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Development of methods for measuring and predicting behavior of rigid rotors with stiff blades at high advance ratios and low rotor speeds
Rotating ring stores momentum in mechanical system. Drag is reduced by suspending rotating part in magnetic field. Combination of composites and forroceramic materials are used, providing a strong stiff device, which is better damped than metals.
The acoustical treatment mass penalties required to achieve an interior noise level of 80 dBA for high speed, fuel efficient propfan-powered aircraft are determined. The prediction method used is based on theory developed for the outer shell dynamics, and a modified approach for add-on noise control element performance. The present synthesis of these methods is supported by experimental data. Three different sized aircraft are studied, including a widebody, a narrowbody and a business sized aircraft. Noise control penalties are calculated for each aircraft for two kinds of noise control designs: add-on designs, where the outer wall structure cannot be changed, and advanced designs where the outer wall stiffness level and the materials usage can be altered. For the add-on designs, the mass penalties range from 1.7 to 2.4 percent of the takeoff gross weight (TOGW) of the various aircraft, similar to preliminary estimates. Results for advanced designs show significant reductions of the mass penalties. For the advanced aluminum designs the penalties are 1.5% of TOGW, and for an all composite aircraft the penalties range from 0.74 to 1.4% of TOGW.
A linearized, single-axis model is presented for bilateral force reflection which facilitates investigation into the effects of manipulator, operator, and task dynamics, as well as time delay and gain scaling. Structural similarities are noted between this model and impedance control. Stability results based upon this model impose requirements upon operator dynamic characteristics as functions of system time delay and environmental stiffness. An experimental characterization reveals the limited capabilities of the human operator to meet these requirements. A procedure is presented for determining the force reflection gain scaling required to provide stability and acceptable operator workload. This procedure is applied to a system with dynamics typical of a space manipulator, and the required gain scaling is presented as a function of environmental stiffness.
This paper describes the design, fabrication, test, and operation of a unique six axis force/torque dynamometer The specimen table used to hold components under test is supported in a full six axis kinematic mount. Support struts fabricated from high strength steel with special integral two axis flexures link the specimen table to the load cell transducers. Realtime force/torque coordinate transform, root mean squared calculation, and data averaging, and color six axis display with controllable scaling are provided by a specially programmed desktop computer. The extensive structural analysis and design optimization required to obtain a stiff, well damped, lightweight structure is described in detail. Geometric optimization of the kinematic mount and fabrication details of the struts are also described.
The present paper concerns itself with the active suppression of mechanical vibrations on a representative future spaceborne optical interferometer. This is accomplished by the incorporation of a set of piezoelectric struts into the truss structure of the interferometer and the use of these active struts to modify the damping and stiffness characteristics of the truss. It is shown that vibration propagation can be significantly reduced through the use of very simple control laws. It is further shown that the force and stroke requirements for the active struts for this application are rather modest and fall well within the capabilities already demonstrated by prototype hardware in ground testing.
In the work reported here it is demonstrated that acoustic radiation pressure may be used in simulated low gravity to produce stable bridges significantly beyond the Rayleigh limit with S as large as 3.6. The bridge (PDMS mixed with a dense liquid) has the same density as the surrounding water bath containing an ultrasonic standing wave. Modulation was first used to excite specific bridge modes. In the most recent work reported here the shape of the bridge is optically sensed and the ultrasonic drive is electronically adjusted such that the radiation stress distribution dynamically quenches the most unstable mode. This active control simulates passive stabilization suggested for low gravity. Feedback increases the mode frequency in the naturally stable region since the effective stiffness of the mode is increased.
A flexible structure was modeled and actively controlled by using a single space realizable linear proof mass actuator. The NASA/UVA/UB actuator was attached to a flexible planar truss structure at an optimal location and it was considered as both passive and active device. The placement of the actuator was specified by examining the eigenvalues of the modified model that included the actuator dynamics, and the frequency response functions of the modified system. The electronic stiffness of the actuator was specified, such that the proof mass actuator system was tuned to the fourth structural mode of the truss by using traditional vibration absorber design. The active control law was limited to velocity feedback by integrating of the signals of two accelerometers attached to the structure. The two lower modes of the closed-loop structure were placed further in the LHS of the complex plane. The theoretically predicted passive and active control law was experimentally verified.
This report describes DIGTEM, a digital computer program that simulates two spool, two-stream turbofan engines. The turbofan engine model in DIGTEM contains steady-state performance maps for all of the components and has control volumes where continuity and energy balances are maintained. Rotor dynamics and duct momentum dynamics are also included. Altogether there are 16 state variables and state equations. DIGTEM features a backward-differnce integration scheme for integrating stiff systems. It trims the model equations to match a prescribed design point by calculating correction coefficients that balance out the dynamic equations. It uses the same coefficients at off-design points and iterates to a balanced engine condition. Transients can also be run. They are generated by defining controls as a function of time (open-loop control) in a user-written subroutine (TMRSP). DIGTEM has run on the IBM 370/3033 computer using implicit integration with time steps ranging from 1.0 msec to 1.0 sec. DIGTEM is generalized in the aerothermodynamic treatment of components.
In the late 1980s, microgravity researchers began to voice their concern that umbilical-transmitted energy could significantly degrade the acceleration environment of microgravity space science experiments onboard manned spacecraft. Since umbilicals are necessary for many experiments, control designers began to seek ways to compensate for these "indirect" disturbances. Hampton, et al., used the Kane s method to develop a model of the active rack isolation system (ARIS) that includes (1) actuator control forces, (2) direct disturbance forces, and (3) indirect, actuator-transmitted disturbances. Their model does not, however, include the indirect, umbilical-transmitted disturbances. Since the umbilical stiffnesses are not negligible, these indirect disturbances must be included in the model. Until the umbilicals have been appropriately included, the model will be incomplete. This Technical Memorandum presents a nonlinear model of ARIS with umbilicals included. Model verification was achieved by utilizing two commercial-off-the-shelf software tools. Various forces and moments were applied to the model to yield simulated responses of the system. Plots of the simulation results show how various critical points on an ARIS-outfitted international standard payload rack behave under the application of direct disturbances, indirect disturbances, and control forces. Simulations also show system response to a variety of initial conditions.
High gain, high frequency flexible responses in gimbaled multibody systems are discussed. Their origin and physical significance are described in terms of detailed mass and stiffness modeling at actuator/sensor interfaces. Guyan Reduction, Generalized Dynamic Reduction, inadequate mass modeling detail, as well as system mode truncation, are shown to suppress the high gain high frequency response and thereby lose system flexibility important for stability and performance predictions. Model validation by modal survey testing is shown to risk similar loss of accuracy. Difficulties caused by high frequency responses in component mode simulations, such as DISCOS, and also linearized system mode simulations, are described, and approaches for handling these difficulties are discussed.
We show that the thermal fluctuations of very soft mechanical oscillators, such as the cantilever in an atomic force microscope (AFM), can be reduced without changing the stiffness of the spring or having to lower the environment temperature. We derive a theoretical relationship between the thermal fluctuations of an oscillator and an actively external-dissipative force. This relationship is verified by experiments with an AFM cantilever where the external active force is coupled through a magnetic field. With simple instrumentation, we have reduced the thermal noise amplitude of the cantilever by a factor of 3.4, achieving an apparent temperature of 25 K with the environment at 295K. This active noise reduction approach can significantly improve the accuracy of static position or static force measurements in a number of practical applications.
In order to be cost effective, space structures must be extremely light weight, and subsequently, very flexible structures. The power system for Space Station Freedom is such a structure. Each array consists of a deployable truss mast and a split blanket of photovoltaic solar collectors. The solar arrays are deployed in orbit, and the blanket is stretched into position as the mast is extended. Geometric stiffness due to the preload make this an interesting non-linear problem. The space station will be subjected to various dynamic loads, during shuttle docking, solar tracking, attitude adjustment, etc. Accurate prediction of the natural frequencies and mode shapes of the space station components, including the solar arrays, is critical for determining the structural adequacy of the components, and for designing a dynamic controls system. The process used in developing and verifying the finite element dynamic model of the photo-voltaic arrays is documented. Various problems were identified, such as grounding effects due to geometric stiffness, large displacement effects, and pseudo-stiffness (grounding) due to lack of required rigid body modes. Analysis techniques, such as development of rigorous solutions using continuum mechanics, finite element solution sequence altering, equivalent systems using a curvature basis, Craig-Bampton superelement approach, and modal ordering schemes were utilized. The grounding problems associated with the geometric stiffness are emphasized.
In order to be cost effective, space structures must be extremely light weight, and subsequently, very flexible structures. The power system for Space Station Freedom is such a structure. Each array consists of a deployable truss mast and a split blanket of photo-voltaic solar collectors. The solar arrays are deployed in orbit, and the blanket is stretched into position as the mast is extended. Geometric stiffness due to the preload make this an interesting non-linear problem. The space station will be subjected to various dynamic loads, during shuttle docking, solar tracking, attitude adjustment, etc. Accurate prediction of the natural frequencies and mode shapes of the space station components, including the solar arrays, is critical for determining the structural adequacy of the components, and for designing a dynamic control system. The process used in developing and verifying the finite element dynamic model of the photo-voltaic arrays is documented. Various problems were identified, such as grounding effects due to geometric stiffness, large displacement effects, and pseudo-stiffness (grounding) due to lack of required rigid body modes. Analysis techniques, such as development of rigorous solutions using continuum mechanics, finite element solution sequence altering, equivalent systems using a curvature basis, Craig-Bampton superelement approach, and modal ordering schemes were utilized. The grounding problems associated with the geometric stiffness are emphasized.
The effect of changing the control sampling period on the performance of the computed-torque and independent joint control schemes is discussed. While the former utilizes the complete dynamics model of the manipulator, the latter assumes a decoupled and linear model of the manipulator dynamics. Researchers discuss the design of controller gains for both the computed-torque and the independent joint control schemes and establish a framework for comparing their trajectory tracking performance. Experiments show that within each scheme the trajectory tracking accuracy varies slightly with the change of the sampling rate. However, at low sampling rates the computed-torque scheme outperforms the independent joint control scheme. Based on experimental results, researchers also conclusively establish the importance of high sampling rates as they result in an increased stiffness of the system.
Vibration in modern structural and mechanical systems can be reduced in amplitude by increasing stiffness, redistributing stiffness and mass, and/or adding damping if design techniques are available to do so. Linear Quadratic Regulator (LQR) theory in modern multivariable control design, attacks the general dissipative elastic system design problem in a global formulation. The optimal design, however, allows electronic connections and phase relations which are not physically practical or possible in passive structural-mechanical devices. The restriction of LQR solutions (to the Algebraic Riccati Equation) to design spaces which can be implemented as passive structural members and/or dampers is addressed. A general closed-form solution to the optimal free-decay control problem is presented which is tailored for structural-mechanical system. The solution includes, as subsets, special cases such as the Rayleigh Dissipation Function and total energy. Weighting matrix selection is a constrained choice among several parameters to obtain desired physical relationships. The closed-form solution is also applicable to active control design for systems where perfect, collocated actuator-sensor pairs exist.
Video system designed for examining interiors of burrows of gopher tortoises, 5 in. (13 cm) in diameter or greater, to depth of 18 ft. (about 5.5 m), includes video camera, video cassette recorder (VCR), television monitor, control unit, and power supply, all carried in backpack. Polyvinyl chloride (PVC) poles used to maneuver camera into (and out of) burrows, stiff enough to push camera into burrow, but flexible enough to bend around curves. Adult tortoises and other burrow inhabitants observable, young tortoises and such small animals as mice obscured by sand or debris.
This report addressed the design, analysis, and testing of the erectable truss structure for the Phase 1 CSI Evolutionary Model (CEM) testbed. The Phase 1 CEM testbed is the second testbed to form part of an ongoing program of focused research at NASA/LaRC in the development of Controls-Structures Integration (CSI) technology. The Phase 1 CEM contains the same overall geometry, weight, and sensor locations as the Phase 0 CEM, but is based in an integrated controller and structure design, whereby both structure and controller design variables are sized simultaneously. The Phase 1 CEM design features seven truss sections composed of struts with tailored mass and stiffness properties. A common erectable joint is used and the strut stiffness is tailored by varying the cross-sectional area. To characterize the structure, static tests were conducted on individual struts and 10-bay truss assemblies. Dynamic tests were conducted on 10-bay truss assemblies as well as the fully-assembled CEM truss. The results indicate that the static and dynamic properties of the structure are predictable, well-characterized, and within the performance requirements established during the Phase 1 CEM integrated controller/structure design analysis.