is modeled as pseudoelastic, where loading and unloading curves are treated as separate elastic materials. The loading and unloading curves will have different scale and shape factors. Creep and stress relaxation are simulated by the presence of the damper which produces a force proportional to the load at any instant. Thus the above mass-spring-damper model is able to simulate viscoelastic behavior of soft tissues. By defining different stress-strain and damping characteristics along different axes, anisotropy can be simulated.

The haptic and graphical simulations were carried out on a PC with 800MHz Pentium III processor, 512 MB RAM, with a 3DOF PHANToM™ device. The programs were written in VC++ 6.0 and used OpenGL as the graphics library.

People
Dr. T. Kesavadas (Dept. of Mechanical Engineering)
Amrita Chanda (Dept. of Computer Science and Engineering)

Sponsors
National Highway Administration
Center for Transportation Injury Research
Tactus Technologies
NYSTAR
UB CAT

Publications
[1] Chugh K., Mayrose J., Kesavadas T., The Atomic Unit Method: A Physically Based Volumetric Model for Interactive Tissue Simulation, accepted for oral presentation, World Congress on Medical Physics and Biomedical Engineering, Chicago, July 2000

[2] Mayrose J., Chugh K., Kesavadas T., A Non-invasive Tool for Quantitative Measurement of Soft Tissue Properties, presented at World Congress on Medical Physics and Biomedical Engineering, Chicago, July 2000

[3] Chugh K., Kesavadas T., Mayrose J., Validating the Atomic Unit Approach to Physically based Modeling for Virtual Reality, poster presentation, Graphics Interface, ay 2000, Montreal, Quebec Canada

[4] Mayrose J., Chugh K., Kesavadas T., Material Property Determination of Sub-surface Objects in a Viscoelastic Environment, paper presentation, Rocky Mountain Bioengineering Society, Colorado Springs, Colorado, April 2000

[5] Chanda A., Kesavadas T., Real-time Volume Haptic Rendering of Non-Linear Viscoelastic Behavior of Soft Tissue Through Dynamic Atomic Unit Approach, Appeared in the proceedings of the Medicine Meets Virtual Reality (MMVR 12) in Newport Beach, CA, January 16-17, 2004.

[6] Kesavadas, T., Chanda, A., “Haptic Rendering through a Dynamic Atomic Unit Approach: Towards Non-Linear, Viscoelastic, Anisotropic Behavior” Poster, Proceedings of Computer Assisted Radiology and Surgery 2003, 17th International Congress and Exhibition, London, UK. (Peer reviewed)