John E. Lloyd

Extending the RCCL programming environment to multiple robots and processors

The Robot Control C Library (RCCL) is a system for developing robot control programs in a UNIX environment, and has proven to be useful in research applications. Work undertaken in expanding RCCL to handle multiple robots, and upgrading its implementation to a multiple-processor microVAX/UNIX system is reported. This includes (1) reworking the primitive set to allow for the specification of multiple robot actions; (2) modifying the trajectory generation mechanism so that several robots may be controlled and coordinated at once; and (3) redesigning the system interface on top of which RCCL is built to allow the creation of multiple real-time robot control tasks interfaced to UNIX.<<ETX>>

A dynamic model of jaw and hyoid biomechanics during chewing

Our understanding of human jaw biomechanics has been enhanced by computational modelling, but comparatively few studies have addressed the dynamics of chewing. Consequently, ambiguities remain regarding predicted jaw-gapes and forces on the mandibular condyles. Here, we used a new platform to simulate unilateral chewing. The model, based on a previous study, included curvilinear articular guidance, a mobile hyoid apparatus, and a compressible food bolus. Muscles were represented by Hill-type actuators with drive profiles tuned to produce target jaw and hyoid movements. The cycle duration was 732 ms. At maximum gape, the lower incisor-point was 20.1mm down, 5.8mm posterior, and 2.3mm lateral to its initial, tooth-contact position. Its maximum laterodeviation to the working-side during closing was 6.1mm, at which time the bolus was struck. The hyoids movement, completed by the end of jaw-opening, was 3.4mm upward and 1.6mm forward. The mandibular condyles moved asymmetrically. Their compressive loads were low during opening, slightly higher on the working-side at bolus-collapse, and highest bilaterally when the teeth contacted. The models movements and the directions of its condylar forces were consistent with experimental observations, resolving seeming discordances in previous simulations. Its inclusion of hyoid dynamics is a step towards modelling mastication.

ArtiSynth: A Fast Interactive Biomechanical Modeling Toolkit Combining Multibody and Finite Element Simulation

ArtiSynth (http://www.artisynth.org) is an open source, Java-based biomechanical simulation environment for modeling complex anatomical systems composed of both rigid and deformable structures. Models can be built from a rich set of components, including particles, rigid bodies, finite elements with both linear and nonlinear materials, point-to-point muscles, and various bilateral and unilateral constraints including contact. A state-of-the-art physics simulator provides forward simulation capabilities that combine multibody and finite element models. Inverse simulation capabilities allow the computation of the muscle activations needed to achieve prescribed target motions. ArtiSynth is highly interactive, with component parameters and state variables exposed as properties that can be interactively read and adjusted as the simulation proceeds. Streams of input and output data, used for controlling or observing the simulation, can be viewed, arranged, and edited on an interactive timeline display, and support is provided for the graphical editing of model structures.

ACME, A Telerobotic Active Measurement Facility

We are developing a robotic measurement facility which makes it very easy to build “reality-based” models, i.e., computational models of existing, physical objects based on actual measurements. These include not only models of shape, but also reflectance, contact forces, and sound. Such realistic models are crucial in many applications, including telerobotics, virtual reality, computer-assisted medicine, computer animation, computer games, and training simulators.

Automatic prediction of tongue muscle activations using a finite element model

Computational modeling has improved our understanding of how muscle forces are coordinated to generate movement in musculoskeletal systems. Muscular-hydrostat systems, such as the human tongue, involve very different biomechanics than musculoskeletal systems, and modeling efforts to date have been limited by the high computational complexity of representing continuum-mechanics. In this study, we developed a computationally efficient tracking-based algorithm for prediction of muscle activations during dynamic 3D finite element simulations. The formulation uses a local quadratic-programming problem at each simulation time-step to find a set of muscle activations that generated target deformations and movements in finite element muscular-hydrostat models. We applied the technique to a 3D finite element tongue model for protrusive and bending movements. Predicted muscle activations were consistent with experimental recordings of tongue strain and electromyography. Upward tongue bending was achieved by recruitment of the superior longitudinal sheath muscle, which is consistent with muscular-hydrostat theory. Lateral tongue bending, however, required recruitment of contralateral transverse and vertical muscles in addition to the ipsilateral margins of the superior longitudinal muscle, which is a new proposition for tongue muscle coordination. Our simulation framework provides a new computational tool for systematic analysis of muscle forces in continuum-mechanics models that is complementary to experimental data and shows promise for eliciting a deeper understanding of human tongue function.

Singularity-Robust Trajectory Generation

A singularity-robust trajectory generator is presented that, given a prescribed manipulator path and corresponding kinematic solution, computes a feasible trajectory in the presence of kinematic singularities. The resulting trajectory is close to minimum time, subject to individual bounds on joint velocities and accelerations, and follows the path with precision. The algorithm has complexity O(M log M), where M is the number of robot joints, and works using “coordinate pivoting,” in which the path timing near singularities is controlled using the fastest changing joint coordinate. This allows the handling of singular situations, including linear self-motions (e.g., wrist singularities), where the speed along the path is zero but some joint velocities are nonzero. To compute the trajectory, knot points are inserted along the path, dividing it into intervals, with the knot density increasing near singularities. An appropriate path velocity is then computed at each knot point, and the resulting knot velocity sequence is integrated to yield the path timing. Examples involving the PUMA manipulator are shown.

Model-based telerobotics with vision

We describe an implemented model-based telerobotic system designed to investigate assembly and other tasks involving contact and manipulation of known objects. Key features of our system include ease of maintaining a world model at the operator site and a task-centric operator interface. Our system incorporates gray-scale model-based vision to assist in building and maintaining the local model. The local model is used to provide a task-centric operator interface, emphasizing the natural and direct manipulation of objects, with the robots presence indicated in a more abstract fashion. The operator interface is designed to work with widely available and inexpensive desktop computers with low DOF input devices (such as a mouse). We also describe experimental results to date, which include performing assembly-like tasks over the Internet.

Fast Implementation of Lemke&#8217;s Algorithm for Rigid Body Contact Simulation

We present a fast method for solving rigid body contact problems with friction, based on optimizations incorporated into Lemke’s algorithm for solving linear complementarity problems. These optimizations improve computation time in general and reduce the expected solution complexity from O(n3) to nearly O(nm + m3), where n and m are the number of contacts and rigid bodies. For a fixed number of bodies the expected complexity is therefore close to O(n) . Our method also improves numerical robustness, and removes the need to explicitly compute the large matrices associated with rigid body contact problems.

Predicting muscle patterns for hemimandibulectomy models

Deficits in movement and bite force are common in patients following segmental resection of the mandible consequent to oral cancer or injury. We have previously developed a dynamic model to analyse the biomechanics of an ungrafted segmental jaw resection with unilateral muscle and joint loss and post-surgical scarring. Here, we describe an inverse-modelling algorithm for automatically predicting muscle activations in the model for prescribed jaw movement and bite-force production. We present the results of simulations that postulate combined muscle activation patterns that could theoretically be used by patients to overcome post-surgical deficits. Such predictions could be the basis for future muscle retraining in clinical cases.

Timbrefields: 3d interactive sound models for real-time audio

We describe a methodology for virtual reality designers to capture and resynthesize the variations in sound made by objects when we interact with them through contact such as touch. The timbre of contact sounds can vary greatly, depending on both the listener’s location relative to the object, and the interaction point on the object itself. We believe that an accurate rendering of this variation greatly enhances the feeling of immersion in a simulation. To do this, we model the variation with an efficient algorithm based on modal synthesis. This model contains a vector field that is defined on the product space of contact locations and listening positions around the object. The modal data are sampled on this high dimensional space using an automated measuring platform. A parameter-fitting algorithm is presented that recovers the parameters from a large set of sound recordings around objects and creates a continuous timbre field by interpolation. The model is subsequently rendered in a real-time simulation with integrated haptic, graphic, and audio display. We describe our experience with an implementation of this system and an informal evaluation of the results.