Derek Rowell

Multi-dimensional treatment planning: II. Beam's eye-view, back projection, and projection through CT sections

Three features of a fully three-dimensional treatment planning program are presented: (1) The beams-eye-view provides the user with an accurate reproduction of anatomic features from the viewpoint of a treatment source. The source can be moved to any feasible position relative to the patient, permitting a choice which allows sensitive organs to be excluded from the beam. In this view a field defining aperture can readily be designed. (2) Back-projection of such an aperture shows the parts of the original transverse CT sections, or reconstructed sagittal or coronal sections, which may be covered by the selected beam. (3) Projection through the CT data from any desired origin provides an alignment film simulation which can be used to confirm accuracy of treatment, as well as help establish anatomic relationships relative to the margins of a treatment field.

Steering Law Design for Redundant Single-Gimbal Control Moment Gyroscopes

Two steering laws are presented for single-gimbal control moment gyroscopes. An approach using the Moore-Penrose pseudoinverse with a nondirectional null-motion algorithm is shown by example to avoid internal singularities for unidirectional torque commands, for which existing algorithms fail. Because this is still a tangent-based approach, however, singularity avoidance cannot be guaranteed. The singularity robust inverse is introduced as an alternative to the pseudoinverse for computing torque-producing gimbal rates near singular states. This approach, coupled with the nondirectional null algorithm, is shown by example to provide better steering law performance by allowing torque errors to be produced in the vicinity of singular states.

Redundant single gimbal control moment gyroscope singularity analysis

The robotic manipulator is proposed as the mechanical analog to single gimbal control moment gyroscope systems, and it is shown that both systems share similar difficulties with singular configurations. This analogy is used to group gimbal angles corresponding to any momentum state into different families. The singularity problem associated with these systems is examined in detail. In particular, a method is presented to test for the possibility of nontorque-producing gimbal motion at a singular configuration, as well as to determine the admissible motions in the case when this is possible. Sufficient conditions are derived for instances where the singular system can be reconfigured into a nonsingular state by these nontorque-producing motions.

Approach to control moment gyroscope steering using feedback linearization

This paper presents an approach to steering control moment gyroscopes. A technique based on feedback linearization theory is used to transform the original nonlinear problem to an equivalent linear form without approximating assumptions. Under this transformation, the spacecraft dynamics appear linearly, and are decoupled from redundancy in the system of gyroscopes. A general approach to distributing control effort among the available actuators is described that includes provisions for redistribution of rotors, explicit bounds on gimbal rates, and guaranteed operation at and near singular configurations. A particular algorithm is developed for systems of double-gimballed devices, and demonstrated in two examples for which many existing approaches fail to give adequate performance.

Quantitative Comparison of Alternative Sensory Displays for Mobility Aids for the Blind

This paper proposes a method to compare quantitatively various auditory display schemes communicating the course a blind traveler should follow to comply with information a blind mobility aid has acquired, and the optimal scheme is sought for that traveler. A computer system emulates different display devices which use amplitude modulation to indicate the error of a subjects location from the indicated course. The real-time Selspot-based TRACK system measures the location of the human subject in real time, and the error is presented to the subject via each of the emulated devices. The indicated course, human trajectory, and error are recorded in computer disk memory. The performance of the human in each task is evaluated by calculation of a transfer function of the human with each display and then using this transfer function as the criterion for comparison. The feasibility of designing the display schemes of blind mobility aids using this procedure is demonstrated. Thus, an optimal choice for the specific blind person can be made via this system before committing a particular mobility aid design to the lengthy development process.

Dynamic Modeling and Simulation of Gaseous Systems

A computer-based model for analyzing air/gas system dynamics has been developed. A set of generic elements are interconnected by generalized junction structures to represent a variety of system configurations. The response of pressure, flow, temperature and heat transfer rate at any point or station in a system is then determined due to a wide range of fluid, thermal and mechanical disturbances. The model has been used to analyze furnace implosions in fossil fuel power plants. A coal-fired plant model has been constructed and simulated. The model simulations of the furnace pressure excursions are in close agreement with the data from the field tests.

Interactive software for dynamic system modeling using linear graphic

A description is given of Lgraph, which is an interactive, graphical software package for creating, modifying, and reducing linear graphs. Linear graphing, a structured method for formulating and representing lumped-element, linear dynamic system models containing connected single-port energy storage, dissipative, and source elements, and two-port transduction elements, is explained through the example of a simplified, quarter-body car suspension. Lgraph enables the user to place elements from several domains in a connected graph, and then computes and displays elemental equations, constraint (loop and node) equations, and state equations for the dynamic system, all in symbolic form. Its operation is described by a second example, the modeling of a DC servomotor drive system. The procedural algorithms are described. The package was implemented using X Windows graphics and runs on Unix-based engineering workstations.<<ETX>>

Evaluation of volumetric differences through 3-dimensional display of distance of closest approach

Treatment planning programs offer the ability to view quantitative information on serial CT sections. Many of these programs also offer the ability to view structures of anatomical interest from a beams-eye view. Such displays may be of wire-frame objects or of shaded surface reconstructions derived from contours drawn in serial CT or MRI sections. Such surface reconstructions are not only shaded to give a 3-dimensional perspective, but can be colored in order to display information about some underlying property of a structure. Color has, for example, been used to demonstrate the distribution of dose on the surface of volumes of interest. We introduce a technique in which color is used to show the spatial differences between volumes of interest by displaying the surface of one volume and encoding in color the distance of closest approach of another volume. Regions in which the two surfaces are within a specified distance are shown in gray; regions in which the first structure lies outside the second are shown in reddish/yellow hues; and regions where the second is outside the first are shown in bluish/green hues with the hue being proportional to the distance apart. Such displays offer insight into anatomical relationships not otherwise easily obtained.

Boiler Implosion Control in Fossil Fuel Power Plants

A computer simulation using the modeling techniques outlined in a companion paper has been used to characterize and evaluate the control systems for furnace and stack implosion problems in fossil fuel power plants. The model simulations have shown that the draft control systems reduce the furnace pressure excursion significantly following a main fuel trip condition. The limitations of these systems are associated with actuator speed and stack pressure excursions during control actions. Preliminary study suggests that a novel additional or alternative solution may be furnished through discharging steam into the furnace and/or stack after a fuel trip.

Current And Future Developments In Radiation Therapy Treatment Planning

The advent of computed tomography (and other imaging techniques), the availability of interactive raster scan graphics display, and the rapidly diminishing cost of memory have made possible new approaches to long-standing problems in radiation therapy. These include: the secure definition (and display) of anatomy in all three dimensions; the synthesis of a variety of diagnostic studies; the identification of a target volume and of critical adjacent structures; the simulation (in three dimensions) of any available radiation field; the development of tools for guiding the treatment set-up and for its confirmation; and the comparison of alternative treatment strategies. Remote interactive planning is also desirable. We describe the implications for image archiving in providing these capabilities.