Raymond C. Montgomery

Subsonic Maneuvering Effectiveness of High Performance Aircraft Which Employ Quasi-Static Shape Change Devices

This paper represents an initial study on the use of quasi- static shape change devices in aircraft maneuvering. The macroscopic effects and requirements for these devices in flight control are the focus of this study. Groups of devices are postulated to replace the conventional leading- edge flap (LEF) and the all-moving wing tip (AMT) on the tailless LMTAS-ICE configuration. The maximum quasi-static shape changes are 13.8% and 7.7% of the wing section thickness for the LEF and AMT replacement devices, respectively. A computational fluid dynamics panel code is used to determine the control effectiveness of groups of these devices. A preliminary design of a wings-leveler autopilot is presented. Initial evaluation at 0.6 Mach at 15,000 ft. altitude is made through batch simulation. Results show small disturbance stability is achieved, however, an increase in surface deflection is needed to offset five degrees of sideslip. This only applied to the specific device group studied, encouraging future research on optimal device placement.

A Distributed System Adaptive Control Strategy

One attitude control device being studied for large spacecraft consists of two counter-rotating rings, each designated as an annular momentum control device (AMCD), that are attached to a spacecraft using several magnetic bearings distributed along the circumference of the rings. For large spacecraft large rings are desirable. Unfortunately, for large rings flexibility is appreciable and it becomes necessary to account for the distributed nature of the rings in the design of the magnetic bearing controllers. Also ring behavior is unpredictably sensitive to ring termperature, spin rate, manufacturing imperfections, and other variables. For that reason a distributed adaptive microcomputer-based control system is being sought for ring stabilization and maneuvering. An original adaptive-control methodology for distributed-parameter systems is detailed and application to spinning ring, i. e., AMCD, stabilization is used as an illustration. The proposed methodology, presented as a step-by-step procedure, combines a lumped-parameter expansion description of distributed parameter systems with a fundamental simultaneous identification and control strategy. Simulations are presented providing preliminary evidence of the capabilities of the proposed procedure.

Identification of the Dynamics of a Two-Dimensional Grid Structure using Least Square Lattice Filters

The basic theory of least square lattice filters and their use in identification of structural dynamics systems is summarized. Thereafter, this theory is applied to a two-dimensional grid structure made of overlapping bars. Previously, this theory has been applied to an integral beam. System identification results are presented for both simulated and experimental tests and they are compared with those predicted using finite element modelling. The lattice filtering approach works well for simulated data based on finite element modelling. However, considerable discrepancy exists between estimates obtained from experimental data and the finite element analysis. It is believed that this discrepancy is the result of inadequacies in the finite element modelling to represent the damped motion of the laboratory apparatus.

Application of analytical redundancy management to shuttle crafts

This paper presents a description of the analytical redundancy management section of an experiment, under development for the space shuttle. The proposed experiment uses a modification of the bank of filters approach, described by Montgomery and Price earlier (AIAA J. of Aircraft, Vol. 13, No. 2, pp. 76-82, Feb. 1976). It is performed by injecting failures into simulated sensor data, generated in real-time or batch simulations of shuttle reentry and on-orbit flight phases. The previous analysis contained a bank of extended Kalman filters running in parallel. In the current study, these filters have been replaced by UD factorized filters. This was done to facilitate a microelectronic implementation. A comparative study of the previous, and current setup, through simulation of the microelectronics is being conducted. Data, obtained using the simulations, are presented. These data indicate that both the Kalman and UD factorized filters suffer from word bit size limitations to about the same extent principally because of modeling error. The principle advantage of the UD factorized filter therefore lies in the stability for serial measurement processing.

A testbed for research on manipulator-coupled active spacecraft

This paper describes a testbed developed to study control of manipulator-coupled spacecraft with active attitude control systems. Pre-design simulation studies are described that were used to size and design the experimental facility and plan testing programs. Results on the dynamics and control of this testbed are presented that were obtained from both simulations and experiments. Initial experimental results have been used to refine the simulators using output matching system identification. Results from this system identification are included in the form of comparisons between simulation and experiment time histories for various model parameter values. These show that the testbed is inherently nonlinear and that parameters, harmonic gear effectiveness, in particular, have appreciable effects on the dynamics.

Simulation of the assembly dynamics and control of Space Station Freedom

A large-angle, flexible, multi-body, dynamic modelling capability has been developed to help validate numerical simulations of the dynamic motion and control forces which occur during berthing of Space Station Freedom to the Shuttle Orbiter in the early assembly flights. This paper outlines the dynamics and control of the Station, the attached Shuttle Remote Manipulator System, and the Orbiter. The simulation tool developed for the analysis is described and the results of two simulations are presented. The first is a simulated maneuver from a gravity-gradient attitude to a torque equilibrium attitude using the Station reaction control jets. The second simulation is the berthing of the Station to the Orbiter with the Station control moment gyros actively maintaining an estimated torque equilibrium attitude. The influence of the elastic dynamic behavior of the Station and of the Remote Manipulator System on the attitude control of the Station/Orbiter system during each maneuver was investigated. The flexibility of the Station and the arm were found to have only a minor influence on the attitude control of the system during the maneuvers.

Nonlinear Modeling of a Long Flexible Manipulator and Control by Inertial Devices

We consider the modeling and control of a planar, long, flexible manipulator that is representative of current space-based robotic arms such as the Space Shuttle Remote Manipulator System. The arm is equipped with three actuators: 1) a shoulder motor; 2) a torque wheel at the tip; and 3) a proof-mass actuator at the tip. The goal is to investigate the potential use of inertial devices as control inputs for maneuvering tasks and vibration suppression. The parameters used for the inertial devices at the tip are comparable to those specified for the Mini-Mast facility at the Langley Research Center. A nonlinear distributed parameter model is obtained by the extended Hamilton Principle. The associated eigenvalue/eigenfunction problem is solved and a finite-dimensional state space model is assembled. A preliminary design of a Linear Quadratic Regulator is used and computer simulations illustrate the benefits of using the proposed actuators.

Testing of a Failure Accommodation System on a Highly Flexible Grid

This paper presents a scheme for on-line failure detection in systems with appreciable structural dynamics. The design is suboptimal because of extensive computational requirements of optimal schemes. To accomplish failure detection, the innovations sequence of a finite order Kalman filter is examined. Because of the heavy dependence of the system on the zero-mean character of the innovations sequence of the filter much attention has been given to the design and evaluation of the Kalman filters used. The filter designs are based on modal models of the structural dynamics. Two modal models were considered, one based on an analytic finite element model and the other based on empirically derived frequency and damping. Experiments using a grid structure are presented which illustrate operation and performance of the filter designs based on these models. The general character of the results presented is that appreciable errors exist in the filter design based on the finite element model. Substantial improvement results if the design model is modified to include empirically derived frequency and damping.

On incorporating reliability considerations into control system designs

This paper considers reliability in designing control systems for large orbital structures that are expected to have appreciable flexibility. Reliability is considered for both the control configuration (i.e. selecting sensor and actuator locations over an admissible set) and the ultimate operation of the system. The approach presented is to construct a cost function that indicates the absolute goal of the system and, for a given structural design, to determine the lowest achievable costs conditioned on the failure states of the system (i.e. which actuators and sensors are operational). These costs are then weighted and summed to provide an overall system performance measure. This measure is then minimized over the set of admissible control configurations. The approach is illustrated using a beam that has four actuators with the goal of achieving a given parabolic shape.

Adaptive control using a moving window Newton-Raphson parameter identification scheme

This paper will describe an adaptive control system for aircraft that is based on a parameter identification on-line scheme. The adaptive mechanism uses two elements: one, a digital batch processing of a data record to obtain identified parameters; and, two, algebraic relations specifying the control law constants given the identified parameters. Concerning the first, a data record representing a time history of the most recent state measurements serves as the input to a computational algorithm for the identifier. It has a finite length and, hence, as new measurements are taken, old ones are pushed out of the data batch. The computational algorithm used to identify the parameters is a modified Newton-Raphson scheme. It uses a linearized model to represent vehicle motions. The model is obtained from continuous-time representations of the aircraft dynamics. The continuous-time representation is converted to a discrete form necessary for the model using a first-order Euler integration scheme. The model contains 10 parameters which are to be identified. Three parameters relate to the longitudinal motions of the vehicle and the remaining ones relate to the vehicles lateral motions. The longitudinal and lateral model parameters affect only longitudinal and lateral model variables, respectively. This permits identification of longitudinal or lateral parameters as subsets when the data record does not contain adequate information for complete identification of all subsets of the model parameter space. Parameter subsets are altered only after a successful test of the rank of the information matrix related to that subset. The second element involves taking the parameter estimates and adjusting control law constants. That operation should be performed only after the process is converged to proper estimates of the parameters. Hence, another test (related to smallness of the model fit error over the data batch) is made to insure that the parameter estimates are meaningful. The design criterion for control law adjustments, given the parameters, is based on maintaining uniform linear response characteristics. Results of tests of the overall system will be presented. The tests are made on a fixed base, six-degree-of-freedom, piloted simulation of the F8-DFBW aircraft. The simulation is resident on the CDC 6600 real-time computer facility at Langley Research Center. The control system is resident on a computing system external to the 6600 facility, an EAI Pacer digital computer system. Control is scheduled using real-time interrupts; but, the processing of the adaptive mechanism is treated as a background job for the system. Records indicating performance of the identifier over a large range of flight conditions will be presented. Results will be presented for different data batch sizes (number of points) and densities (spacing in time between points).