M. Seetharama Bhat

Experimental Demonstration of H∞ Control based Active Vibration Suppression in Composite Fin-tip of Aircraft using Optimally Placed Piezoelectric Patch Actuators

The goal of this study is the multi-mode structural vibration control in the composite fin-tip of an aircraft. Structural model of the composite fin-tip with surface bonded piezoelectric actuators is developed using the finite element method. The finite element model is updated experimentally to reflect the natural frequencies and mode shapes accurately. A model order reduction technique is employed for reducing the finite element structural matrices before developing the controller. Particle swarm based evolutionary optimization technique is used for optimal placement of piezoelectric patch actuators and accelerometer sensors to suppress vibration. H∞ based active vibration controllers are designed directly in the discrete domain and implemented using dSpace® (DS-1005) electronic signal processing boards. Significant vibration suppression in the multiple bending modes of interest is experimentally demonstrated for sinusoidal and band limited white noise forcing functions.

Vibration control in a composite box beam with piezoelectric actuators

This work presents a theoretical and experimental investigation of vibration control of composite box beams using distributed, surface mounted piezoelectric (PZT) patches as actuators. The finite element modeling of the box beam is done by formulating a first-order shear deformable active composite thin walled beam element. The degrees of freedom at each node are axial, bending in spanwise and chordwise directions, corresponding shears and twist. The superconvergent element derived uses higher order interpolating polynomials obtained by solving electromechanically coupled static governing differential equations. The open loop response is validated with experimental and analytical results available in the current literature. A system equivalent reduction expansion process (SEREP) is implemented for reduced order modeling. Open and closed loop responses to electrical bending actuation are obtained both experimentally and analytically using state space modeling. A proportional-integral (PI) controller using acceleration feedback is implemented for control of vibration due to single-frequency excitations. Experimental and numerical results correlate very well in the above cases. Eigenstructure assignment through output feedback is designed for multimodal control of transient responses.

Longitudinal Hinfinity Stability Augmentation System for a Thrust-Vectored Unmanned Aircraft

The development of discrete longitudinal H infinity stability augmentation system to improve the handling qualities of a radio-controlled unmanned aircraft is presented. The aircraft features thrust-vector control by deflection of a cold jet to expand the operating envelope. Output sensitivity and a control sensitivity minimization problem are considered to suppress low-frequency gust disturbance and high-frequency noise disturbance and to ensure proper control allocation between the elevator and the thrust flap. Gain scheduling is avoided by designing a single controller at the operating point most sensitive to perturbations in actuators and the aerodynamic model. To this end, a systematic method to choose the design operating point is presented. The closed-loop system is comprehensively tested for robust stability using mu analysis and for robust performance over the entire cruise envelope. Disturbance and sensor noise rejection capabilities are assessed for realistic gust and sensor noise using nonlinear simulation and hardware in loop simulation. The entire design procedure, is performed in discrete time.

Incremental-angle and angular velocity estimation using a star sensor

A method is described for estimating the incremental angle and angular velocity of a spacecraft using integrated rate parameters with the help of a star sensor alone. The chief advantage of this method is that the measured stars need not be identified, whereas the identification of the stars is necessary in earlier methods. This proposed estimation can be carried out with all of the available measurements by a simple linear Kalman filter, albeit with a time-varying sensitivity matrix. The residuals of estimated angular velocity by the proposed spacecraft incremental-angle and angular velocity estimation method are as accurate as the earlier methods. This method also enables the spacecraft attitude to be reconstructed for mapping the stars into an imaginary unit sphere in the body reference frame, which will preserve the true angular separation of the stars. This will pave the way for identification of the stars using any angular separation or triangle matching techniques applied to even a narrow field of view sensor that is made to sweep the sky. A numerical simulation for inertial as well as Earth pointing spacecraft is carried out to establish the results.

Robust design of multiple trailing edge flaps for helicopter vibration reduction: A multi-objective bat algorithm approach

The objective of this study is to determine an optimal trailing edge flap configuration and flap location to achieve minimum hub vibration levels and flap actuation power simultaneously. An aeroelastic analysis of a soft in-plane four-bladed rotor is performed in conjunction with optimal control. A second-order polynomial response surface based on an orthogonal array (OA) with 3-level design describes both the objectives adequately. Two new orthogonal arrays called MGB2P-OA and MGB4P-OA are proposed to generate nonlinear response surfaces with all interaction terms for two and four parameters, respectively. A multi-objective bat algorithm (MOBA) approach is used to obtain the optimal design point for the mutually conflicting objectives. MOBA is a recently developed nature-inspired metaheuristic optimization algorithm that is based on the echolocation behaviour of bats. It is found that MOBA inspired Pareto optimal trailing edge flap design reduces vibration levels by 73% and flap actuation power by 27% in comparison with the baseline design.

An Optimal g-Guidance Scheme for Satellite Launch Vehicles

A closed-loop steering logic based on an optimal (2-guidance is developed here. The guidance system drives the satellite launch vehicle along a two- or three- dimensional trajectory for placing the payload into a specified circular orbit. The modified g-guidance algorithm makes use of the optimal required velocity vector, which minimizes the total impulse needed for an equivalent two-impluse transfer from the present state to the final orbit. The required velocity vector is defined as velocity of the vehicle on the hypothetical transfer orbit immediately after the application of the first impulse. For this optimal transfer orbit, a simple and elegant expression for the Q-matrix is derived. A working principle for the guidance algorithm in terms of the major and minor cycles, and also for the generation of the steering command, is outlined.

Output Feedback-Based Discrete-Time Sliding-Mode Controller Design for Model Aircraft

Model aircraft provide a low-cost test bed for the design and validation of control algorithms. The control system should improve the transient response and have good disturbance-rejection properties. The stability derivatives of the aircraft vary due to in-flight velocity fluctuations and the controller should be invariant to such perturbations. Weight and power requirements of the aircraft payload restrict the number of onboard sensors and so a limited number of output measurements can be used for feedback.

Fast access method for onboard star catalog

A new method to retrieve the star data from the onboard catalog is presented. A query to find stars that lie within a cone of uncertainty angle of a given unit vector is answered quickly. If stars are present, a set of catalog numbers is returned along with the nearest neighbor to the center of the cone. An index to the star catalog is formed with a hash function that maps the given unit vector to an index in a hash table. The hash function preserves the stars’ closeness even after their translation to the hash table and thus facilitates the query of a star with a noisy measurement. The proposed organization of the star catalog, other data structures used, and their access methods are described. When subjected to a numerical simulation test with 10,000 random star vectors corrupted with noise, all of the queries are answered correctly. A hash table of 5773-word (16-bit) size is required for a star catalog containing 1613 stars (10,485 words). This method requires only 2.2 average catalog accesses and 2.0-μs process time per query, compared to a traditional binary search that requires 24 accesses and 3.8 μs.

An experimental and numerical study of piezoceramic actuator hysteresis in helicopter active vibration control

An aeroelastic analysis is used to investigate the rate dependent hysteresis in piezoceramic actuators and its effect on helicopter vibration control with trailing edge flaps. Hysteresis in piezoceramic materials can cause considerable complications in the use of smart actuators as prime movers in applications such as helicopter active vibration control. Dynamic hysteresis of the piezoelectric stack actuator is investigated for a range of frequencies (5 Hz (1/rev) to 30 Hz (6/rev)) which are of practical importance for helicopter vibration analysis. Bench top tests are conducted on a commercially available piezoelectric stack actuator. Frequency dependent hysteretic behavior is studied experimentally for helicopter operational frequencies. Material hysteresis in the smart actuator is mathematically modeled using the theory of conic sections. Numerical simulations are also performed at an advance ratio of 0.3 for vibration control analysis using a trailing edge flap with an idealized linear and a hysteretic actuator. The results indicate that dynamic hysteresis has a notable effect on the hub vibration levels. It is found that the theory of conic sections offers a straight forward approach for including hysteresis into aeroelastic analysis.

A novel autonomous orbit determination system using Earth sensors (scanner)

With a pair of conical Earth scanners operating as attitude sensors, four horizon points are obtained during each scan. Using three of the four points, a unique method is derived to obtain the distance and direction of the Earths centre with respect to the satellite position. The fourth point, used in a least squares or averaging manner, improves accuracy. Combining this data with on-board 3-axis attitude, it is shown that a position accuracy of better than 4 km could be achieved. A simple digital low pass filter incorporated in the measurement path improves the accuracy to better than 2 km, and also improves the transient response of the Kalman filter during sudden attitude disturbances due to panel rotations. A simple state prediction (position, velocity vectors) based on f, g series makes the state dynamics linear without deteriorating the performance. The average behaviour of the errors are based on 20 simulation runs.