Stephen L. Canfield

Optimal Synthesis of Compliant Mechanisms Using Subdivision and Commercial FEA

The field of distributed-compliance mechanisms has seen significant work in developing suitable topology optimization tools for their design. These optimal design tools have grown out of the techniques of structural optimization. This paper will build on the previous work in topology optimization and compliant mechanism design by proposing an alternative design space parametrization through control points and adding another step to the process, that of subdivision. The control points allow a specific design to be represented as a solid model during the optimization process. The process of subdivision creates an additional number of control points that help smooth the surface (for example a C 2 continuous surface depending on the method of subdivision chosen) creating a manufacturable design free of some traditional numerical instabilities. Note that these additional control points do not add to the number of design parameters. This alternative parametrization and description as a solid model effectively and completely separates the design variables from the analysis variables during the optimization procedure. The motivation behind this work is to create an automated design tool from task definition to functional prototype created on a CNC or rapid-prototype machine. This paper will describe the proposed compliant mechanism design process and will demonstrate the procedure on several examples common in the literature.

Velocity analysis of parallel manipulators by truss transformations

Abstract A method of velocity analysis of general parallel manipulators is presented. The method consists of transforming the parallel manipulator to an equivalent variable-geometry truss (VGT), and then generating the set of linear velocity equations that govern the transformed manipulator. VGTs are a special class of parallel manipulators. Their actuation is provided by extensible links terminating in spheric joints. A method for velocity analysis of VGT manipulators will be briefly reviewed. This method employs a connectivity chart to generate a system of linear algebraic equations for the manipulator velocity. In addition to the spheric joints and two-force member links that make up truss actuators, general parallel manipulators commonly contain revolute, prismatic, and cylindric joints and multi-force links, links that are more than two-force members. Further, a number of actuation schemes are possible. This work shows how such general parallel manipulators can be conceptually transformed into equivalent VGTs and thereby allow a straightforward velocity analysis approach. While this approach is applicable to general parallel manipulators, the method is most effective when applied to parallel manipulators with truss-like structures.

Multi-Objective Optimization of Compliant Mechanisms Including Failure Theories

This paper will present a version of failure theory suitable for designing optimal compliant mechanisms. The resulting theory will be incorporated as design objective functions within a multi-objective optimizing engine with the purpose of producing optimal and robust compliant mechanisms suitable for manufacture. Combining these failure-based objective functions with the classical ones measuring efficiency in performing a task, in the context of diversity promoting multiobjective optimization (see [1]) will demonstrate the tool’s ability to produce optimal compliant mechanisms that are failure-proof as well as provide insights into the complexity of particular design problems.Copyright

Similarity Criteria and Associated Design Procedures for Scaling Solar Sail Systems

T HE NASA In-Space Propulsion program has promoted the development of solar sail propulsion technology through the development of subsystems, operations tools, analytical and computational models, and ground-based testing. Solar sails can potentially provide low-cost propulsion and operate without the use of propellant, allowing access to non-Keplerian orbits through constant thrust. A solar sail is a gossamer structure: a membranebased large, lightweight structure. The primary objective of the sail is to convert emitted solar pressure into thrust on a spacecraft. This solar pressure is extremely small, however, on the order of 9 N=km at 1 astronomical unit, resulting in a need for the sail to bevery large and lightweight to achieve reasonable accelerations. The solar sail development process consists of a progression through ground-based testing and flight demonstration to yield a system ready to perform a specific science mission. The science missions envisioned will require sails on the order 2000 to 10; 000 m (or larger); sail size will be dictated by the mass and mission scope of the craft, while flight demonstrations and ground testing will be conducted at significantly smaller sizes [1,2]. For this reason, the ability to understand the process of scalability, as it applies to solar sail system models and test data, is crucial to the advancement of this technology. Because of the significant size of solar sails, the search for scalable sail designs or scaling laws that assist in testing and analyzing solar sail structures are fairly common in the literature. For example, scalable solar sail designs have been proposed by researchers, including Murphy [3], Gaspar et al. [4], Murphy and Murphey [5], and Greshik [6]. Here, the term scalable design seems to imply solar sail designs that retain a basic geometric relationship over various sizes, each tested to validate analytical models [3,4], or architectural design based on a set of sail panels sized to meet engineering requirements and numbered to meet the mission requirements [6]. Alternatively, several researchers develop scaling laws that govern the design of solar sails at larger sizes while maintaining similarity; that is, amodel can be built that represents the behavior of a prototype existing at a different scale. For example, Holland et al. [7] observed a set of scaling properties for inflatable structures (boom) based on geometric parameters, while Greschik et al. [8] andZeiders [9] offer scaling laws for dimensional analysis of solar sail structures. One way to perform dimensional analysis is through a process of nondimensionalizing a set of governing equations. None of the preceding contributions appear to use this approach. This Note is intended to provide an illustration of deriving the similarity conditions or criteria based on nondimensionalization of the governing equations for a model of the solar sail system. The similarity criteria will define the requirements needed to achieve similarity between a prototype and model solar sail design. The complete number of similarity criteria will be demonstrated through nondimensionalization of the governing equations for the solar sail system following the method of dimensional analysis as demonstrated for a variety of applications in [10–12]. This model will apply to a four-quadrant sail design as presented in [3,6,9]. The model will account for arbitrarily large sail deflection, sail–boom interaction, and the onset of buckling in the boom. The effects of wrinkling in the sail and nonlinear buckling behavior of the boom are two examples of higher-order effects not considered in this model. The results show a set of four independent similarity criteria that must be satisfied. A procedure is offered to demonstrate the use of these similarity criteria to guide the design of a model for ground testing.

A model for optimization and control of spatial compliant manipulators

This paper will present a kinetoelastic model appropriate for spatial compliant manipulators that will be used for size optimization and motion control of these devices. This model will be applicable to a class of compliant manipulators based on a parallel architecture that combines the characteristics of parallel manipulators with the low-cost, small-scale capabilities resulting from a compliant structure design. The model will address both the forward and inverse kinematic analysis of such devices, as well as form a design tool for dimensional synthesis in an optimal sense based on sensitivity to joint strain limits and manufacture, parameters that are critical in the performance of compliant manipulators. The model will then be applied to a specific compliant 3-degree-of-freedom manipulator topology to demonstrate its use in size optimization of the dimensional parameters of the selected compliant manipulator. The ability of the model to accurately solve the forward and inverse kinematics will also be evaluated through testing with the prototype. The authors provide a general discussion geared to the future implementation of this model in control of positioning compliant manipulators.

Optimization and Control of a Spatial Compliant Manipulator

This paper will present a model for force and motion control appropriate for spatial compliant manipulators that will characterize the elastic nature of these devices. This model will be applicable to a class of such compliant manipulators that combine the characteristics of parallel manipulators with the low cost, small-scale capabilities resulting from a compliant structure design. This model will be applied to a specific compliant three-degree-of-freedom manipulator topology and its use will be demonstrated as a design tool to perform size optimization of the dimensional parameters of the selected compliant manipulator. A general discussion towards future implementation of this model in control of micro positioning manipulators is given. The work is then demonstrated through development and fabrication of several miniature compliant manipulators (MCM) designed for pointing and precision motion control applications.Copyright

Developing Metrics for Comparison of Mobile Robots Performing Welding Tasks

Developments in mobile robotic systems are leading to new methods and techniques for manufacturing processes in fields that traditionally have not seen much automation. Some of these tasks require process validation prior to use in the manufacturing process. One such example process is welding. However, there is a lack of industry standards for mechanized or robotic welding that can impede the introduction of mobile robotic welding systems in the market place. There is also a lack of generalized fitness measures that gauge the suitability of mobile robot topologies or dimensional designs to a set of tasks and can be used in the design or verification process. This paper will propose such a metric and demonstrate its use in evaluating mobile robot designs for welding tasks. The approach will be based on the representation of a general task as a pair of n-dimensional subsets in the Euclidean n-space. Similarly, the robot capabilities are represented as n-dimensional subsets (manipulability and torque ellipse) in the Euclidean n-space. The motivation is to enable a direct geometric comparison of the capabilities of the robot to the requirements of the task yielding a quantitative measure of fitness. This method is suggested to be well suited to tasks comprised of a relatively short sequence of well-defined motions, called gaits, which are performed repeatedly or in a periodic manner. Some examples are welding, swimming, painting or inspection. The paper will demonstrate the use of this metric in the evaluation and design of mobile robots for welding tasks with a desired set of weld pattern motions. Three mobile welding platforms having different topological kinematic arrangements will be evaluated based on this design verification metric. This metric will further be shown to supplement the weld qualification process through verification of the motion control portions of the weld process based on a specific robot design. The method will contribute to the design and development of mobile robotic welding systems to become viable and accepted manufacturing processes.Copyright

Modeling and Design of a Linkage-Based Suspension for Tracked-Type Climbing Mobile Robotic Systems

Skid steer tracked-based robots are popular due to their mechanical simplicity, zero-turning radius and greater traction. This architecture also has several advantages when employed by mobile platforms designed to climb and navigate ferrous surfaces, such as increased magnet density and low profile (center of gravity). However, the suspension design plays a critical and unique role in track-based climbing systems relative to their traditional counterparts. In particular, the suspension must both accommodate irregularities in the climbing surface as well as transfer forces to the robot chassis required to maintain equilibrium. Furthermore, when properly designed, the suspension will distribute the climbing forces in a prescribed manner over the tractive elements. This paper will present a model for analysis and design of a linkage-type suspension for track-based climbing robot systems. The paper will further propose a set of requirements termed “conditions of climbing” that must be met to ensure stable (no falling) climbing for a given robot design over a range of climbing surface geometries. A recursive strategy is proposed to implement these conditions and yield a factor of safety in the current climbing state. This model will be compared through empirical testing with several prototype climbing robot systems. A method will also be demonstrated to use this model in the design of a preferred suspension system.Copyright

Developing a Kinematic Estimation Model for a Climbing Mobile Robotic Welding System

Skid steer tracked-based robots are popular due to their mechanical simplicity, zero-turning radius and greater traction. This architecture also has several advantages when employed by mobile platforms designed to climb and navigate ferrous surfaces, such as increased magnet density and low profile (center of gravity). However, creating a kinematic model for localization and motion control of this architecture is complicated due to the fact that tracks necessarily slip and do not roll. Such a model could be based on a heuristic representation, an experimentally-based characterization or a probabilistic form. This paper will extend an experimentally-based kinematic equivalence model to a climbing, track-based robot platform. The model will be adapted to account for the unique mobility characteristics associated with climbing. The accuracy of the model will be evaluated in several representative tasks. Application of this model to a climbing mobile robotic welding system (MRWS) is presented.Copyright

Instruction of introductory programming course using multiple contexts

This paper describes the experience of redesigning a traditional CS1 programming course, utilizing traditional coding practices as well as microcontroller units (MCU) based coding, to provide multiple programming environments. The objective of this redesign is to improve the programming skills for engineering students by 1) providing them with program development experience in multiple contexts and 2) relating the initial programming experience to the typical notion of engineering through significant hardware experience. Typical CS1 courses are designed with an instructor led lecture focusing on the introduction of specific computer skills and languages while programming assignments and laboratories help strengthen these skills in the students. For this remodeling, in addition to the typical programming exercises, supplementary MCU based lab exercises were used to provide an additional, different programming target for increased learning and highlighting the complementary relationship between hardware and software. The outcomes of this effort demonstrate that the addition of a MCU to an introductory programming course can work as an effective motivator, providing the students with a secondary context to reinforce programming skills developed during the course, and that providing multiple contexts (traditional desktop programming and hardware-based programming) together can aid in learning and the transfer of knowledge.