Ben K. Wada

An Intelligent Control System for Multiple Degree-of-Freedom Vibration Isolation

Multiple degree-of freedom (DOF) vibration isolation and suppression capabilities are essential for precision control of a wide range of space-borne structures as well as earth-based systems. This paper presents the system design and implementation, control algorithms, parallel computing architecture, and real-time experiments of a six DOF Hexapod Active Vibration Isolation (HAVI) system developed at Intelligent Automation, Inc. The system design of the HAVI is based on the hexapod (Stewart Platform) concept. A novel geometric arrangement of a hexapod, called the Cubic Configuration is used. Magnetostrictive material Terfenol-D actuators are employed. A two-layer control system architecture incorporating local force feedback control and a robust adaptive filter control for active vibration isolation are employed. A high performance parallel computation engine MDSP-100 with multiple DSP processors and maximum 566 MFLOPs throughput is also developed for the HAVI applications and was used to implement proposed control algorithms. The unique architecture and functionality of the MDSP-100 facilitate the data acquisition and control law computation operations in a parallel computing fashion. Mechanical hardware for the HAVI system has been fabricated and tested. The six channel simultaneous active vibration control experiments were performed to demonstrate HAVI systems 6 DOF vibration isolation and suppression capabilities. A wide band (below 250 Hz) active vibration attenuation for all six channels was obtained in the real-time experiments.

Milling machine for the 21st century: goals, approach, characterization, and modeling

Ingersolls Octahedral Hexapod--a milling machine for the future--is described. The specific target applications and the performance goals for an enhanced version of the machine are illustrated. The approach to achieving the goals by incorporating of advanced composites and active chatter and vibration control using smart materials is discussed. The machine characterization performed on an existing machine, the FE models developed and the plans to use the characterization and the validated models in designing an enhanced machine are described.

Shape Control of Inflatable Reflectors

In this work studied are the means of maintaining the target design geometry of inflatable membranes that are used for low frequency antenna collectors. Considering the deviations from the target state as the linear combinations of a set of given n normalized undesirable displacement patterns (e.g., Fourier-Zernike functions), the linearized thin shell and membrane theories are used to obtain expressions for the components of the corresponding n number of induced macro strain states. Then the problems associated with the failure in implementing by piezofilm actuators all the components of the geometrically compatible macro strain states are studied. As a result of these studies several recommendations are given.

Inverse dynamics of adaptive structures used as space cranes

As a precursor to the real-time control of fast moving adaptive structures used as space cranes, a formulation is given for the flexibility induced motion relative to the nominal motion (i.e., the motion that assumes no flexibility) and for obtaining the open loop time varying driving forces. An algorithm is proposed for the computation of the relative motion and driving forces. The governing equations are given in matrix form with explicit functional dependencies. A simulator is developed to implement the algorithm on a digital computer. In the formulations, the distributed mass of the crane is lumped by two schemes, viz., trapezoidal lumping and Simpsons rule lumping. The effects of the mass lumping schemes are shown by simulator runs.

Advanced reconfigurable machine for Flexible Fabrication

In a recently awarded ARPA program to advance the state of the art of parallel actuated next-generation machine tools, a vertically integrated team led by Martin Marietta is applying recent advanced in electroceramic smart materials and advanced composites to achieve leapfrog advanced in precision, flexibility, and speed of machine tools. Specific approached to achieve these advanced include active vibration cancellation, improved control technology, and design optimization using advanced structural and dynamic models. In this program, the team will integrate large high-force actuators, composites, and active vibration control with the Ingersoll Milling Machine Companys innovative Octahedral Hexapod machine to develop to Advanced Reconfiguration Machine for Flexible Fabrication. The enhanced Octahedral Hexapod machine will provide new levels of machining flexibility while still retaining precision and low cost. This technology will have widespread impact on the flexible fabrication of materials--especially those that are tough to machine traditionally--in several industries, e.g., aerospace, defense, aircraft, and automotive.

Optimal Actuator Placement in Adaptive Precision Trusses

Precision structures, used in space, are usually fabricated from truss concepts and support precision equipment/objects that require a high degree of accuracy for reasonable performance. Maintaining this precision, in the face of disturbances that displace the positions of these precision points, can be achieved by the use of highly sensitive actuators, such as the JPL Active Member (Fanson et al., 1989, Anderson et al., 1990). These actuators are attached at their vertices (i.e., end points) to the nodes of the precision truss structure and are capable of length extension/contraction.

Adaptive Structures in Japan

A review of some of the research and development activities in Japan in the area of adaptive structures is given. The article is based on the authors participation in the First and Second U.S./Japan and Japan/U.S. Joint Conferences on Adaptive Structures in Maui. Hawaii (November 1990) and Nagoya, Japan (November 1991), their review of the papers presented in these meetings, personal contacts with the researchers, and on-site visitation of the laboratories both in the industry and also in the universities. The review covers the period ending January 1992.

Real-time control of geometry and stiffness in adaptive structures

Abstract Adaptive structures are those which can adjust their geometry, stiffness and damping on demand to] meet the changes in their loading environment. With the advents in the piezo-electric device and composite material technologies, such structures are already being considered seriously in aero-space industry. The success of these structures depends largely on the effective control of the structure through the actuators, and the sensors embedded in their load carrying members. This work outlines the basic theory for the geometry, stiffness and damping control. Necessary and sufficient conditions for stress free geometry control in statically determinate and indeterminate adaptive discrete structures are given. In particular, for the subset of discrete adaptive structures viz. adaptive truss structures the equations similar to the forward and inverse kinematic equations of open loop mechanical manipulators are derived. These equations describe the large geometry control under show motion (i.e., no inertia forces) assumption. Two criteria for choosing the optimum control from among the possible ones are established. A fast algorithm based on variable order variable step multistep method is given that can compute the controls for a large maneuver in real-time. Numerical results from the algorithm are presented. As an example of damping and stiffness alteration on demand, the vibration control in adaptive trusses by means of elongations and elongation-rates of the active elements is also given.

Effect of Joint Imperfections on Static Control of Adaptive Structures as Space Cranes

Effect of imperfections in the joints of an adaptive structure on its slow (no inertia forces) motion along a prescribed trajectory as a space crane is studied. Two mathematical models to predict the effect of joint imperfections are proposed. The two models are used to obtain estimates of the deviations of the node of the space crane to which the end-effector is attached, from its prescribed trajectory. An application of the models to a two-section space crane is given.

PZT/PVDF flexible composites for actuator and sensor applications

Currently, smart structures utilize both polymeric sensor and PZT-based actuators. The benefit of self-sensing calls for the development of integrated actuator/sensor composites. This paper addresses the development and properties of this new class of smart materials. Controlled porosity (amount, size, shape and distribution) were fabricated using sintering, blending and spin- on-disc. New flexible, thin polymer based composites were obtained. The optimization between d31 and g31 was performed to provide both actuator and sensing capabilities. It was found, that processing conditions including poling can be adjusted to provide the contribution from both PZT and VDF.