Richard G. Cobb

Vibration isolation and suppression system for precision payloads in space

This paper describes the design and performance testing of a vibration isolation and suppression system (VISS) which can be used to isolate a precision payload from spacecraft borne disturbances. VISS utilizes six hybrid isolation struts in a hexapod configuration. Central to the concept is a novel hybrid actuation concept which provides both passive isolation and active damping. The passive isolation is provided using a flight proven D-strut design. The passive design is supplemented by a voice coil based active system. The active system is used to enhance the performance of the passive isolation system at lower frequencies, and provide the capability to steer the payload.

Sensor Placement and Structural Damage Identification from Minimal Sensor Information

A method of prioritizing sensor locations on a flexible structure for the purpose of determining damaged structural elements from measured modal data is presented. This method is useful in applications where practicality dictates only a small subset of the total structural degrees of freedom can be instrumented. In such cases, it is desirable to place sensors in locations yielding the most information about the damaged structure. No a priori knowledge of the damage location is assumed. The prioritization is based on an eigenvector sensitivity analysis of a finite element model of the structure. In addition, the dual problem is presented and solved, which determines the observability of change in the measured elgenstracture from the instrumented degrees of freedom. This analysis is used to determine the extent to which damage can be localized. An analytical example is presented that illustrates the relationship between the number of measured modes, the number of instrumented degrees of freedom, and the extent to which damage can be localized. Additionally, an analysis of an experimental cantilevered eight-bay truss assembly consisting of 104 elements instrumented with eight single-axis accelerometers is presented. The extent to which structural damage can be localized from the measurement data is limited by the number of measured modes.

Three-Dimensional Trajectory Optimization Satisfying Waypoint and No-Fly Zone Constraints

To support the U.S. Air Forces global reach concept, a Common Aero Vehicle is being designed to support the global strike mission. Waypoints are specified for reconnaissance or multiple payload deployments and no-fly zones are specified for geopolitical restrictions or threat avoidance. Because of time critical targets and multiple scenario analysis, an autonomous solution is preferred over a time-intensive, manually iterative one. Thus, a real-time or near real-time autonomous trajectory optimization technique is presented to minimize the flight time, satisfy terminal and intermediate constraints, and remain within the specified vehicle heating and control limitations. This research uses the hypersonic cruise vehicle as a simplified two-dimensional platform to compute an optimal analytical solution. An up-and-coming numerical technique is a direct solution method involving discretization and then dualization, with pseudospectral methods and nonlinear programming used to converge to the optimal solution. This numerical technique is first compared to the previously derived 2-D hypersonic cruise vehicle analytical results to demonstrate convergence to the optimal solution. Then, the numerical approach is applied to the 3-D Common Aero Vehicle as the test platform for the flat Earth three-dimensional reentry trajectory optimization problem. The culmination of this research is the verification of the optimality of this proposed numerical technique, as shown for both the two-dimensional and three-dimensional models. Additionally, user implementation strategies are presented to improve accuracy, enhance solution convergence, and facilitate autonomous implementation.

Multiple Method 2-D Trajectory Optimization Satisfying Waypoints and No-Fly Zone Constraints

Minimum time to target is one of the primary goals of a global strike mission. The Hypersonic Cruise Vehicle and the Common Aero Vehicle are currently being investigated for mission effectiveness. Additional mission requirements include passage through intermediate waypoints and avoidance of no-fly zones. Thus, a real-time or near real-time autonomous trajectory generation technique is desired to minimize the flight time, satisfy terminal and multiple intermediate state constraints, and remain within specified control limitations. The research herein presents a baseline technique, an analytical geometric trajectory optimization technique, and a dynamic optimization technique. Numerical examples for constant speed trajectories as well as decelerating flight are used to demonstrate and compare the presented techniques. These results show the significant time savings achievable through optimization, the accuracy and computation efficiency of the geometric solution, and the robustness and application of the dynamic optimization technique.

Scramjet Isolator Shock Train Location Techniques

Within a scramjet, the isolator can contain a shock train in which the static pressure increases from the inlet to the combustor. In order to control the location of the shock train, the shock train leading edge location must be detectable. The purpose of this research is to compare the accuracy of several shock train leading edge detection techniques to measurements made using high-speed shadowgraph photography. A cold-ow, direct-connect high-speed wind tunnel was used to collect all the data, which was then post-processed, for this research. Six methods were considered to locate the shock train leading edge. The rst four use linear interpolation along with pressure transducer measurements and locations. The measurements used include the ratio of static pressures, static pressure increase, static pressure standard deviation, and static pressure power spectral density. Additionally, two static polynomial models were developed which related the sum of the static pressures to the shock train location and the back pressure to the shock train location. The research validates that the spectral content and the pressure rise along the test section can accurately detect shock train leading edge locations. Further, the sum of the pressure transducers method provides a highly accurate leading edge location measurement that is computationally ecient.

Structural Damage Identification Using Assigned Partial Eigenstructure

A method of identifying damaged structural elements from measured modal data using an incomplete measurement set is presented. The method uses a mathematical optimization strategy to minimize deviations between measured and analytical modal frequencies and partial mode shapes. Damage is identified by determining the stiffness change to a finite element model required to match the measured data of the damaged structure. Damaged elements are obtained directly from the results of the cost-function minimization because the allowed structural changes are consistent with the original finite element formulation. The cost-function minimization is based on an assigned partial eigenstructure algorithm in which the physical properties of the structural elements are treated as control variables, which are chosen to achieve the measured partial eigenstructure. An iterative solution is used to solve the nonlinear optimization problem. Experimental results are reported for a cantilevered eight-bay truss assembly consisting of 104 elements.

Survey of state-of-the-art vibration isolation research and technology for space applications

A broad survey of current literature detailing the current state-of-the-art and future research trends in vibration isolation for current and proposed space systems is presented. Terminology for vibration control is first defined. Next, the vibration isolation problem is discussed taking into account nonlinearities of variable parameters. It is followed by a discussion of some current space specific applications, illustrating the tremendous importance of vibration control technology for space and the challenges these applications present. Next, a discussion of vibration control elements is provided. Finally, a summary of semi-active control strategies is presented and conclusions are drawn about the state of the art.

Fuel-Optimal Maneuvers for Constrained Relative Satellite Orbits

This research investigates strategies to enable a deputy satellite to hover within a defined volume fixed in the vicinity of a chief satellite in a circular orbit for an extended period of time. Previous research developed initial methodologies for maintaining restricted teardrop hover orbits that exist in a plane fixed within the chiefs local reference frame. These methods use the natural drift of the deputy satellite in the relative frame and impulsive thrust to keep the deputy in a bounded volume relative to the chief, but do not address fuel optimality. This research extends and enhances that work by finding the optimal trajectories produced with discrete thrusts that minimize fuel spent per unit time and stay within the user-defined volume, thus providing a practical hover capability in the vicinity of the chief. The work assumes that the Clohessy―Wiltshire closeness assumption between the deputy and chief is valid. Using the new methodology developed in this work, feasible closed- and nonclosed-relative orbits are found and evaluated based on a fuel criterion and are compared with an easily calculated continuous-thrust baseline. It is shown that in certain scenarios (generally corresponding to a smaller total time of flight) a discrete-thrust solution provides a lower overall fuel cost than a continuous-thrust solution. A simple check is proposed that enables the mission planner to make the correct strategy choice.

Toward Flapping Wing Control of Micro Air Vehicles

Research into insect-sized flappingwingmicro air vehicles has exploded over the last decade, yetmost of this work has focused on simply achieving flight, while leaving the issue of how to control it to future researchers. Here, previous work in the field of flapping wing control is summarized and each proposed technique is evaluated according to how many body forces and moments it can directly influence on the micro air vehicle and how complicated a wing flapping mechanism is needed to implement it. Though some promising techniques have been proposed, none satisfactorily address all of the design criteria; therefore, a new technique is proposed, biharmonic amplitude andbiasmodulation. This technique is analyzedwith quasi-steady blade-element techniques and shown to provide direct influence over five micro air vehicle body forces and moments in hover while requiring only two actuators. Furthermore, it is applicable to a wing flapping mechanism operating at its resonant frequency, which reduces the energetic cost of control for the micro air vehicle.

Closed-loop performance of a vibration isolation and suppression system

This paper describes the initial closed-loop isolation testing of the vibration isolation and suppression system (VISS) which can be used to isolate a precision payload from spacecraft borne disturbances. VISS utilizes six hybrid isolation struts in a hexapod configuration. The VISS experiment has recently been tested at the Phillips Laboratory and significant closed-loop vibration isolation was achieved. The control approach, the performance testbed, and the initial test results are described.