Journal of Aerospace Engineering

In a general sense, control can be described as a cyclic sequence of observing, planning, and executing new actions. Similar to the human body, robotic devices are being equipped with a wide variety of sensors for the purpose of acquiring the needed information for planning and replanning. Because of the harsh work environment, construction robots, such as autonomous excavators, depend on hardened and reliable sensory systems, thus limiting the choices for selecting sensors. For control purposes, the sensory data need to be evaluated for decision‐making purposes. The intelligence for this endeavor has to be based on a thorough understanding of the meaning of the sensory data and high confidence in the appropriateness of future actions. In addition, a special obstacle‐avoidance concept is needed to prevent collisions and other types of accidents. The present paper discusses methods for both motion control and path planning for backhoe excavation. Experimental laboratory data are used to verify theoretical findings and to discuss the use of pattern recognition for the purpose of intelligent reasoning about the sensed environment. It is shown how patterns found in collected data could be used for automatic control of an autonomous excavator.

The introduction of robots and automation technology to construction operations will require considerable efforts to understand the mechanics of the many tasks better as they relate to building structures. To control arms, grippers, nozzles, and other production devices automatically, the mechanics of tools as they interact and work with the different construction materials requires extensive knowledge. One approach to learn about the behavior of all the components of an automated system for construction is to build scaled prototypes that are able to handle the actual material. This paper describes the design of an automated masonry system for prefabricating brick panels. It discusses the conditions that affect the bond strength of the masonry. It then describes the design and development of a Cartesian robot actuator and scaled‐down mortar spreader system built to test the bond strengths and accuracy of robotically placed bricks. The actual fabrication of the partially functional prototype helped the researchers to gain hands‐on familiarity with the many real‐world obstacles in using automation and robotics in construction. However, the learning experience also included an appreciation of the potentials offered by using advanced technologies in construction.

A National Aeronautics and Space Administration (NASA) research and technology program called the large deployable antenna (LDA) program was initiated in early 1989 by NASA Langley Research Center to investigate and demonstrate the availability of critical technologies for passive microwave imagers. The NASA Office of Aeronautics, Exploration, and Technology (OAET) sponsored this research program to provide advanced sensor technologies for mission to planet Earth remote sensing applications. The goal of the OAET program is to provide the technology needed to enable and enhance the long‐term observations, documentation, and understanding of the Earth as a system. The LDA program involved university, industry, and NASA researchers and technologists with a range of skills and experience from electromagnetic theory to advanced materials and deployable structures. The study approach involved determining basic operational parameters and configurations for a geosynchronous wide‐scanning radiometer from which specific structural requirements were used as goals (rather than specifications) with which specific technologies could be evaluated. The study team performed a detailed technology review, evaluated the feasibility and technology readiness for a large dual‐reflector radiometer, and developed a system concept for a 25 m deployable radiometer. This paper describes the science objectives, reviews the current state of the art, presents electromagnetic configurations under consideration, and discusses the mechanical systems development effort.

The results of a mathematical investigation of the dynamic response of the Sandia/Department of Energy/U.S. Department of Agriculture 34‐m‐diameter Darrieus rotor wind turbine erected at Bushland, Tex., are presented. The formulation used a double multiple stream‐tube aerodynamic model combined with a turbulent airflow and included the effects of (linear) aeroelastic forces. The structural analysis was carried out using previously established procedures with the computer program MSC/NASTRAN. A number of alternative expressions for the spectrum of turbulent wind were used. The modal loading represented by each did not differ significantly; a more significant difference was caused by imposing full lateral coherence of the turbulent flow. Spectra of the predicted stresses at various locations show that without aeroelastic forces, very severe resonance is likely to occur at certain natural frequencies. Inclusion of aeroelastic effects greatly attenuates this stochastic response, especially in modes involving in‐plane blade bending. Comparison is made with preliminary field data collected at medium‐low wind speeds (11.3 m/s) from critical locations on the blade. This comparison shows generally good agreement between measured stress spectra and model predictions based on 10% turbulence intensity and including aeroelastic forces.

Simultaneous design of a structure/active control system in which the mass of the actuators required to implement the active control is considered is addressed. The inclusion of required actuator mass essentially tightens the coupling between structure and control dynamics and introduces an inner iteration loop in the overall design process. Because this inner iteration loop significantly increases the computational burden, an algorithm for estimating the required actuator mass, given the control law and the desired maneuver, is presented. Additionally, a secant‐based actuator mass update algorithm is investigated in order to improve convergence characteristics of the inner iteration loop. A nonlinear optimization algorithm is used to direct the design process. The methodology is demonstrated on the design of a large two‐dimensional L‐shaped structure in which it is desired to minimize the line‐of‐sight‐pointing error between the two ends of the L after a worst‐case slew maneuver.

The conventional finite element method shows its weakness in determining the stress distributions along fiber matrix interfaces. This is because the chosen element displacement functions do not explicitly and/or implicitly satisfy the equilibrium and compatibility conditions that prevail at the interfaces. A procedure has been proposed to modify the conventional finite element method so as to allow the elastic stresses along the fiber matrix interfaces of two‐ and three‐dimensional fiber‐reinforced composites to be determined with improved accuracy. The results obtained by this modified method for a two‐dimensional problem are compared with both a photoelastic and a traditional finite elelment solution. The comparison shows that the modified solution is in better agreement with the photoelastic test data than the traditional solution. The results obtained by the modified method for a three‐dimensional problem are compared with a traditional finite element solution and the modified method is found to be superior.

This paper presents an iterative finite-difference technique for the analysis of axisymmetric spherical shells with variable wall thickness. The formulation is based on thin elastic shell theory. One-dimensional finite-difference points are used to discretize the shell into strips along the meridian, and an iterative technique is employed to determine the normal and meridional displacements. The stress resultants and bending stresses are then evaluated. Unlike existing analytical and finite-difference techniques, the proposed method is applicable with ease to any variation in rigidity along the meridian, to general loading conditions, and to steep and shallow shells. Results are presented and compared with those of the finite-element method.